the acidity and basicity of salts going to be the topic of this lesson my name is chad and welcome to chad's prep where my goal is to take the stress out of learning science now in addition to high school and college science prep we also do mcat dat and oat prep as well i'll leave links in the description for where you can find those courses now this lesson is part of my new general chemistry playlist i'm releasing several a week throughout the school year so if you want to be notified every time i post one subscribe to the channel click the bell notification all right so the acidity and basicity of salt now first off a salt is an ionic compound so it's gonna be a little different than what we've seen in the rest of the chapter we've been dealing with acids uh you know h and then something and they're made of all non-metals they're molecular compounds and technically though the strong bases we dealt with the group one metal hydroxides and the group two metal hydroxides those are technically ionic compounds you could call them salts but we're going to be talking about all the rest of the salts in this lesson and it turns out some salts are acidic salts when you add them to water they cause the ph to go below seven some salts are basic salts when you add them to water they cause the ph to go above seven and then some salts are neutral salts and when you add them to water the ph stays right around seven and you've got to be able to identify each of these three classes of salts whether it's going to fall into the acidic basic or neutral salt category and you definitely want to have memorized your strong acids and your strong bases because that's going to basically be our key to remembering this so big thing we should look at here is we're going to take a look at cations and we're going to look at anions and treat them separately and go from there and what you should realize is that you know if we look at the lewis definition for acids and bases they'll kind of give us an idea of whether we should expect cations and anions to be acids or bases so a lewis acid is an electron acceptor and if i was looking at cations or anions and saying which one's more likely to accept electrons electrons are negative it's the cations that are positive and so you'd expect cations to be electron acceptors and so cations are usually lewis acids we talked about this back in the first lesson in the chapter we talked about acids that might qualify under lewis's definition but all you know not under any of the rest we talked about boron containing compounds which have an empty orbital and then metal ions metal cations now on the other hand anions having a negative charge they are much more likely to be electron pair donors so lewis bases or in you know many cases uh bronsted-bases or even iranian bases as well so we're going to kind of look at just the lewis definitions for this because it's easy to associate them with cations and anions cations might be great electron acceptor i'm sorry yes electron acceptors lewis acids anions might be great electron donors lewis bases and so when you see a cation you should think good chance it's an acid when you see an anin it's a good chance it's a base because most cations are acidic and most anions are basic and the key is you're going to remember which ones aren't because there's only a few cations that are not acidic and there's only a few anions that are not basic as long as you remember which ones aren't then you'll know that all the rest are either acidic for cations or basic for anions and knowing your strong acids or bases that is your key to remembering which ones are not acidic and not basic so we're actually going to start with the anions here so if you take a look at your strong acids so we've emphasized this a couple times throughout the chapter that the stronger an acid the weaker the conjugate base and so it turns out you know if you've got a strong acid at the top of the list its conjugate base is so weak that it has zero ability to act as a base in water i'm going to call it a negligible base so and so in this case if you take a look at chloride bromide iodide nitrate perchlorate and chlorate we'll talk about him special bisulfate in here in a little bit so but the first six chloride bromide iodide nitrate chlorate i'm sorry perchlorate and chlorate so it turns out they are such weak conjugate bases that they have no ability to act as a base in water they're negligible bases and so i'm going to put those special here we're going to talk about chloride bromide iodide nitrate perchlorate and chlorate and these are the ones that we are going to call negligible they are your negligible conjugate bases they don't affect the ph of water at all so again when you see an anion you should think it's probably a base just not these guys they're the conjugate bases of six out of the seven strong acids and they're such weak bait you know weak conjugate bases that they're not going to act as a base in water okay now h2so4 is a little bit special here so h2so4 is the only one of these that's polyprotic and you might recall that only the first h dissociates 100 percent the second one only partially the first h is a strong acid the second one is only a weak acid well once he donates one h he's going to look like hso4 minus and we just said he's a weak acid so that's an exception of all exceptions right so he's an anion almost all anions are bases here's the ones that are neutral but here's one that actually is acidic and he's not the only one but probably the only one you're likely to encounter so there's a couple uh one of note like uh turns out oxalic acid is also a diprotic acid and the monoprotic version is also acidic but you're not likely to encounter that so this is the most likely one you're gonna encounter and so the way you should treat anions then is you should know that these are the neutral ones there's one acidic one and any other anion outside of these is going to be a base so an electron donor that's the way it works now cations on the other side so it turns out that the group 1 metal ions and the group 2 metal ions those are going to be your negligible ones over here and so in this case i'm just going to put group 1 metals group two medals these are now the negligible ones so but it turns out they're not alone so we're gonna make a little room i guess in this box and put transition metals that are only plus one so i'm going to call it a transition metal with just a plus one charge we'll add them in here as well technically there's some other oddball ones under the staircase on the periodic table that are even when they're plus two but you're just not likely to encounter this so i'm gonna sum it up like this so group one metals like lithium sodium potassium ribbidium cesium and then the group two metals like magnesium calcium strontium barium when you encounter them in a salt they are negligible but pretty much any other cation you're likely to see outside of the group one the group twos and if a transition metals only plus one they're going to be negligible as well and one thing you should realize about this there's a couple of trends involving these lewis acids for metal ions so if you were to compare say like fe2 plus an fe3 plus and just your gut which one of these is more likely to act as a stronger lewis acid a stronger electron pair acceptor well the one that's more positive for sure and that is definitely the way it is so if you got two of the same metal but different charges it turns out the more positively charged they are the stronger the acid now the other comparison you're going to want to be aware of is based on size so if i have fe2 plus and ruthenium 2 plus so it turns out a smaller one can bring those electrons closer to its nucleus because it's just got a smaller size and that's going to make it a better electron acceptor as well and so in this case fe 2 plus is better than ruthenium two plus right below it on the periodic table so both charge can play a role and how strong the lewis acid is but size can play a role as well and that's just a quickest side on a couple trends there for these lewis acids so what you might be asked now then is we're going to present you with some salt and you're going to have to identify if these salts are acidic salts basic salts or neutral salts and the idea is you know if you've got negligible cations and negligible anions that's going to be a neutral salt but if you've got an acidic cation with a negligible anion that's going to be an acidic salt but if you've got a negligible cation with a basic anion that's going to be a basic salt so an odds are you're probably not going to have one of each you're probably not going to have both an acidic cation and a basic anin not the most likely scenario the only case you might see that is if you have a hydroxide salt and you should just think that hydroxide salts are going to be bases because it's hydroxide all right so let's take a look at a few examples all right so here we've got five ionic compounds these are the ones we're going to run through so and when you look at these you should realize oh well magnesium hydroxide is one of your strong bases so right off the bat if you've got a strong base for when you're on it compounds well then that's going to be a base and in fact even if it wasn't a strong base hydroxide salts just tend to be basic so regardless of what the cation they're probably going to be base in fact i can't think of an example that's not going to be the case so the other four though we're going to evaluate here and we'll come back through him and just verify some things but for the other four what you want to do is look at your cations and oftentimes you get this in a context of a multiple choice question where you're going to get several answer choices and rather than looking at each answer choice separately i'm just going to go through and examine all the cations at the same time so because when i look at cations i think oh group one and group two or if it's a transition metal that's only plus one those are the ones i'm gonna eliminate so if you look then we can say that sodium is group one barium's group two magnesium's group two and but aluminum is not group one or two and it's uh got a plus three charge we can't ignore it and then ammonium is not even a metal it's a polyatomic ion so he's definitely not in the negligible category here and is therefore an acid anybody that we didn't cross out as negligible is going to be an acid for a cation then we'll move on to the anions and analyze it the same way anybody that was the conjugate acid of 6 out of those 7 strong acids that conjugate base is going to be negligible hcl is a strong acid so we can cross off chloride hf is not a strong acid though and f minus doesn't appear in the negligible list here hclo4 is a strong acid and that's it we crossed off a couple that's the way it goes so if you notice then some people actually say you know split your ionic compound up and with your cation add an o h and if what you just made is a strong base like naoh is then cross off your cation so and then on the other side they say to your anion add the h first and if what you write is a strong acid then cross him off he's going to be a negligible conjugate base that's another way to approach this not personally the way i do it but if it works for you fantastic all right so the way this works now if you've crossed off your cation is negligible and your anion is negligible well then that right there is a neutral salt you can add it into water and the ph is going to stay right around seven now if you've crossed off your cation but you have not crossed off your anion well almost all anins are bases and this is going to be a basic salt cool you stick barium fluoride in water and the ph is going to go up above seven all right on the other side of the equation here we've got aluminum perchlorate and again we've crossed off the anion but aluminum we did not cross off and any cations you didn't cross off are acids and you put aluminum perchlorate in water and the ph is going to go below seven it is definitely an acidic salt as we said before you know with magnesium hydroxide we crossed off the cation but we have an anion that's a base so obviously the base and in this case it was a strong base cool now the last one is not fair so with this last one we've got ammonium we didn't cross him off he's an acid and then we've got cyanide on the other side we didn't cross him off so he's a base so you didn't have a negligible for either the cation or the alien if you recall a second ago i said you're not going to see this and then i put it on the board well i just want to give you an example of what you're probably not likely to see and and you could actually answer this if you could look up some values in a table and stuff like that and what you'd really need is you'd need the ammonium ions ka and you'd need the cyanide ions kb so and you'd probably have to look up some related values in a chart and then actually determine them as we'll see in a little bit here so but basically whoever's got the higher ka or kv that's the one that's going to win so if ammonium's ka is higher than cyanide's kb then it's going to be an acid but if cyanide's kb is higher than ammonium's ka then overall it would be more basic salt so but if you don't have kas and kbs then you just can't answer this question it's not something you're likely to encounter in that scenario so just keep that in mind so now we know how to identify neutral salts acidic salts basic salts so what if you're actually asked to calculate the ph of one of these salts let's take a gander all right so now we're going to calculate the ph of a couple of salt solutions one of them is going to be 0.1 molar sodium fluoride one of them is going to be 0.1 molar ammonium chloride so and if you take a look here so sodium is a group 1 metal so he's negligible but fluoride is not the conjugate base of a strong acid he's not negligible and so overall this is a basic salt we should expect its ph to be higher than seven now if we look at ammonium chloride on the other hand so in this case chloride is the conjugate base of a strong acid so he's negligible but the ammonium ion is not a group one metal it's not a group two metal it's not a metal at all so it's not a transition metal with a plus one charge either it's a polyatomic ion here definitely a weak acid and so overall this is an acidic salt and we should expect the ph to be below seven so how would we actually go about calculating the ph here well in this case we have to ask ourselves well we've learned how to calculate the ph of strong acids and strong bases weak acids and weak bases well what do we have well in this case we've got the fluoride ion as a base well the question then is is it a strong base or a weak base well it's not a strong base because the strong bases were either like lithium hydroxide sodium hydroxide potassium hydroxide erbidium hydroxide cesium hydroxide or the group two metal hydroxides and he's not any of those so he's a weak base well how do you calculate the ph of weak base well we we had a little shortcut for it right so we technically like ice chart and you know kb and stuff but the shortcut was it equals you know the hydroxide concentration is equal to the square root of the kb times the initial base concentration so you need a kb value that is a problem so what's going on here is your sodium fluoride is dissociating to give sodium and then and fluoride ions and then your fluoride the sodium again is not to do anything from that point so but sodium salts are completely soluble it dissociates in ions and then that fluoride is going to further dissociate in water as a base to produce hydroxide and hf that's where your hydroxide is going to come from so and again the key is we said that he was a basic anion what do bases do they dissociate in water to form hydroxide that's where that's coming from and the shortcut we got for calculating the hydroxide concentration of weak base is this guy so we need a kb and so you look in the back of your textbook and you try to find a kb value for f minus and you're not going to find it so kas and kbs are most commonly reported for neutral species neutral acids or neutral bases when i say neutral i don't mean ph seven i mean no charge not ions and so in this case you're not going to find one for hf and that kind of sucks what you are gonna find though is you're going to find a ka value for hf given just like i've provided it in this question and so the ka for hf is 6.8 times 10 to the negative 4. and hf is a neutral species you could totally look up his ka in the back your textbook and it would say something similar to this value so what you're supposed to know from here is something we determined in the last lesson that ka times kb equals kw for a conjugate acid conjugate base pair and that's what we've got here hf is the conjugate acid f minus is the conjugate base and so here we need to know kb for f minus but it's not going to be found in a table most likely so you've got to get the ka for hf instead and rearrange this and kb is simply going to equal kw over ka and that's ultimately what i'm going to substitute in to that expression right there for kb all right so from here it's just plugging and chugging with our shortcut so we've got hydroxide equals the square root and again kb is going to be kw over ka so that's going to be 1 times 10 to the negative 14 all over the 6.8 times 10 to the negative 4. whether you want to calculate it first and substitute it in or just put you know the formula and that's fine either way and then we've got a 0.1 molar concentration because point one molar sodium fluoride one to one ratio here should give point one molar fluoride if for some reason we had a coefficient of two here well then this would be point two molar instead but in this case it is point one molar and from here it's plug and chug to get that hydroxide concentration let's take 1 times helps when you turn your calculator on so 1 times 10 to the negative 14. i need my hand divided by 6.8 times 10 to the negative 4 times point one and that's going to be one point four seven times negative twelve i'm gonna take the square root of that and get one point two times ten to the negative six molar and then we'll take the negative log of the hydroxide to get the poh so the negative log of this number comes out to 5.9 and if we subtract from 14 and a number like this i just highly recommend use your calculator 14 minus that last answer for whatever reason subtracting from 14 with numbers of decimals paying the butt we're going to get 8.1 a lot of students would have done this and come out with 9.1 so i'm saying to use your calculator cool and that is your ultimate answer there's your ph so most salts are going to typically end up just being either a weak acid or a weak base depending on if it's the anion then it's a weak base or in this case it's going to be the cation so ammonium chloride is completely soluble in water to a very very significant extent and you're going to get ammonium ions and chloride ions and it's the ammonium that is a weak acid so it's going to dissociate to give h plus an ammonia and once again we have an issue though ammonium ions have a positive charge they're cations and again for ions you're pretty much not going to find k or kbs in most tables for them what you are going to find though is that his conjugate base ammonia is a neutral species and you will find a kb for him and in fact we had it earlier we had the kb was 1.76 times 10 to the negative 5 and it's in this question as well so so the key is we're not going to find a ka for him directly we'll have to calculate it using the k uh b for ammonia instead and so in this case instead of solving for kb we're solving for ka so ka is going to equal kw over kb and that's what i'm going to substitute in into our shortcut here and so in this case our h plus concentration is equal to the square root of 1 times 10 to the negative 14 all over 1.75 times 10 to the negative 5 times 0.1 molar for our acid concentration so because we get one ammonium from one ammonium chloride and from here it's going to be plug and chug to get our h plus and then we'll take the negative log to get the ph pull out that calculator again all right so 1 times 10 to the negative 14 divided by 1.75 times 10 to the negative 5 times 0.1 gets me 5.7 times 10 to the minus 11 and i'll take the square root of that and get 7.6 times 10 to the negative 6. again we'll take the negative log of this to get the ph so negative log of my last answer gets me 5.1 and voila so one thing to note i've kind of worked these out i'm trying to make it look straightforward these are the absolute trickiest ph calculations typically on this exam and the reason they're tricky is i give you a salt like the naf here in this case and then i give you the ka for hf a lot of students the minute they see they're given a ka they're like oh i got a ka and i got a concentration oh whenever i get a k and a concentration i multiply them together and take the square root and i get the h plus and then i'll take the negative log and get the ph the problem is is that's assuming you have the concentration of that actual acid well i've got the ka of hf given to you in this first example but we don't have an hf solution we have a sodium fluoride solution we have a solution of the conjugate base and so you know there's only a handful of calculations we have to know how to do so far strong acid strong base weak acid weak base so we'll add a couple more in the next chapter but that's it for this chapter four different calculations that you might be on the hook for the problem is you've got to figure out which one you actually have you have to be able to recognize do you have a strong acid do you have a strong base do you have a weak acid do you have a weak base and if you're given a salt then you have to figure out which one of those that turns into after it dissociates and again in this first example the cation was negligible it was the anion that was a weak base in the second example it was the annie and that was negligible it was the cation that was a weak acid and the moment you say oh i've got a weak base there's your shortcut the moment you say you've got a weak acid there's your shortcut and you have to know what to do and then look at what you've been given to figure out how you get there so in this first example you were given a ka but you should have before you even evaluated that said oh this is the salt's a weak base for a weak base this is my formula i need a kb value well it didn't give you a kb value so you need to take and solve for kb with uh as equal to kw over the ka of the conjugate acid they gave you so that's kind of the the methods and again there's only so far four different types of ph calculations in this chapter at least four different solutions strong acid strong base weak acid weak base but identifying which you have and then having it real methodical what process you're going to use to solve for that ph that's the rub and it's going to get more challenging because in the next chapter we're going to introduce buffers and then titrations which are going to be even more ph calculations and the water is going to get pretty muddy so you want to get this down really really good before we get into the next chapter now if you found this lesson helpful thumbs up button is the best way you can support the channel let youtube know that they need to share this lesson with other students and if you're looking for acid and based practice ph calculation practice or general chemistry practice in general check out my general chemistry master course over 1200 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