in this lesson we're going to focus on the basics of acids and bases so one of the first things that you need to be able to do is you need to be able to identify an acid or base if you're given the chemical formula of it acids typically have a hydrogen in front of them so hcl that's hydrochloric acid hf hydrofluoric acid as you can see there's a hydrogen in front of it or acidic acid hc2h3oh those are acids bases typically have a hydroxide ion like naoh koh those are bases if you see a hydrogen next to a metal like sodium hydride then it's a base not an acid but if the hydrogen is attached to a nonmetal typically it's an acid so whenever hydrogen has a positive charge it's an acid but if the hydrogen has like a negative charge then it's a base acids tend to be positively charged bases are usually negatively charged now you need to understand the arrhenius or arrhenius definition of acids acids are basically they're substances that release h plus ions into the solution hydrogen ions are equivalent to hydronium ions in water this really doesn't exist by itself in water in fact it's actually bonded to water and so it exists as h3o plus the iranian definition of bases is that a base releases hydroxide ions into solutions while acids they release h plus ions into solution so keep that in mind now you also need to know the bronsted-lowry definition of acids acids are proton donors bases are proton acceptors so let's say if we put hydrochloric acid in water what's going to happen hcl is the bronzing lary acid h2o is the bronzolary base the acid is a proton donor so hcl is going to lose the hydrogen and turn into chloride the base is a proton acceptor water is going to accept the h plus ion and it's going to convert into the hydronium ion this is called the conjugate acid because we added a hydrogen to water to make it and this is the conjugate base because we took away hydrogen to make it so the acid always turns into the conjugate base and the base turns into the conjugate acid in the course of an acid-base reaction let's look at another example ammonia when it reacts with water it produces nh4 plus and oh minus in this example identify the acid the base the conjugate acid and the conjugate base so notice that nh3 it gained a hydrogen turning into nh4 so therefore it's the proton acceptor which means nh3 is the base in this example water lost the hydrogen so water was the proton donor it gave away hydrogen which makes it the acid now the base is always going to turn into the conjugate acid because we added a hydrogen to it so nh4 plus is the conjugate acid which means hydroxide has to be the conjugate base because water lost the hydrogen to make it now let's say if you're given water and you want to write the conjugate acid of water and at the same time the conjugate base what is the conjugate acid of water whenever you want to find the conjugate acid of something all you need to do is add h plus to it so you're going to add a hydrogen and increase the charge by 1. so this is going to turn into h3o plus the charge increases from zero to one now whenever you wanna write the conjugate base of something take away a hydrogen and decrease the charge by one so instead of having two hydrogens we now have one and the charge is going to decrease from zero to negative one so the conjugate base of water is hydroxide the conjugate acid is h2o plus so given ammonia write the conjugate acid and the conjugate base of nh3 so the conjugate acid we got to add a hydrogen it's nh4 with a plus charge to write the conjugate base we need to remove a hydrogen so it's nh2 with a negative charge now for the sake of practice try this one too let's say we have h2po4 minus right the conjugate acid and the conjugate base so we gotta add a hydrogen this will turn into h3po4 so that's the conjugate acid of this thing the conjugate base we got to take a hydrogen and then decrease the charge by one negative one minus one that's negative two so now you know how to identify and how to write the conjugate acid and conjugate base of a substance now let's talk about the ph go i probably should have went over this earlier but better late than never so typically in most textbooks you'll see the ph scale between 0 and 14 however it can go beyond those numbers so keep that in mind at 7 the solution is neutral at a ph that's less than 7 it's acidic so the ph was negative two it's very very acidic above seven the solution is basic now if you want to calculate the ph of the solution you need to know the concentration of the hydronium ion or the h plus ions it's a negative log of h3o plus and the poh of the solution it's negative log of the hydroxide concentration the ph plus the poh always adds up to 14 at 25 degrees celsius now for some reason if you ever need to find the h3o plus concentration it's 10 to the negative ph and if you need to find the hydroxide concentration it's 10 raised to the negative poh so those are some equations that you might find useful when learning about acids and bases now let's talk about strong acids and weak acids you need to identify if an acid is going to be strong or if it's going to be weak strong acids ionizes completely weak acids they partially ionize they don't ionize completely so a strong acid basically almost all of it would ionize in solution a weak acid less than five percent strong acids form strong electrolytes in water the solution will conduct electricity weak acids form weak electrolytes when dissolved in water so you need to know the six most common strong acids and these are hcl hbr hi hf is a weak acid the others are nitric acid hno3 sulfuric acid h2so4 and perchloric acid hclo4 those are the most common strong acids that you'll see in a typical general chemistry course now other weak acids include the ammonium ion nh4 plus acidic acid and basically almost any acid that is not on his list so like cyanic acid nitrous acid and sulfurous acid notice that sulfuric acid is stronger than sulfuric acid and nitric acid is stronger than nitrous acid what pattern do you see here when dealing with oxyacids the acid that has more oxygen atoms is the one that's going to be more acidic so sulfuric acid is more acidic than sulfurous acid and nitric acid is more acidic than nitrous acid perchloric acid is more acidic than chloric acid which is more acidic than chlorise acid and that is more acidic than hypochlorous acid now hcl is not an oxyacid so it doesn't fit this trend in fact hcl is more acidic than hclo so keep that in mind the trend only works for those that actually have oxygen if it doesn't have oxygen it's not going to fit the trend nicely now you need to know how to write chemical reactions with strong acids and weak acids we went over the example with hcl and water because hcl is a strong acid it ionizes completely into chloride and hydronium ions so notice that i have a single arrow so that's the way you need to write the chemical reaction if you're mixing a strong acid in water now if you're mixing a weak acid with water it doesn't ionize completely so you have a reversible reaction so you need to use a double arrow symbol rather than a single arrow whenever you have a weak acid but to identify the products is going to be very similar to the reaction above so the only difference is just use a double arrow instead of a single arrow when dealing with weak acids because they exist in equilibrium the reaction is reversible now let's talk about bases you need to identify or distinguish a strong base from a weak base strong bases are soluble ionic compounds like potassium hydroxide sodium hydroxide barium hydroxide these are soluble in water they ionize virtually completely think of the strong acids like hcl hbr hi the reason why they're strong is because they ionize almost 100 percent so these are soluble and as a result they form strong electrolytes and so they make strong bases a weak base the ones that have hydroxide in them they're associated with insoluble compounds aluminum hydroxide doesn't dissolve very well in water only a small amount of it dissolves and so it's insoluble and it doesn't ionizes it ionizes less than one percent which makes it a weak base so i'm going to put here 100 ionization less than 5 percent ionization some other examples of weak bases include ammonia and the conjugate bases of weak acids like fluoride nitrite acetate cyanide all of these are weak bases even h so3 minus other examples of strong bases besides hydroxide are oxide if you have oxide in solution that's a strong base and hydride if you put an oxide ion in water it's going to grab a hydrogen from water creating two hydroxyl ions so oxide is a stronger base than hydroxide because it has less hydrogens hydroxide is a stronger base than water water is neutral and water is a stronger base than h2o plus because h2o plus is acidic so as you can see the less hydrogens that are on an atom the more basic it is so oxide is more basic than everything else in this list whereas h2o plus is most acidic hydride is also a strong base when it reacts with water it's going to produce hydrogen gas and hydroxide so notice that hydride produces hydroxide typically a stronger base will produce a weaker base so hydride is a much stronger base than hydroxide let's talk about the mechanisms of these reactions so this is oxide it's basically an oxygen atom with four lone pairs and i'm going to draw the lewis structure of water this oxygen has two lone pairs so oxide is going to use one of its lone pairs to take a hydrogen atom from water the arrow represents the flow of electrons so this arrow tells us that this lone pair is going to be used to form a bond between the oxygen atom and the hydrogen atom when the hydrogen atom breaks away from water the two electrons in this bond is going to go back to the water which is now going to be hydroxide so this oxygen gained hydrogen so now it looks like this it lost the lone pair so instead of having four lone pairs it now has three and it no longer has a negative two charge but it has a negative one charge now this oxygen it lost a hydrogen so now it has one but it gained a lone pair as indicated by this red arrow so now it has a negative one charge so as you can see whenever you mix oxide in water if if it's not bonded to anything it's going to produce two hydroxide ions now let's consider this reaction where we said hydride plus water produces hydrogen gas and a hydroxide ion so hydride is basically a hydrogen atom with a lone pair so it's it's an ion in this case and it's going to react with water now the hydrate ion is attracted to this hydrogen and the reason why it's attracted to it is because that hydrogen has a partial positive charge it's partially positive because it's attached to a partially negative oxygen atom oxygen is electronegative and as a result it pulls electrons towards itself so as it pulls electrons towards itself it leaves a partial positive charge on the hydrogen and we know that opposites attract so this hydrogen ion with a negative charge is attracted to this hydrogen atom with the partial positive charge so those two will connect and once they get together this bond is going to break those two electrons are going to go back to this oxygen so when these two get together they form hydrogen gas h2 leaving behind a hydroxide ion so now this oxygen now has three lone pairs instead of two and so that's the mechanism for this reaction so whenever you add hydride or oxide to water it causes the solution to become basic it produces hydroxide ions now let's briefly review some properties of acids and bases acids taste sour if you think of lemons lemons contain a lot of citric acid they have a sour taste to them bases on the other hand they taste bitter and if you ever were to touch a base it would make your skin feel slippery now with regard to indicators acids they turn blue litmus paper red bases they turn red limits paper blue now we've covered the ph of acidic solutions for an acidic solution the ph will be less than seven for a neutral solution the ph is equal to seven and for basic solution the ph is greater than seven now both acids and bases can conduct electricity in solution strong acids are strong electrolytes and the same is true for strong bases they're strong electrolytes as well they both ionize completely weak acids and weak bases are weak electrolytes so if you were to place a strong acid or a strong base in solution they would conduct electricity very well due to the free flow and ions in solution weak acids and weak bases they don't ionize completely therefore even though they can conduct electricity they don't do it very well so they can conduct a small amount of electricity in solution so the electrical conductivity of a strong acidic solution is greater than the electrical conductivity of a weak acid solution now acids they react with active metals to produce hydrogen gas a good example is this reaction zinc metal will react with aqueous hydrochloric acid to produce aqueous zinc chloride and hydrogen gas not all metals react with acids only the active ones such as zinc aluminum iron metal nickel sodium but sodium is too reactive it'll react with water as well so any active metal will react with an acid to produce hydrogen gas copper is not an active metal copper doesn't react with acids silver or gold those metals are not reactive so copper silver gold they don't react with acids to produce hydrogen gas now let's review some of the definitions of acids arrhenius acids they release h plus ions in solution arrhenius bases release hydroxide ions in solution bronsted-lowry acids the definition for that is they're proton donors brassileary bases are proton acceptors and here's another one lewis acids are electron pair acceptors but lewis bases are electron pair donors so those are some other definitions or ways of defining assets now let's consider the reaction between hydrofluoric acid and water hydrofluoric acid is a weak acid so only a small amount of it will convert into the h3o plus ion or the hydronium ion and the conjugate base f minus now this is a reversible reaction so we need both arrows the big arrow tells you that it's reactant favored the small arrow tells you that only a small amount you know converts into the products that you see in the right that is hydronium and f minus so that's an unequal reversible reaction now we can calculate the ka of this reaction ka is the acid dissociation constant of the asking question which is hf now everything is in the aqueous phase except water what is a liquid to write the expression of the acid dissociation constant it's going to be the concentration of the products which is h3o plus times f minus divided by the concentration of the reactants now what is a liquid so you don't include liquids and solids in the equilibrium expression when you're dealing with acids and bases so that's ka as the ka value increases the strength of the acid increases the pka value decreases so strong acids have large ka values but have small pka values now just as ph is the negative log of the hydronium ion concentration pka is the negative log of ka now make sure you add these equations to your notes because we're going to use them later now let's say if we have a weak base let's use ammonia ammonia is going to react with water but because it's a weak base it's going to produce hydroxide ions in solution now ammonia being the weak base as a brought to larry base it's the proton acceptor it accepts a proton to turn into the conjugate acid and h4 plus water is behaving as a weak acid it's donating the proton to ammonia and so when it loses a proton it turns into the conjugate base hydroxide so this is the bracillary base and this is the bronstellari acid this is the conjugate base that's the conjugate acid now water can act as a base or an acid we saw in the last example where hf reacted with water to produce h3o plus and f minus hf was the broscillary acid it donated the proton to become the conjugate base fluoride water was the bronzing base it accepted a proton to turn into the conjugate acid so notice that water depending on the circumstance can behave as an acid and sometimes it can behave as a base whenever you have a substance that can behave as an acid or as a base that substance is known as an amphoteric substance it can accept a proton or it can give away a proton so a good example of an amphoteric substance or at least another example in addition to water is h2 po4 minus this can act as a base it can react with an acid like hf and become h3po4 and then you get the fluoride ion or it can act as a base well i take that back here it's acting as a base because that's acting as an acid but in other circumstances h2po4 minus can act as an acid if we react it with ammonia it's going to become we're going to get nh4 plus and then dihydrogen phosphate will lose the hydrogen turn it into mono hydrogen phosphate so here it's acting as the acid ammonia is acting as the base so this is another amphoteric substance it can act as an acid or it can act as a base depending on what it's reacting with now looking at the products on the right which ones are conjugate acids and which ones are conjugate bases hf lost a hydrogen any time you remove a hydrogen from something you create the conjugate base dihydrogen phosphate lost the hydrogen to become mono hydrogen phosphate so this is the conjugate base as well on the other reaction ammonia acquired a hydrogen to become ammonium so this is the conjugate acid and here dihydrogen phosphate acquired a hydrogen to become phosphoric acid so anytime you add an h plus ion to something you create the conjugate acid when you remove an h plus ion from something you create the conjugate base now going back to the reaction above let's write the expression associated with kb the base dissociation constant like ka kb is going to be the products over the reactants so we have the concentration of hydroxide times the concentration of ammonia i mean ammonium rather and we're going to divide that by the concentration of ammonia so that's the equilibrium expression for kb when you have a base reacting with water now just as the poh is equal to the negative log of the hydroxide concentration pkb is equal to negative log kb now here's another example that shows how water can behave as an acid in the base water can react with itself i need to put a small arrow to the right and a big arrow towards the left when you have pure water it's not exactly pure water a small amount of it will ionize with itself one of the water molecules will act as the bronchillary acid and the other one will act as a base now if this acts as an acid it's going to lose a proton so it's going to turn into the conjugate base hydroxide if this acts as a base it's going to acquire a proton turn it into the conjugate acid hydronium ion now hydroxide and h2o plus they're dissolved in water so they're aqueous but water is a liquid thus the equilibrium expression for this reaction is not ka or kb but it's kw it is the r the auto ionization constant for water it's equal to the concentration of the products divided by the concentration of the reactants but because both reactants are in a liquid state we just divided by one so we could simply write it like this now kw is an equilibrium constant that is dependent on temperature if you increase the temperature kw increases which means water more of the water molecules will ionize into these ions at higher temperatures at lower temperatures these ions convert back to the liquid form but at higher temperatures they will ionize more into these two now at 25 degrees celsius kw is one times 10 to negative 14. but if you were to increase the temperature to let's say 60 degrees celsius kw increases it's 1 times 10 to negative 13. so it's dependent on temperature but for most chem problems if you don't see a temperature it's assumed to be 25 degrees celsius so that's we have this equation hydroxide times atrial plus is equal to 1 times 10 to negative 14 which means if you know the hydronium ion concentration you could use that formula to calculate the hydroxide ion concentration because of that formula the ph plus the poh adds up to 14. likewise the pka plus the pkb at this temperature adds up to 14 as well so those are some additional formulas that you want to add to your list of formulas in your notes now there is another one that we can add as well since hydroxide times atrial plus is equal to 1 times 10 to negative 14 at 25 degrees celsius it turns out that ka times kb it's also equal to kw which at 25 degrees celsius that's one times 10 to negative 14. so if you know the ka value you can easily calculate the kb value and remember the pka is a negative log of ka so if you have the ka value you can calculate the pk the pka value and then if you have the pka value you could use this equation to get the pkb value so there's a lot of equations that you could use to find one thing from another now let's work on some practice problems the h3o plus concentration in a solution is four times ten to the minus three what is the ph of the solution we know that the ph is equal to negative log of the h2o plus concentration so that's going to be negative log 4 times 10 to the negative three now without a calculator if you were to estimate the range of the ph value of the solution what would you say looking at the exponent which is negative three the ph is going to be somewhere close to three would you say it's going to be two to three or three to four it's always going to be the lower of the two ranges so it's going to be between two to three now let's plug it in to get our answer negative log of 4 times 10 to the minus 3 it's 2.3979 so that's going to be the ph of the solution so we could say it's approximately 2.4 now let's calculate the poh of the solution the ph plus the poh adds up to 14. so if you rearrange the equation the poh is going to be 14 minus the ph so 14 minus 2.4 14 minus 2 is 12 12 minus 0.4 is 11.6 so the poh is going to be 11.6 now what about part c what is the hydroxide ion concentration of the solution to calculate the hydroxide ion concentration we could use this formula it's 10 raised to the negative poh so that's 10 raised to the negative 11.6 so that's going to be approximately 2.5 times 10 to the negative 12 and the unit will be molarity or moles per liter so that's the hydroxide ion concentration of this solution number two the hydroxide concentration in the solution is 5.3 times 10 to negative 4. what is the poh of the solution by the way feel free to pause this video if you want to try this problem the poh is going to be negative log of the hydroxide ion concentration and so that's going to be negative log 5.3 times 10 to the negative 4. so if you were to estimate the range of the poh of the solution what would you say so looking at this exponent negative four we know it's going to be between three to four now let's go ahead and plug it in the answer is going to be three point two seven five seven so we could say that it's approximately 3.28 now let's calculate the ph of the solution the ph is going to be 14 minus the poh so that's 14 minus 3.28 40 minus 3 is 11 11 minus 0.28 if you were to take 100 and subtracted by 28 you would get 72. so this is going to be 10.72 now let's move on to part c what is the h2o plus ion concentration of this solution so we could use this formula it's 10 raised to the negative ph so it's going to be 10 raised to the negative 10.72 and that answer can be rounded to 1.9 times 10 to the negative 11. so that's going to be the concentration of h3o plus in the solution now what about this one we're given the hydronium ion concentration and we want to calculate the hydroxide ion concentration feel free to try this problem so if you recall h3o plus times the concentration of oh minus that's going to be equal to kw the autoionization constant of water and at 25 degrees celsius it's 1 times 10 to the minus 14. so to get hydroxide by itself we need to divide both sides by h2o plus thus the hydroxide ion concentration is going to be kw which is this divided by h3o plus so it's 1 times 10 to the negative 14 divided by two point five times ten to negative five so the hydroxide concentration is going to be 4 times 10 to the negative 10. so that's the answer for number three number four if the ka of acetic acid hc2 h3o2 is 1.8 times 10 to the negative five what is the pka of the acid now just as ph is negative log of h2o plus pka is negative log of the acid dissociation constant which is ka so it's going to be negative log of 1.8 times 10 to the negative 5. so looking at the exponent we know this is going to be between 4 to 5. so the pka is four point seven four five that's the pka of acetic acid now let's calculate the pkb and then we'll calculate the kb just as poh is 14 minus ph pkb is going to be 14 minus the pka at 25 degrees celsius we're going to assume it's that temperature unless specified otherwise so this is going to be 14 minus 4.74 now let's see if we could do this mentally 14 minus 4 is 10. if you were to subtract 1000 by 745 what would you get you should get 255. so 10 minus 0.75 is going to be 9.255 so we're going to round it and say well actually this is a good round the answer when you plug it in you actually get 9.25527 so we'll stick with that answer now let's get kb so kb is 10 raised to the negative pkb so that's going to be 10 raised to the negative 2.55 the final answer is going to be 5.56 times 10 to the negative 10. so that's the value of kb and remember ka times kb is equal to kw so you can get kb by taking kw and dividing it by ka so kw is 1 times ten to the negative fourteen ka is one point eight times ten to the negative five and this will give you the same answer 5.56 times 10 to negative 10. so that's it for number four number five which of the following statements is false so let's analyze each one let's focus on a bases taste bitter and feel slippery and that's a true statement so we can eliminate that answer choice now looking at b acids taste sour if you think of an acid like lemon lemon tastes sour and acids react with active metals to produce hydrogen gas so if we were to take an active metal like iron and if we were to react this with hydrobromic acid this would form febr2 plus hydrogen gas the hydrogen gas will escape the solution so active metals like zinc iron aluminum they can react with strong acids to produce hydrogen gas so b is a true statement now what about c hcl is a strong electrolyte this is true if you were to dissolve hydrochloric acid in water because hcl is a strong acid it will dissociate almost completely into these ions and whenever you have free flow and ions in solution it can conduct an electric current so what we have here is a strong electrolyte strong acids are strong electrolytes strong bases are also strong electrolytes now what about d acids turn red litmus paper blue now this is not correct this is a false statement the reverse is true acids turn blue limits paper red so d is the false statement as for e sodium hydroxide is strong base like hcl is a strong acid both of them are strong electrolytes and they both can conduct an electric current therefore e is a true statement so answer choice d is the correct answer for this problem number six which of the following solutions will have the highest ph is it going to be hbr hydrobromic acid hf hydrofluoric acid nacl nh3 ammonia or koh potassium hydroxide what would you say well first let's identify each substance hydrobromic acid is a strong acid hydrofluoric acid is a weak acid nacl is a neutral salt nh3 is a weak base koh is a strong base now let's create our ph scale so we're going to put 7 in the middle 0 on the left 14 on the right a 0.1 molar solution of hydrobromic acid it's going to have a ph of around one so i'm going to put hbr here now weak acid like hf the ph is going to be somewhere around two to three sodium chloride is neutral so the ph is going to be seven now keep in mind acids have a ph that's less than seven now because hbr is a stronger acid than hf given the same concentration hbr is going to produce a solution with a lower ph so the strongest acid is going to have the lowest ph assuming the concentration is the same now what about the bases here we have a weak base and a strong base with the same concentration this solution is going to have a ph of around 11. a 0.1 molar koh solution will have a ph of around 13. so this is going to be nh3 and then koh but what you want to gather from this is that the strong base is going to have the highest ph the strong acid is going to have the lowest ph so remember bases have a ph that's greater than seven acids have a ph that's less than seven thus because we're looking for the solution with the highest ph we're looking for the solution with the strongest base given that the initial concentration is the same therefore answer choice e is the correct answer number seven the ka value for hf and acetic acid are 7.2 times 10 to the negative 4 and 1.8 times 10 to the minus five which acid is stronger hydrofluoric acid or acetic acid what would you say well first let's write down our two acids on the board so we have hydrofluoric acid and acetic acid and let's write down their corresponding ka values now as the k a value increase what's going to happen to the string for the acid does the acid strength go up or down you need to know that acid strength is directly related to ka which means that the acid with the higher ka value is the stronger acid so which one is higher negative four or negative five on a number line negative four is higher than negative five the higher numbers are to the right and negative 4 is here and here's negative 5. so you can see negative 4 is to the right of negative 5 on a number line therefore hf is the stronger acid so even though both of these acids are considered weak acids in general this acid hf is stronger than acetic acid it's a stronger weak acid now what about part b which conjugate base is stronger the conjugate base for hf is fluoride the conjugate base for acetic acid is acetate so which one is stronger so here's what you need to know the stronger acid creates the weaker conjugate base and the weaker the acid the stronger the conjugate base so because hf is the stronger acid the base is going to be weaker so because h c2 h3o2 or acetic acid because this is the weaker acid the conjugate base is going to be stronger and here's why ka times kb is equal to a constant kw kw doesn't change if the temperature stays constant therefore if you increase ka kb is going to decrease just as ka reflects the acid strength kb reflects the bass drift this one has a higher ka value this is going to have a higher kb value let's calculate the kb value for each conjugate base so it's going to be kw or 1 times 10 to negative 14 divided by that number and so for fluoride the kb value is going to be 1.39 times 10 to negative 11. now let's do the same for acetate if we take 1 times 10 to the negative 14 divided by 1.8 times 10 to the minus 5. we're going to get this familiar value which we calculated in an earlier problem 5.56 times 10 to the negative 10. 10 to the negative 10 is higher than 10 to the negative 11. therefore because acetate has a higher kb value it's going to be the stronger base so remember the stronger the acid the weaker the conjugate base hf is a stronger acid fluoride is the weaker conjugate base this has a higher ka value so it's going to have a lower kb value for the conjugate base here this is the weaker acid but this is going to be the stronger conjugate base as you decrease the ka value for an acid the kb value for the conjugate base increases so they're like they're inversely related so to speak now let's talk about the pka which acid will have the lower pka value is it going to be hf or acetic acid now remember pka is equal to the negative log of ka so because of that negative sign there's an inverse relationship between ka and pka acid strength increases with high ka values but it increases with low pka values so the acid with the low pka value is the stronger acid and let's calculate pka so let's take the negative log of that number negative log of 7.2 times 10 to negative 4 is 3.14 so this is the pka value i'm just going to write the number for hf it's 3.14 now the pka value for acetic acid if we take negative log of 1.8 times 10 to the minus 5 we get this familiar value it's 4.74 it's 4.7447 so you can round it to 4.74 or 4.745 now notice that the stronger acid has the lower pka value whereas the weaker acid has the higher pka value therefore acid strength and pka are inversely related acid strength increases with decrease in pka values so let's summarize what we've learned here today the stronger acid is going to have the higher ka value and the stronger acid is going to have the lower pka value so just keep that in mind and the stronger the acid the weaker the conjugate base now let's work on this exercise go ahead and match each term with the correct letter so let's begin by focusing on the arrhenius definition of acids and bases so what is the arrhenius acid an arrhenius acid is an acid that releases h plus ions in solution so number one is c the uranius base releases hydroxide ions in solution it's number two is e now what about the bronsted-lowry definition of acids and bases what can we say about that the broxillary acid is a proton donor the bronsted-lowry base is a proton acceptor so number three is b number four is f now for the lewis definitions or the lewis definition of acids and bases rather it has to do with a transfer of electrons a lewis acid is an electron pair acceptor a lewis base is an electron paired donor so number five is a number six is d now let's go ahead and talk about this so to understand it from the arrhenius point of view imagine if we have hydrochloric acid hydrochloric acid in water can dissociate into hydrogen ions and chloride ions so hcl releases hydrogen ions in solution which makes it an arrhenius acid now let's consider a strong base like sodium hydroxide sodium hydroxide can release hydroxide ions in solution so that makes it an iranian space in that reaction now let's focus on the bronze salary definition of acids and bases let's say if we have hf reacting with water where we're going to get fluoride and h3o plus hf is the brassillary acid i'm just going to write acid because it donates a proton to water in order to become fluoride so as hf becomes fluoride it loses a proton it donates a proton so the proton donor is the broncillary acid water in this reaction is acting as the bracillary base water accepts a proton from hf and as it absorbs that proton it turns into the hydronium ion atrial plus so a proton acceptor is a bronsolari base now let's consider an example that highlights the lewis definition of acids and bases so for this example we're going to use aluminum chloride and we're going to react it with ammonia and h3 ammonia has a lone pair and as it donates that lone pair to aluminum a bond is going to form between aluminum and the nitrogen atom and so we're going to get a product that looks like this whenever aluminum has four bonds it's going to have a negative formal charge and whenever nitrogen has four bonds it will have a positive formal charge but notice that ammonia donates a pair of electrons to become this product because it donates a pair of electrons it is the lewis space and we know that ammonia is a weak base but in this reaction is the lewis base therefore the electron paired donor is lewis base so six is d the aluminum in alcl3 it accepts a pair of electrons and that makes it the lewis acid the lewis acid is the electron pair acceptor by the way if we were to put sodium chloride in water the ph of the solution will be seven this is what is known as a neutral salt but if we were to put aluminum chloride in water the ph is not gonna be seven the ph is less than seven so this is what is known as an acidic cell whenever you have a metal with a very high positive charge that metal has a strong affinity for electrons it's going to be a lewis acid an electron pair acceptor and so it could bond to water in such a way that it can split hydrogen and hydroxide from water it'll absorb the hydroxide releasing hydrogen ions in the solution so when you see metal ions with very high positive charges like al 3 plus fe3 plus or even pb4 plus these strongly positively charged ions are acidic in nature acids tend to have positive charges bases tend to have negative charges or a lot of lone pairs here this is a neutral base with a lone pair and here this has a negative charge but a lot of lone pairs keep in mind lone pairs represent electrons which are negatively charged so acids are associated with positive charges bases are usually associated with negative charges you