[Music] [Music] hello my name is Chris Harris and I'm from Olivia chemistry and welcome to this video on acids bases and buffers this is for the OCR a specification and so the content that's in this video is specifically designed and tailored towards the specification so if you are studying OCR then this this has everything that you need to know and nothing more and is designed as a revision tool so you may have already looked at this already and it just refreshing your knowledge on this topic so that's what it goes to clearly is really really important that you practice as well using exam questions because it's one thing knowing the content it's another thing athlete applying it and and doing it an exam question so make sure that you're able to answer questions properly and there are actually videos on a lyric chemistry YouTube channel that has that goes through pass paper walkthroughs and looks at the exam technique side so have a look around there and all the videos on our e 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topic so um so there's a lot to cover as you can see okay right so let's start with the discovery of acid-base theory okay so scientific theories develop over many years and they are improved upon by different Sciences that's just how science works science evolves constantly even the science today it's constantly been adapted or tweaked or improved or sharpened or applied to to everyday life because that's the whole point of Sciences that explains things around us and allows us to develop new technologies and theories so what we're doing here is we're looking back we're going down down the timeline and trying to look at how our current way of thinking has been adjusted and adapted over time so this is the history part of it so what we're going to look at is the link between oxygen and acids and this person a French man called Antoine Lavoisier's the Razia was famous for the study of oxygen and discovered that oxygen was responsible for the combustion for combustion such as burning fuels for example and named the gas in 1778 and Lavoisier's all knew at the time he was aware of formulas of acids such as sulfuric acid was h2so4 and nitric acid hno3 so was way back in 1778 so he knew that already he suggested that all acids must have oxygen in that's what that was his theory that's what he came up with Budi didn't know the formula of other acids such as hydrochloric acid so you could kind of excuse him for coming up with that theory because the acids that he was aware of they all contain oxygen so he made that link between the two and of course he found oxygen so you had this some kind of bias towards that as well but he wasn't aware of other acids so the proton and hydroxide line theory this was a little bit later on 1884 and you may have seen especially in the previous topics which is Arrhenius equation so this would have been in the rates of reaction section which is the how fast section of this of of module 5 but Savanti arenas was Swedish and he came up with a theory to do with physical chemistry so he's a physical chemist that's what he was and so he came up with a proton and hydroxide ion theory for this and Arrhenius said that acids donate protons that's H+ ions and bases release hydroxide ions which is always smile design so that was his thinking and he also suggested that when an acid and a base react they form salt and water so okay so he's a little bit closer to where we what we understand it to be currently at the moment and but he didn't explain why chemicals like ammonia a bases and we know that ammonia is a base and but the problem is he said that base has released hydroxide ions but ammonia doesn't have hydroxide ions so his theory was much closer to where we are now so it took these two people you need the two heads to do this which was bronsted-lowry who were danish and british chemists and basically they collaborated and improved the theory that Arrhenius did in 1923 so that's much much closer on just under 100 years ago so there's the picture so Larry Thomas Larry was British British scientist and your hands bronsted was was danish and they came up with an idea that acids are proton donors and they also said that when we mix acids with water that hydrogen ions that H+ ions are actually released into you into solution now H+ ions they also come with said they don't exist on their own in water they actually form hydroxo neum ions which is h3o plus sometimes you might see them as hydronium ions and basically it's these lines that make something acidic but for simplicity purposes what you will see much more common is just the use of H+ it's a lot easier to write h+ rather than h3o plus but just be aware that what makes something acidic is not just hate plus ions it's actually h3o plus so here's an example so H a represents an acid and you'll see that used quite a lot in this video reacting with water and it forms h3o plus that's your hydroxo Nia mine or hydronium i'm and that produces a - so this is the generic question generic equation so bronsted-lowry bases are proton acceptors so that's what he said so he said that acids are proton donors so they said should I say they said that acids of proton donors and bases are proton acceptors so when we mix bases with water they react with the H+ ions to form hydroxide ions that's a CH - ions so an example is when you react to base with water as you can see here that produces B h+ + o h- and that solved the problem with the ammonia because ammonia doesn't have a h- ions but it's actually the ammonia reacts with water to form the o h minus ions and indeed so do other bases as well so this really kind of answered some of the the issues of with the anomalous chemicals that didn't fit them theories so this is as you may have seen already this is the theory that we predominantly based chemistry around now until another scientist comes along and explains other issues and then this is the theory that we use so it's they'll call it the bronsted-lowry theory okay so that's on topic of acids so let's look at different types of acids so polyprotic acids is a as an example of acids that you will see in everyday everyday chemistry so and some acids donate more than one proton so it's not just exclusively one proton somehow - now we call them polyprotic or even poly basic if they accept if they accept protons so for example hno3 is a monoprotic or mono basic mole of basic substance so for example one mole of h2 no.3 will produce one mole of H+ ions so here's another one which is sulfuric acid sulfuric acid is diprotic one mole of sulfuric acid h2so4 will produce 2 moles of h plus ions because it's got it's got two protons for every molecule of sulfuric acid phosphoric is triprotic so it has one mole of phosphoric acid will produce 3 moles of h plus ions so you've got to be able to be familiar with these acids these are acids these are strong acids but these are acids that you see quite regularly in chemistry so you'll be familiar with them already but you do need to know the formulas follow okay so acids and bases acids react to the bases to form salts which are pH neutral so we call these neutralization reactions so h+ ions that are produced from acids they react with the o h minus ions that are produced from alkalis such as sodium hydroxide and these make neutral substances such as water is a classic example so a simplified ionic equation is h+ reacting with o x- to form h2o and this is a reversible reaction so let's look at a specific one so this is hydrochloric acid reacting with lithium hydroxide so this is your base and this is your acid this forms lithium chloride which is the salt and water which is the neutral byproduct that's produced from this now salts are made from the metal from the base or if it's the ammonium ion if it's a if it's an ammonium substance ammonia for example you reacting it with or and it's made from the nonmetal other than hydrogen from the acid so you can see here that we have our non metal which is chlorine from heat CL and a metal from the base and that forms lithium chloride which is the sauce here's another example so this is nitric acid reacting with potassium hydroxide and that forms potassium nitrate and water again so we're taking the non metal part from the from the acid and we're taking the metal part from the salt form from the base to form the salt we've got to be careful with ammonia because I'm only doesn't have an O H in there so it doesn't produce the O which minus sign so it uses water to produce the O H minus signs on its behalf so it does that by accepting a proton so you've got nh3 here accepts a proton from water and it forms ammonium ions which is NH 4 plus and Oh H minus science of disks remember anything like this in solution is what makes it basic so if we can even if it's directly or indirectly we can produce a rich minus ions then we produce a basic absolution apply so an example it'd be ammonia reacting with acids to make ammonium salts but no water so unlike these ones here where they produce water when we get when we react ammonia with an acid we don't produce water we just produce a salt which is neutral as well so that's fine so this is ammonia reacting with sulfuric acid producing ammonium sulphate which is the the name of the soul there okay so let's look at acids and metals and metal compounds and how they react you need to know a lot of reactions with acids acids form an integral part of chemistry so it's no surprise that they appear in various different reactions so let's look at the first one and so metals reacting with acids so got metal plus an acid will produce salts plus hydrogen so this is just standard equation you may have seen that GCSE you certainly need to remember at a-level and here's an example so we're calcium methyl reacting with sulfuric acid forming calcium sulfate and hydrogen gas and we also need to know our ionic equations as well so on equations all we do is remove spectator ions from the aqueous solution a spectator ion is something that doesn't get involved with the reaction in terms of it doesn't produce anything which is valuable and necessarily valuable to us so for example you've got calcium and sulfuric acid calcium sulfate and hydrogen so the integral parts here were calcium because there's the ions there and and the hydrogens because that's what makes it a cilix so H+ got a form h2 clearly the sulfate ions in there but the sulfate doesn't change from the left it doesn't change to the right so it's exactly the same so we call it a spectator ion is there it exists but it doesn't actually fall integral part of the reaction so methyl oxide plus acid will form salt plus water okay so we treat the metal oxide as a base because it is a base that's right so we form salt and water so an example would be magnesium oxide which is a metal oxide plus hydrochloric acid which is your acid will form magnesium chloride and water so this is your salt that's being produced and again we write ionic equation down an ionic equation shows us that we have MgO and which is a solid h+ from HCL and then we have mgcl2 and h2o now you'll notice here that we haven't split mg or the reason why is we can only form ions which are in solution so because magnesium oxide here is a solid we can't split it into its ions so anything that's like water which is liquid or solid must remain as it is you can't split them up the only things we can break break apart into ions alright anything which has a cube next to it so they're aqueous substances or anything dissolved in water so that's why the chloride bit here and the chlorine here is no there's no difference there there's no change there whereas from the magnesium there is we've gone from an mg 2 plus and obviously the water and water is produced as well but again we don't split water for that same reason okay so let's look at metal hydroxides and metal hydroxides react with acids to form salt and water again the very similar they were base so for example sodium hydroxide reacting with sulfuric acid will form sodium sulfate and water and the ionic equation and they're all aqueous we can all split them all up apart from the water so oh it's - is there is the integral part here the h+ is the most important bit here and this bit forms water so effectively it's a neutralization reaction so we've got Oh H minus and h plus okay so methyl carbonates and again these are these are bases as well but the difference is that this produces carbon dioxide as well so if you salt water but also carbon dioxide so an example would be lithium carbonate reacts with nitric acid to form lithium nitrate carbon dioxide and water so you'd see fizzing with this reaction the ionic equation would be h+ again we split that the nitrate as the spectator ion it hasn't changed left and right it's exactly the same and then we form carbon dioxide and water remember because these are not aqueous they still go into the equation but we don't split them up for example we don't split water up into oh it's minus and hate applause because this is liquid okay conjugate pairs so and a conjugate pair is linked by the transferring of a proton okay so basically what we're doing here is were following the proton where is it coming from and where is it going to and we pair their molecules up so any species that is gained a proton is the conjugate acid and a species that has lost proton is the conjugate base okay so this is not to be confused with a standard acids for example an acid is a proton donor and a base is a proton acceptor so a conjugate is the opposite of that so bronsted-lowry acids and bases they react according to this fot the following equilibrium so we've got beat eh-eh which is your acid B which is your base and that forms BH plus and a minus so this is the general acid base equation that you've just seen before all we're doing is we're putting it into simplified letters instead so H a is an acid in the forward direction as it donates a proton so remember that's what we said there a minus is a base in the reverse direction so that's Stephanie on the end so in the reverse direction as it accepts a proton from BH plus to form H a yeah so though it is there so there's the a minus so that would accept a proton to form h a which is here so that is classed as a base because it would accept proton to form that remember this is a reversible reaction so go forwards or backwards so we have a conjugate pair with H a and a minus so there you have a conjugate acid which is H a and your conjugate base is a minus so we also have a conjugate pair with B and BH plus so the conjugate acid is BH plus because that's the proton donor and the conjugate base is B so water reacts with acids to form h3o plus which is the conjugate acid and it reacts with the base to form a conjugate base which is oh it's - so we've got to be aware of that because you'll see that quite a lot and you've got to be careful that sometimes something might be called an acid in its name it's common name but he might not behave as an asset what you've got to remember is that that anything to be an asset it must donate a proton and anything anything that is a base must receive or accept a proton according to that bronsted-lowry Theory here so you just got to keep that in your mind and that's what you're looking for so here's an example so a base for example with the water so a base reacts with water to form be H+ and OH it's - so this is the effectively the the base is the B bit here is actually acting as an acid because it's accepting a proton to form BH plus whereas here H a which is the acid reacting with water water acts as an acid so water acts as a base because it's receiving that proton and hey CheY is acting h a effective e is breaking up to form a - okay so let's look at acids and bases being strong or weak so what we've got what we've got to distinguish and what you've got to be able to do is distinguish between weak acids and strong acids so when weak acids and bases react with water they form a reversible reaction so acids for example if we had an acid with water we form h3o plus remember that's your Hydrox only wine and a - that can be represented as just H + and a base and a base will react with water to form BH + and do H - and that's what makes something basic strong bases dissociate or ionize readily okay so they almost they pretty much dissociate completely in other words strong base such as a sorry a strong base such as sodium hydroxide will when you add it to water it will break up into its ions which is na plus and RH - and it will do that really easily weak bases won't and likewise with strong acids exactly the same strong acids dissociate really well so for example hydrochloric acid h HCL and their HCL breaks up readily in water to form H+ and Cl minus signs there's very little HCl and molecules left and you see them in the examples in this table so weak acid these are normally organic acids so things like ethanoic acid and other carboxylic acids that form the fall into that category anything with COOH on the end and the backwards reaction is favored in the in these types of reactions so we don't get many H+ ions produced and you'll see in this reaction here so in this case we're using ethanoic acid and the in terms of equilibrium it lines well over to this side which is the left hand side so we don't get much of these ions here which makes them a weak acid okay so strong acids on the other hand they are strong because they dissociate readily so hydrochloric acid sulfuric acid nitric acid the forwards reaction is favored strongly in these types of reactions so this breaks up readily to form haste plus and cl- ions strong bases and these are such as sodium hydroxide and potassium hydroxide these are cluster strong bases equilibrium lies well over to the right-hand side here so the strong bases dissociate readily produce lots of these ions here not there's not many of these left if any weak bases ammonia is a classic example of a weak base the backwards reaction is not favored so the backwards reaction is favored heavily in terms of ammonia so in other words we have lots of this on the left-hand side not so much of this on the right-hand side so because we don't have much oh it's minus ions therefore it's a it's classed as a weak base you need to be able you need to be able to associate strong bases strong weak bases and there's your acids so you need to be able to identify and categorize different acids and bases in terms of strong or weak because that's going to have a big impact on what type of reactions and what type of chemistry going to be looking at so you must you must be able to remember them okay so acid-base reactions so when acids and bases react with each other protons are exchanged so we we looked at that just a little bit before so what we need to do is now symbolize this in terms of an equation and see what's happening so here we've got a generic example of hey CheY which is the acid donating a proton to the base which is be a positive negative ions are produced so you can see here here's your acid there's your base that produces BH plus and a minus so you can see here that the base has accepted the proton from H a to 4b h plus and then the a is then left behind which is the a minus so this is what we classed as an acid so this is from year one chemistry so you would have listen I mean you would have seen this the reactions always in equilibrium okay so it's a reversible reaction and so if we add more H a to this and you would have seen this in the previous I've done a previous video on this to do with equilibria so basically it's how far that's that's the name of the video so how fast have a look at that but and this is to Alisha Tania's principal so if we add more H A or B than equilibrium will shift to the right to use it up so will produce more of this if we increase the amount of B h plus or a minus equilibrium will shift to the left so you need to be aware of that in terms of Lycia Tillie as principal so a lot of these topics interlink with each other although the videos are done in separately because it would take we know you probably a whole week to do the full lot but they're done separately just to break it up a bit but they do interlink and I think that helps quite a lot because instead of seeing them in isolation you've seen it as one and that helps you to remember it because you're making connections so water behaves as a base and when acid is add to it so notice in this reaction that the water molecule has accepted a proton to form the hydroxyl in your mind so here it is yeah so here's the acid H a reactant with the water okay the water molecule and water here is actually accepting a proton to form H three o plus so water is actually a base in here and hey CheY is the acid okay so remember always look for where that protons going so with strong acids remember equilibrium lies well over to the right okay and with weak acids equilibrium lies well over to the left so we it doesn't dissociate that much okay so now we've got to the the most of the acid stuff out of the way so the the basic principles of what an acid is what a base is how that correlates with H+ and are which - ions and what a strong and weak acid is what we can now do is really get into the flesh of this and we start to look at some of the some of these phenomena such as kW for example which is the ionic product product of water and you'll also see for example ka a little bit later on as well so there's a lot of these don't get them confused with KC which is to do with them which is not well not nothing to do with acids and nothing to us in the basis in in this sense here and within this topic so water exists in equilibrium with its Hinds so we've seen that already so in other words a glass of water doesn't just contain h2o molecules okay so water dissociates into hydroxide ions and hydroxyl ions as illustrated in this equation but also have seen that before as well so for example two molecules water will break down into the hydroxyl you mine which is hate to the O plus and the hydroxide ion which is oh it's - remember this can be simplified to h+ as well so that's yes this is probably easier to work with but you've just got to be aware that h+ really exists as h3o plus okay so the equilibrium law can be applied to this we're said regarding kc so this is a an element of kc but it's specifically to do with water so the equilibrium law can be applied as we have an equilibrium reaction so we can use the kc expression that we'd seen in a previous video to do with equilibria and to represent the reaction so we've got h plus a which - and history also remember it's always products over reactants when we're looking at equilibrium laws so water dissociates into its iron very weakly it doesn't break up very readily most of if you had a glass of water most of it would still contain the full h2o molecule you won't have many of these ions so the fact so in fact there's so little oh it's - an h+ ions compared to the water molecules the whole water molecules that we assume that the concentration of water has a constant value because it dwarfs the amount of water molecules in there compared to which - and H+ completely dwarf that so we need another expression to represent to take this into account effectively so if we multiplied the two constants KC and hey CH - oh so the constants of them and then we get a new constant which is called kW and we call it the ionic product of water and the units of kW are moles - DM to the minus 6 so you must remember that so we use a different constant for water it's always a little bit different something so simple has to have its own ethyl as the House of its own constant effectively so it's still to do with equilibrium but it's just a special type and here it is here so K W equals hate plus or which minus sign the h2o bit that was in the KC expression you can see there has been removed because of this assumption that we make and so this is the formula that we used for kW okay so let's delve into kW a little bit more so important points do with the value of KW so the value of K W is the same in a solution at a given temperature so there's no difference there it's the same with KC's you've seen before and kW always has a value of 1 times 10 to the minus 4 moles to DM to the minus 6 okay so that's the value of kW and this value changes if the temperature changes so that's the standard value that we use the benchmark value that we use for kW unless the jam board will tell you otherwise if it's in you example so pure water has an equal amount of hate plus or no H minus ions because if you're gonna from one water molecule you're gonna have for every H+ ion you're obviously going to have a low it's minus sign so we have an equal amount so when we're referring to pure water in other words there's no other elements there they're just 100% pure water we can simplify kW to h plus squared okay because we can assume that the amount of heat plus ions is the same as the amount of Voyage minor signs okay so let's look at the pH equation so pH is a logarithmic scale that measures the concentration of hate signs in solution so you would have seen this from GCSE probably well before before GCSE the pH scale so it starts from 0 to 14 0 being very acidic so a strong acid 14 being very basic so this is a a strong base and 7 is neutral so that's banging in the middle so the equation we use is pH equals minus log to the base 10 h+ ions h+ ions come from the solution and the logarithmic scale is what you'll find on the calculator but we'll look at that in a moment so pH can be calculated when we know the concentration of h+ ions so if we know the concentration of h+ ions in solution we can work out pH so if we example calculate the pH of naught point naught 3 moles p.m. cubed of hydrochloric acid so we know that hydrochloric acid is a strong acid so therefore we assume that the HCL molecule dissociates fully into H+ and Cl minus ions so what this means is that the concentration of H+ ions will equal the concentration of the HCL where it's come from so we can see here HCL dissociates form h plus and Cl minus so using the equation we can calculate pH so pH is minus log to the base 10 of naught point naught 3 and again you'll have that onto your and you calculate the automatic automatically is based M and the pH is 1 point 5 2 which seems reasonable because it's a strong acid so the concentration of hydrogen ions can also be calculated when we know the pH so we can effectively work backwards so calculate the concentration of hydrogen ions of nitric acid with a pH of 1.7 so we've given the pH we have to work out the H+ ions so all we do and here's the calculator is we just inverse the pH equation to give h+ equals 10 to the minus pH so how you get this bit is on the calculator so if you click shift press the shift button first then this bit there's your normal log button so you press that in see it comes up with ten and not come up with a little square that flashes and in that square you put in minus the pH that you've been given so that's one point seven so what you should get is an answer which is naught point naught to zero and if you put that in your calculator you should be able to get that same answer as well okay so let's calculate the pH of strong acids so when we calculate the pH of strong acids we we assume that they dissociate fully so this makes it really simple okay you'll see you'll do pH of weak acids and it's a little bit more complicated but strong acids are fairly straightforward so for monoprotic acids such as hydrochloric and nitric acids they dissociate one proton for every molecule of acids okay so this means that the concentration is you've seen just before the concentration of the acid equals the concentration of the H+ ions so for example the pH of so for example the pH of an or point two five moles per diem cubed of hydrochloric acid is h plus equals the concentration of acid so that equals the same because that's the assumption we're making so therefore we can say that the pH is the minus log of naught point two five that's just the concentration of the acid and that gives us a pH of naught point six because this is hydrochloric acid that's sensible because that pH is in the strong acid region if you're getting something like 13 or 14 something hasn't gone right because you can't get hydrochloric acid that's got a pH like that so you'll always check you answer the number has a meaning so relate it back so diprotic acid is a little bit more complicated but you've just really got to watch out for it's not too difficult just be just be really vigilant when you get these types of questions so for uric acids what that does is that produces two H+ ions for every acid molecule so softly OH acid produces two H+ ions so that means the concentration of the acid is two times the concentration of the H+ ions okay because we're producing two two times the amount of H+ ions so for example if we want to work out the pH of not 0.25 Sentry's cubed of sulfuric acid then we have two H+ ions e cause one acid molecule the concentration of so naught point two five cent moles p.m. cubed of acid produces double the amount of H+ ions so it's naught point five remember you've got to do that first then all we do is we plug that into our normal formula then we get a pH and again the pH is is not 0.3 so that's that's to be expected because sulfuric acid is a strong acid okay so let's look at the calculate the ph of strong bases so similar to strong acids but what we're doing is we're calculating the ph of a strong base we've got to assume that the dissociate fully okay which is the same as a strong acid but we're dealing with the base instead so for example his sodium hydroxide which is a strong base that dissociates fully into sodium ions and hydroxide ions and most of these strong bases these dissociates to produce one o h minus sign for every base molecule so it's a pretty straightforward ratio so it means whatever the concentration of the bases will be the same as the concentration of O h minus ions that it produces so to calculate the pH of a base we still need the h plus sign we still need that but to get this what we have to do is use that kW expression that you've just seen before so to work out h plus we need to know kW and we need to know the Oh H minus concentration at a certain temperature because member katyo readers change if the temperature changes so once we know the h plus ions are the h+ concentration we can work out pH so for example if we calculate the pH of naught point one five moles DM cube of sodium hydroxide solution at 298 Kelvin so that's the temperature kW remember has a has a value of one times 10 to the minus 14 unless it's specified otherwise so first what we're going to do is substitute the figures into the kW expression so the kW expression is 1 times 10 to the minus 14 equals H plus x by naught point 1 5 s and 0.15 is your concentration that's Kate ibly we then rearrange expression to find h+ so h plus is 1 times 10 to the minus 14 / not put 1 5 and that gets is six point six seven times by 10 to the minus 14 so that's the concentration of h+ and then now we've got the h+ we then look at our ph equation minus log of the H+ and then we get our pH of 13 point one eight again this is sensible because this is a strong base sodium hydroxide so that means so that means that the pH has got to be somewhere in the region of 13 to 14 and it does come up with that so make sure your answers are sensible that's the key thing if it's not then go back and see if we can work out where you've gone wrong okay so K a acid dissociation constant so weak acids dissociate slightly now you'll use this formula when you're working at the pH of a weak acid so the ones that you've seen there just before is a strong acid and a strong base which is fairly straightforward because you can assume they fully dissociate the problem is with weak acids they don't fully dissociate and so therefore it's really difficult to it's a little bit more trickier to calculate the the pH of that so they only dissociate slightly in aqueous solutions so we have to use another constant to help work out their pH values so what we can do is wear with a strong acid like you say we can assume that the amount of h+ ions equals the so the concentration of h+ ions equals the concentration of acids we can't do this for weak acids and so we use this acid dissociation constant which is ka and so for weak acids they exist in this following equilibrium so it's H a h+ a - no different from any other acids it's the same it's the position of equilibrium which makes weak acids different to strong acids so the first assumption we make when work when dealing with weak acids is that only a small amount of the weak acid that's h a dissociates so what we can assume is that the concentration of H a our equilibrium that's the amount of acid is roughly equal to the concentration of hitch here at the start of the reaction so that's before anything goes so the minute there will be a slight differ in reality but because the difference is so small is difficult to measure we can make this approximation this assumption so the equilibrium law can be applied here as we have an equilibrium reaction so we use the KA expression to represent the reaction so ka is hate plus a minus divided by H a which is at the start remember the assumption that we're taken is that the concentration of H a at the start is approximately equal to the concentration at equilibrium so we can use this even though it's in an equilibrium reaction the units are moles per diem cubed here and so therefore what we do is we put products over reactants that's the equation that we use the second assumption is the dissociation of acid is greater than the dissociation of water present in the solution and so we can assume that all haitch plus ions come from the acid okay so it's not coming from the water obviously they will being they will exist across the G because it's in solution but we assume that they all come from the acid and so therefore what we can assume is the amount of h+ ions is approximately equal to the amount of a minus ions so we can now simplify this KA expression that we've got here to something a little bit easier so this is a good thing about chemistry you just keep simplifying everything it doesn't have to be complicated so ka is hate plus squared over H a so basically we've just got rid of the a- and squared to the h+ so it's a lot easier to use when it's in that format okay so let's calculate the pH of a weak acid so remember we've got to use that KA expression we can't use the assumptions like we did for a strong acid okay so we use that KA expression so here's the question so calculate the pH of naught point naught 3 moles fleein cubed of ethanoic acid 298 kelvin the KA for ethanoic acid at 298 Kelvin is one point seven six times by 10 to the minus 5 moles per diem cubed so first of all what we're going to do is write down our KA expression so ka equals h+ squared over ch3cooh which is your ethanoic so the second thing is we rearranged to calculate the H+ squared that's what we want to work out because remember to calculate pH we need to know the H+ concentration so H+ squared equals K a multiplied by the concentration of ethanoic acid so h+ squared is one point seven six times by 10 to the minus 5 times by naught point naught 3 0 0 that gives us a h+ squared of five point two eight times by 10 to the minus 7 so that's the concentration of acid ions squared so that's the key thing because it's squared we got to take the square root of that answer to get just h+ because that's all we want so we take the square root of that answer and we get seven point two seven times by 10 to the minus 4 moles per diem cubed that's the concentration of h+ ions that this acid is produced but we need to work out pH so to calculate the pH we put it into that formula pH equals minus log of H+ ions that's what we've just worked out so minus log of seven point two seven gives a times 10 to the minus 4 gives a pH of three point one four again look at the number is that sensible yes it is because it's a weak acid so therefore our pH has got to be three point one four still the same okay so let's use that KA expression but this time what we're going to do is we're going to calculate ka or the concentration of a weak acid so instead of working out pH we're going to work out something else so here's another example so we're going to calculate the concentration in moles vidiian cubed of methanoic acid h co h at 298 Kelvin with a pH of 3.14 the KA for methanoic acid at 288 Kelvin is at one point seven seven times by 10 to the minus 4 multi and cubed Wow there's a lot of information there right so let's start the first one calculate H+ that the concentration of H+ in the pH equation that's what we've got to do first so the pH is minus log H+ and we rearrange that to give us H+ equals 10 to the minus pH so we've got work out the H+ first so remember would looked at the previous slide use your calculator shift log you gave me 10 you're flashing box just above the 10 you put in - Eph which we know is 3.14 so I've been told that so therefore the concentration of h+ ions here is 7 point 2 4 times by 10 to the -4 moles per decimetres cubes okay so then we write down the KA expression so ka equals h plus squared divided by methanoic acid then we rearrange that to calculate the concentration of the acid because that's what we want to know so that's 8 plus squared over ka we know what h+ is because we've just worked that out there so we do seven point two four times by tenth minus four squared divided by ka which we've been told so that's one point seven seven and so the concentration of weak acid is two point nine six times by ten to the minus three so calculating ka is exactly the same we just repeat steps one and two but we just substitute the numbers into into the straightaway we don't actually need need to rearrange so we just do h plus and divide it by the concentration of the acid which in this example you would be given if you're asked to work out and ka so it's them that's even more straightforward so you can see all we're doing is we're we're using a lot of these equations and just pooling them pooling them up so you'll have them equations in your mind and try and think right which one do i need to use and you know some of them for example contain h+ pH contains h+ k w contains h+ + k egg contains h+ so there's a lot of equations to concept h+ so a good idea is if in doubt calculate h+ because the likelihood is you're going to be able to put it in some form for question if you're unsure okay so this one's a new one this is called PK or calculating it so PK is another way of measuring the strength of an acid similar to pH and a notice is a little bit of a similarity that both start the little P so the lower the value the stronger the acid again just like pH the lower the value the stronger the acid so PKA is the minus log of ka now that brings about because pH is the minus log of h plus so in theory you could say that the P bit stands for minus log of something so that's how you can remember it so PKA is the minus log of ka pH is the minus log of h plus C so so there is logic behind it is a so calculate the value of PK value of an acid with a KA value of seven point five two times return 2 minus 3 this is dead easy and we just do minus log of ka and this gives us a pKa of two point one two so remember the lower the value the more siddik it is we can also rearrange that so we can do ka equals 10 to the minus PK again that's the same thing shift log on your calculator and we put minus PK in so for example calculate the pH of naught point naught 2 5 0 moles p.m. cubed of ethanoic acid at 2:00 298 Kelvin and as a pKa value of four point seven five at 298 Kelvin so the first one calculate the K first that's the first thing we need to do so ka equals 10 to the minus PK so 10 to the minus four point seven five so our K value is 1.78 times by 10 to the minus 5 it's not it's fairly straightforward when you look at it it's just there's a lot of things in there which think crikey that's quite that's quite scary unless of course you do they love all math of course and that may make it a little bit more easier and so then what we've got to do is we then go to calculate h+ from the k expression so this is the second thing we need to do so we know the value of KX you've just worked that out and this is the KA expression that we use here so h+ squared so we rearranged it to get h+ squared which is ka times by arthur Noack acid which is effectively one point seven eight times 10 to the minus 5 x naught point naught 2 5 0 because we've got that there h+ squared is four point four five times 2 10 to the minus 7 and then heat plus which is which is this bit here concentration of h+ square root so remember to square root it because this is tamas about H+ squared a lot of people miss that bit and they end up just putting that as the answer and so no that's it so be really careful the final thing now we know the concentration of H+ we can then work out pH so pH is minus log H+ so pH in this example is three point one eight again check to make sure that sensible don't just take the number as a as a you know for its for its own worth this is ethanoic acid it's a weak acid so it expect we weak acids to be around pH three four so so this is a this would be looks about right is quite strong weak acid but it's still a weak acid nonetheless okay so how do we actually measure the pH experimentally because a lot of this is about calculation a theory etc but the end of the day we've got to carry out things practically and we're going to know how we can measure the pH of a substance to either work out the concentration of something or to use the formulas as we've suggested there so what we do is we use pH meters and pH meters they measure the pH of a solution and but they've got to be calibrated now if you've used one of these in the lab and you'll know what a nightmare that can be and you've got to use and buffer solutions and what kind of buffers in a minute that's another thing but buffer solutions and you basically calibrate it using that so the buffer solution would have a buffer of three say three seven and nine let's say and you'll put your pH meter in and just make sure it is reading what them chemicals are so the pH probe must be placed in distilled water first you've got to clean the end of it and you've got to be careful they aren't quite delicate the meter should be reading seven if it's in pure water so you've got adjust that if it's not then what we do is repeat the process with standard solutions at pH four and pH ten and make sure that you rinse it with distilled water between each pH solution so in other words make sure it's pH seven if this is in a if this is in a if there's one of the testing while this is tap water on it but if this is in a buffer solution of pH for let's say and we put the pier probe in and it comes out as three we need to adjust we need to calibrate that because we know that that's four okay and you'll you'll see in a minute why pH meters they should be calibrated for use because you know if you it's a bit like getting on the scales if the skills is already showing to store and let's say and you step on it then you're going to be to store in heavier the other scales is gonna see your to stone heavier than what you actually are so so this is exactly the same except this is for pH meter okay and we always wash the probe after each test so we do not just stick it into different substances because you'll just contaminate it you've got to wash the little end of that of the of the probe to make sure it's completely washed of contaminants okay so let's look at titrations so titrations are are vital them in acid-base chemistry and titrations and basically allow us to work out the concentration of an acid or base and you must have done titrations before and they are very fiddly because you've got to make sure you're reading it accurately you've got to be making sure that you have the writes substances and the right places and that you're monitoring for an endpoint but anyway so in the burette you have an acid or a base with a known concentration in in the burette and you have an acid or base depending on which way around you doing a table see if we got an acid in the burette you'd have a base and the conical flask and vice versa but in the conical flask the substance whatever substance in there you have an unknown concentration but a known volume in the conical flask and you add you indicator in there so it might be for example 25 centimeters cubed of solution in the conical flask and then what we do we add the chemical in the burette into the conical flask until we get a change in the color of the indicator and this is known as the endpoint this is the point where you stop titrate and you measure how much of the substances you matters and we add it drop by drop near the endpoint you don't just you've got to go really really slowly otherwise because this one drop can make could be the difference between the endpoint and not being at the endpoint so that's how precise it is you've really got to go quite slowly towards the end and you read how much chemical was added from the burette to neutralize whatever chemicals in there and we always read from the bottom of the meniscus so you'll always read an I level so here's a diagram here and you can see that we have a meniscus here which is just the it looks like a semicircle you've got a top layer and you've got a semicircle underneath so you always read at the bottom of that meniscus so you can see here this is reading 20 centimeters cubes not 19.8 okay so that's really really important okay so and record your results to two decimal places and repeat until you get to two results that are concordant with each other and what we mean when we say concordant is that they're within naught point one centimeters cubed of each other so you've got to get results that are fairly close you can't have one at twenty one at twenty one and the one at twenty seven because you haven't you've got results that are all over the place and you can't be confident of what the actual value is so you keep on doing it until you get two results so if you get to 20s for example then you know the 22 and 27 are just anomalies you shouldn't count them okay so let's look at titration curves and so the following graphs these show a pH against the volume of base added from a titration so we've seen the mechanics of a titration and what this shows is different combinations of weak and strong acids and bases okay so what we're going to look at is the first one which is strong acid strong base the graph starts at pH one so you can imagine in your conical flask you have an acid solution in this example because what we're doing is we're adding a base to this conical flask and the pH is rising and rising and eventually it jumps up and becomes a basic because we're adding the base to it so there comes a point when we get to an end point so this is a very strong a strong base a very pronounced SS shape here so a strong acid weak base so we're adding a weak base to a strong acid that's in the conical flask so it starts off low pH because it's a strong acid and we're adding the weak base and because it's a weak base it doesn't rise as high as a strong base would so we get a shorter a shorter length a weak acid strong base so because we start with a weak acid the curve is a lot higher up the graph because a weak acids were around a six five or six somewhere around there so we start from there we have a strong base and that rises to a high level higher than what it was here before because it's a strong base the next one is a weak acid weak base now there's barely any change there's there's not a very well-defined endpoint here a weak acid in a weak base there isn't much difference in their PHS so when it's going to be really difficult to get an endpoint a nice clear endpoint on titrations like this so you can see what you've got to be able to do is identify what what type of titrations they are just from these graphs here so let's have a look at some of the features so we have something called on these graphs here something called an equivalence point or an endpoint and this is on the vertical part of the S shaped graph so at this point the acid has been neutralized fully by the base and the sharp vertical rise shows a rapid change in pH and that's why you've got to add your substances drop by drop because it takes a very small amount when you get near to that point there and it pH completely changes and you can try it next time you can see if you have a pH meter in the actual conical flask you'll see the change in pH and at this point we assume that the amount of H+ ions in there is the same as the amount of o H minus ions so we have the the same concentration of the respective ions at that point and that that means it's neutralized we don't have more H+ ions than which minus signs the change in pH is smallest when using a weak acid a weak base together in a titration so we've seen that on the previous graph there was a very small change and so in this example the blue line and what we're seeing is we're starting off with a strong base in the conical flask and we're adding an acid so a strong base is obviously got a high pH and that starts off high and as we add the acid from the burette it starts to drop and you get a massive change in pH and eventually it becomes all of a sudden it becomes more acidic so this is just showing the other way around so it is very important you know which way the graph goes will tell you what's in that conical flask so an indicator is quite a useful thing then because if you have two colorless liquids you have no idea where that endpoint is because it's just users having two colorless liquids together so an indicator is really useful because it tells us where that endpoint is we have to choose the right indicator you can't just be picking any old indicator here so a suitable indicator must change color entirely within the vertical part of your titration curve and obviously the vertical part depends on what type of titration you're doing as you've seen with the four different graphs before and so you can see here the in this example here we must choose an indicator that changes completely between a pH range of three and ten and in the outside that range and we're not necessarily going to find the endpoint so it's a variety of indicators that's can be used however two of them which are the most common are methyl orange and phenolphthalein so transit out one quick so phenolphthalein so um you can see here we've got color change from methyl Orange is somewhere between pH three and ph 4.5 which fits perfectly within our range here phenolphthalein changes between eight point two and ten so that one changes as well so we can use either of them indicators within this titration and so what you can see is a color change that's happening between them so for methyl Orange methyl orange is red at low pH and yellow at high pH values and so this can be used for strong acids and strong bases or strong acids and weak bases because they fit within that range phenolphthalein is colorless at low pH and pink at a high pH values so it can be used for the following which is a weak acid and a strong base so it can be used for that type of indicators you see the choice of indicator is important so when you're doing this in an exam what you've got to look for is that zone that Zone in that vertical part and select the right indicator now you see weak and week basis they have no sharp pH change and so no indicator is suitable so we must use a pH meter for that type of titration it's not the best titration to do it on because the the very limited scope that you have to measure the pH so really what we've got to do is measure the pH plot that on a graph of record it down and then plot the graph to see where your endpoint was okay so you wouldn't have to do but it wouldn't be titrations without calculations so we're gonna look through some of the calculations of titration methods here so this is just a bit of a reminder because you would have seen this in year one so remember the equivalence point is the point that when neutralization occurs so a pH meter is really used when you want to plot a graph to show this so let's look at this example so you've got fifteen point seven centimeters cubed of naught point four five zero most DM cubes of sulfuric acid was required to neutralize naught point 1 to 0 moles per diem cubed of sodium hydroxide calculate the volume of sodium hydroxide been neutralized in centimeters cubed so the first thing is we write out our balanced equation so this is h2 so4 which is sulfuric acid reacting with sodium hydroxide and that will form sodium sulfate and water you see here we're going to put sulfuric acid in the burette sodium hydroxide in the conical flask then what we have to do is calculate the number of moles of sulfuric acid so moles is concentration that's moldy and cubed times by volume so that's naught point four five times by fifteen point seven times ten to the minus three so what we've done here is you've got naught point four five which is the concentration and the volume is fifteen point seven what I've done is I've added times ten to the minus three on the side there because what that does is it shows us but by adding that effect you're dividing it by a thousand and that converts centimeters cubed into decimeters cubed so there we are okay so you can just divide that by a thousand if you want I find it easier to just put times ten to the minus three on eight on the end of anything that you want to divide by three because it just keeps a cleaner because the third thing was then we used the equation to find out the molar ratio in order to work out moles of sodium hydroxide and we call this stoichiometry so for example we have a 1 to 2 ratio between sulfuric acid and sodium hydroxide so what that means is the number of moles of sodium hydroxide so which is which is this but here is 2 times the moles of sulfuric acid so the number of moles of sodium hydroxide is 2 times 7 point zero seven times by 10 to the minus 3 because that's what we've just worked out here and that's going to give us naught point naught 1 4 1 moles of sodium hydroxide so basically we're working at that and then the fourth one the last step is we now calculate the volume using volume divided by moles absolute volume equals moles divided by concentration and we need to divide that by a thousand to get centimeters cubed at the end as well so the volume in decimeters cubed is moles divided by concentration so the volume is naught point naught 1 4 1 divided by naught point 1 2 0 because that's the concentration that we've that we've been told which is here and that gets us no point 1 1 8 decimate is cubed we need to find it in centimeters cube because that's what the questions after so it times it by a thousand to get centimeters cube that should say times by thousand rather than divide okay so this is looking at the the last section of this topic which is the introduction to buffers so a buffer is a chemical that resists the change in pH when small amounts of acid or base are added it's not it doesn't stop the change it just resists it ok that's really important buffers do not stop the change like we said and there's two types of buffers we have an acidic buffer and we have a basic buffer and we need to understand both buffers can be quite complicated so oh its listen carefully to this element in particular looking at the mechanics of a buffer and try to explain it in the best way possible I think the key thing in an exam though with a buffer is being able to articulate yourself to give a reason why buffer works or why you know explaining your evening and you know some of the you'll see here the way I described it in the right in the written part here is is to try and get as many keywords and there as possible and you see what I mean okay so acidic buffers so acidic buffers resist the change in pH in order to keep the solution the low pH 7 because acids are pH 7 so that's key resist is the key word here they are made from a weak acid and it's salt okay not any salt it's got to be the salt with a weak acid that you're using okay so an acidic buffer is using ethanoic acid so that's ch3cooh which is your weak acid and sodium athon oh it which is its salt okay so basically we take that and that and mix it and we've got a buffer okay they're really simple to make but they're they're really complicated to understand so right in any buffer solution but I'll try make it simple as a count of course so in any buffer solution there are two equilibrium equations at play okay so the two equations coexist in the same beaker so we've got one beaker with two reactions going on at once okay so just pause that a moment right here's the first equation okay so equation one is the acid the weak acid which is dissociating to form the ion and the H+ ion so we've got large amounts of this acid remember a weak acid dissociates Perley so we have lots of this and not much of this so equilibrium is well over to the left you have a large amount of them acid molecules the other reaction that's going on in this beaker remember this to the other one is the salt because we've chucked in a salt into the beaker as well the salt is dissociating it because it dissolves and dissolves in water so it breaks down to form this in this case it's a nathaniel iron and the sodium and but because of salt dissolves really well we don't have much of the original salt at all it pretty much breaks down fully into your two ions here so we say we have a high amount of each of their minds so imagine with both of these going on at once okay so that's the first difficult bit right to get your head round so this is where we're going to throw a little bit of a curveball into it this we get a little bit complicated just constantly keep in mind them two reactions at once they're both in the same beaker and this will help so what happens when we add an acid to this buffer and acid H+ remember so the H+ ions will react with the ch3coo minus ions in solution yeah why do they do that because positives will react with negative ions yeah and we do have and ch3coo minus signs we've got them here so thankfully we've got an abundance of them dissolve got loads of them here we've got a salt that produced loads of these Ives so that's fine so you can add loads of h+ ions and there's loads of co o minus signs in there so what that means this is where it's a little bit complicated so there's now going to interlink these two equations together because they're happening in the Beco at once remember what is that so you've got more CC h3 so you've got more ch3cooh is produced which means equilibrium shifts to the left okay so we are producing the H+ is reacting with this so what we're producing is more of this so by default we're shifting it to the left because of this this bit when we write them above each other is to the left of that we are moving that arrows pointing left isn't it okay what happens when we add a base to this buffer so have a think oh it's minus the minus is going to be attracted to a positive iron isn't it something that's positive so in this case it's going to be attracted to this which is H+ now you might think why wouldn't go with sodium it will react with sodium but it will form sodium hydroxide which is NaOH that is dissolved very readily okay in water so you'll just go back to square one you'll just produce na plus or no it's - so so it will react it just won't hold for very long it will break down straight away the moment is formed it'll just break down back into it again so what we're concentrating on here is a CH science now you might think well there's not much of these there's a low amount of these so I'd only have to add a handful of them in and it's already can be overwhelmed well this is where it gets quite clever so let me out with h+ ions but but there is a low concentration of these but they can be reproduced this is where lesser tilia comes in so the Oh h- reacts with the h+ removes them but this reaction is not just going to sit there and think oh well I'll just leave that what it's going to do is I'm going to replace them you've removed effectively you've removed some of the h+ ions from the solution so the acid is going to break down and form more h+ ions because and the reason why we can do that is because got loads of this i've got a high concentration of that there's plenty of them to replace that it's a bit like a stockroom you've got a stockroom with all your goods in yeah and that stockroom has to serve twenty supermarkets let's say so all them supermarkets order save some of the items it might be I don't know and tomatoes trays of tomatoes let's see so the Lord of all them tomatoes and the stockroom has to replace that it has to get them order some more in from the farm but it can replace it because it's got a source where it can replace them from if you see what means so so this is Lissa Tilly is principal so the equilibrium shifts to the right in this case and it replaces them H+ ions now this is tricky so if you understand this bit and the mechanics of it and see the equations are separate but conjoined and intertwined at the same time then you'll do well and you get your head round it so these keywords here in this bit is really important all about equilibrium it's all about militia Tilly is principal of a place and to the right all these keywords and mentioning concentration is very important okay so with that in mind we're now going to take that concept and we're going to use it to calculate the pH of a buffer so to calculate the pH of a buffer you need to know the KA value and the concentration of the weak acid and its salt so we need to know a lot of information here so let's look at this example so calculate the pH of a buffer that contains two point three five times by ten to the minus two moles of methanoic acid and one point eight four times by ten to the minus two moles of sodium meth and weight in one decimetres cubed of solution the value of ka at 25 degrees Celsius for methanoic acid is one point seven eight times by ten to the minus four moles p.m. cubed Wow like there is a lot of information there these are probably say it now these are probably going to be one of the most complicated calculations you'll ever do with chemistry okay add a little chemistry and so if you can get your head around this you'll be fine the rest of it is not so bad okay so this is so don't worry too much about it just lots of practice right so the first thing we need to do is write the equation and the KA expression so here we've got HC o h and producing h plus ions and HCL - so this dissociates ka equals h plus h co - h ce o-- h so we just write the KA expression we've seen that before okay so that's the first thing that we must do buffers might have a little bit of a difference so buffers h+ doesn't equal a minus the reason why is because we've got a salt and an acid it's coming from two different sources so we can't use this bit h plus squared c h0 - we must leave in the a - part a spit of a pain and but it it will come into use promise okay so we can't use it's the only time when you can't use this expression is when you're dealing with buffers okay so just remember that so we've got to use equilibrium concentrations here not initial concentration so that's important it must be equilibrium and and we do assume salts dissociate fully and weak acids dissociate poorly so what we assume here is the concentration of the salt that we have equals the concentration of a minus ions okay because the salt we assume the salt fully dissociates to produce all of the saltines that it makes em and that the concentration of the acid at the start okay because it dissociate it's a weak acid dissociates Perley equals the concentration at equilibrium in other words the amount of C if the concentration of the weak acid was 1 let's say for example right at the start the reaction we set the reaction away and even equilibrium it'll still be 1 it will be it'll be a little bit different but because it doesn't really dissociate so there the assumptions there critical them to assumptions there so the second thing is we rearrange the expression to get h+ okay so h plus equals ka times by h co h divided by the h cor - okay remember this has come from the salt that is the asset okay that's the ad dissociated i'm from the salt just to remember that okay so we have the number of moles in the question ok so concentrations must be used remember we're not using moles we've got to use concentrations so the concentration is moles divided by volume and in this case in this example its moles divided by 1 decimeter cubed okay so the number of moles equals concentrations in this example but you must use concentrations okay so we're dividing this by 1 decimeter cubed so third one is 2 then we then calculate h plus so h plus equals one point seven eight times 10 to the minus 4 so that's the value of ka okay times by two point three five times by 10 to the minus two which is the value of the concentration of our acid and the concentration of our methanol we attained from the salt is one point eight four so the amount of h+ is two point two seven times by 10 to the minus four so then the final thing is we then need to work out their ph so calculate the ph so pH is minus log h plus so pH equals minus log two type two point two seven times by 10 to the minus four the pH is three point six poor okay so that's quite tough okay so let's work out the pH change of a buffer now we've really got to be thinking carefully about this one okay so this is when you a small amount or acid or base to a buffer we need to calculate the change in pH and this one's a bit more tricky again and what I'm gonna say is the assumptions do apply from the previous slide but what I'm gonna say is again this has probably got to be one of the most trickiest calculations at a level chemistry so don't worry too much about it lots of practice and you'll be fine okay so here's the question calculate the pH change when 10 centimeters cubed of one mole of per diem cubed hydrochloric acid is added to one decimetres cubed of a buffer solution that contains naught point 1 moles with the N cubed of ethanoic acid and all point 1 moles dmq disodium ethanoate with a pH of 4 point 7 3 Susan acidic buffer the value of ka at 25 degrees Celsius for ethanoic acid was one point seven times 10 to the minus 5 moles per decimetres cubes right here we go ok first thing we need to calculate the number of moles of ethanoic acid its salt and hydrochloric acid before anything okay so if in doubt work out the moles that's what I always say so first of all ethanoic acid we have naught point one moles of ethanoic acid okay so remember moles is concentration times volume okay so before we mix anything we've got a beaker of ethanoic acid it has that amount in there we've got a bigger of its salt which is naught point 1 moles of that salt and we have a beaker of the strong acid that we're going to add to the buffer which is 1 times 10 to the 1 1 times 10 or point naught 1 which is naught point naught 1 roles so work out the moles of everything ok so then this is where it gets a little bit tricky so concentrate really hard on this so when we add the strong acid we assume that all the hate plus ions react with the Stanaway ions which is and the a minus to form a Stefano ik acid H a so after adding the strong acids so basically if we imagine it we now have a beaker with our ethanoic acid and sodium ethanoate that's our buffer we're now going to add our acid our strong acid to that beaker and we're going to see what the pH difference is okay so remember a buffer is going to try and resist the change in pH we're going to add an acid to that and the buffer is going to try and stop that from changing the pH and so this is the mechanics of it so the amount of acid okay so we add remember we add a h+ h+ will react with the will react with the Saltine to produce the acid remember that back in the other slide so the amount of acid will increase by the same number of moles as what you've added in your acid in your strong acid so we've added naught point naught 1 moles of strong acid to the buffer and that will produce an extra naught point naught 1 moles the same number of moles of weak acid because remember H+ ions have an H+ ions to a buffer will increase the amount of weak acid okay and the amount of salt which is a - ÿà will decrease by the same amount of moles of strong acid that you've added because that's being used up so remember when you add hich +2 an acidic buffer it will use up some of them saltines that's the a - and it will use up so if we've got naught point naught 1 moles of acid strong acids going in it's going to use off nor naught 1 moles of the salt ein that was produced - that's going to decrease that's quite important because that's now told us how many new moles we've got okay and it's a bit like having a hundred people so you've got a hundred people lined up in a hall and you're gonna add you're gonna add 50 other people into that hole that's like 50 HCl molecules they're gonna pair up with 50 of them and you're gonna have some left over so in this case this is what we're doing we're adding some molecules of acid in there but they're going to react with some of the ions that are already exist in there there's still some left over look there's some there but were formed extra paired molecules were formed an extra pair haven't we so now we need to get concentrations for all of them so the concentration of H a so we now know how many moles of H a there are in there is naught point 1 1 divided by i 1.01 now this is tricky the reason why we're dividing it by one point zero one can you might think well hang on I've only got one I've only got a fixed volume which is 1 decimeter skewed but remember we've added 10 centimeters cubed of acid so the volume is a little bit bigger than us don't forget that bit so that divided by that gets you the concentration of your acid and then your concentration your salt is naught point naught 9 divided by that bigger number which is not point naught 9 moles per diem cube so now we've got our concentration this gets a little bit easier now from now on so that was the tricky bit use the KA expression to rearrange to get your h+ okay so h+ is ka times by h m so it's ch3cooh so that's the ethanoic acid divided by your salt so we put that in concentration of h+ put the figures in and we get that two point zero eight times by 10 to the minus 5 calculate the ph so we've got the amount of h plus there's the ph there but remember it's after the ph change so we're going to subtract the to difference so the pH change is from four point seven three to four point six eight so that's the change that is tough but if you get your head around this bit in the red box then you're flying okay so where do we use these buffers they seem awfully complicated but incredibly useful so buffers have many uses in household products naturally occurring in living things as well and blood is a classic example of buffer if you didn't have a buffer system in your blood you would not be alive it's as simple as that and it works day in day out and it's constantly rebalancing the ions in your blood to make sure it's at the perfect pH if it's not the right pH it's simply you enzymes wouldn't be able to function cells wouldn't be able to adapt so it is vitally important so it is absolutely worth knowing about it so it must be as close to 7.4 that's the pH of blood we need that to be the same like say to allow bodily functions and book carbon dioxide plays a massive role here and within blood plasma so we have what we call a carbonic acid and hydrogen carbonate which is hco3 system so we have your carbonic acid which is here this dissociates into H+ and h Co Co 3 - so that's your acid bit and then we have this system going on as well where you have a CH 2 CH 2 Co 3 and water and carbon dioxide so you got your carbon dioxide that already exists in your blood and so this is the buffer system that occurs in your blood with carbon dioxide now the amount of carbonic acid in the blood is controlled by respiration in your cells so we breathe out carbon dioxide and the level of carbonic acid reduces and is equilibrium shifts to the right to replace them so when we breathe out we're breathing out some of that carbon dioxide because we've produced it from ourselves perspiring okay and that carbon dioxide reduces and so therefore the equilibrium shifts to the right to replace them so some of this acid moves across - to replace the carbon dioxide that's been lost so we do need carbon dioxide in our body we don't have to get rid of all of it hydrogen carbonate which is hco3 minus M so this carbonate ion here and it's controlled by your kidneys and if there's too much of this in your blood that's just removed air through urine so it's a fine balance between them and your body's got numerous ways to either get rid of this or to retain it and that's it so that was a bit of a marathon and a tough topic it's no it's it's not an easy talk if you did find it difficult then there's a reason for it it's not an easy one and but and please subscribe to my channel there's loads more videos on this that go through all these and some of these complicated areas and hopefully it'll go a little bit a little bit of a way to try and help to explain it this is V available to purchase as well so you have your own copy if you click on the link in the description box and that will get there but please subscribe these are all for free and always will be all I ask is that you just click hit the subscribe button and that keeps you in the loop regarding videos and but that's it okay bye-bye