 Yeah, shall we start now guys? Yeah, okay So first three cases we had discussed Now the last one is hydrolysis of salt salt of weak acid and weak base weak acid and weak base for example We are taking Cs3 COOH weak acid and a base is an H4 OH weak base, okay So when this reaction takes place It forms CH3 COOH NH4 That is a base and water ions we have in this salt are We have CH3 COO minus and NH4 Plus right and both will get hydrolyzed over here Right, both will get hydrolyzed so the reaction would be for hydrolysis if you see the reaction CS3 COO minus plus NH 4 plus dissolved in water Forms CS3 COOH and NH4 OH This is what the reaction we have Okay If the concentration of this salt, I am assuming C and C initially Because it comes from the same salt ions. So we have the same concentration zero and zero Then it becomes C minus CH C minus CH This is CH and this is CH K H if I write down It is concentration of CH3 COOH divided by Concentration of CS3 COO minus Okay, N2 Will have concentration of NH4 OH concentration of NH4 H4 plus Okay, and Just to get an expression. I am multiplying here H plus Multiplying here OH minus and to cancel out this will multiply H plus and OH minus in the numerator Can we do this? Yeah, can you do this guys? What is this then? This is 1 by this is 1 by k a This is 1 by k B for a base and And this is KW. Okay, so the expression of K H would be KW by K into KB Remember like I said in the beginning that whatever is weak that will come here in down expression of K H Here it will come down since we have both acid and base weak over here. So both will be here K into KB Yeah, one second. I'll go back. Yeah done Okay, okay, this is one thing now when you write down the expression of K H in terms of H and C concentration will write down this as CH into CH like this, okay Concentration will get cancelled. So it is H is Square by 1 minus H is square Okay, so if you find out H from this Okay, so we'll write down here 1 H by 1 minus H equals to root over of K H, which is further equals to K into KB. Is it clear? So this is also important if you find out K H H in terms of K H here Then what we can write? root over of K H Is equals to H by 1 minus H So H equals to what we can write. So the hydrolysis degree of hydrolysis equals to root under K H by 1 plus K H all these expressions are important, correct? Since we need to find out the pH here so H plus concentration we require So for H plus concentration, you need to write down the expression for K K is equals to CS3 COO minus H plus divided by CS3 COOH Okay, CS3 COO minus the concentration you see we have C into 1 minus H H plus you let it be as it is CS3 COOH is CH we have here. C and C will get cancelled We'll write K A is equals to concentration of H plus 1 minus H by H Now you see 1 minus H by H Here we have H by 1 minus H So if you revert this you'll get this expression here So this equals to H plus concentration you let it be and 1 minus H by H from this if you write down it is K A into K B by K W root over of it When you solve this because we have this expression you see I'll write down this in the next page We have K A is equals to H plus concentration K A into K B by K W root over of it This K A and this K A will get cancelled. We are left with root over of K A here Hence that H plus concentration equals to K A into K W by K B root over of it Right important relation this one also now from this you can easily find out pH So pH is equals to 7 plus half of pK A minus pK B Copy this down Should I go back? Did you write down this? Done No derivation is not uh, you don't have to memorize derivation formula. You must memorize the final one Because since we have three four formulas, no like this one is important H plus K H formula is important H formula is important So that's why I did this right Now you'll see in the previous one In the previous one We had either strong acid or a strong base based on that we can say The solution is acidic or basic But here in this case both acid and base are weak Correct So we cannot say directly whether the solution is acidic or basic It depends upon obviously you see the pH value we have this So whether the solution is acidic or basic it depends upon the value of pK A and pK B Correct So we have three possibilities here based on the value of pK A and pK B What is that possibility? I'll tell you there are three possibilities case one if pK A equals to pK B In this case pH is equals to what? seven and hence neutral solution case two if pK A Is greater than pK B This is true then we can also write k A is less than k B We know pK A and k A are inversely proportional So if k A is less than k B pK A is greater than pK B If pK A is greater than pK B then we can say pK A minus pK B is greater than zero hence pH is what? Is greater than seven because seven point seven plus something we'll get no Seven plus something pH is greater than seven So the solution is what? The solution is basic another thing Since k B is more It is basic Right, we know more value of k more acidic the compound is more value of k B more basic the compound is Similarly if k B is more then basic and the last case is the other one like the third possibility Where k A is less than k B greater than k B So the case three we have here k A is greater than k B When this is true This is also true this further means pK A minus pK B Less than zero It means pH is less than seven once again pH is less than seven which makes the solution acidic That is also true because k is more than k B Is it clear? Okay Now you try this question You need to find out the pH Calculate the the pH of solution formed by mixing First question 100 ml of 0.1 molar CS3 COOH And and 100 ml of 0.1 molar NaOH First one is this The second one Of 0.1 molar NH4 OH With 100 ml of 0.1 molar HCl Third one I'll go back wait 100 ml of 0.1 molar HCl And 100 ml of 0.1 molar NaOH Last one Is 100 ml of 0.1 molar NH4Cl 100 ml of 0.1 molar CS3 COOH This is the four question we have One more data is given over here like Ka value is given Ka for acid That is CS3 COOH Is 2 into 10 to the power minus 5 And k B value for the base Which is NH4 OH Is 2 into 10 to the power minus 5 Try this Done The first one 8 point something close I guess Okay, see in the first one See the number of Equivalence or millimoles whatever you say If you find out for this the number of millimoles It will be 10 millimoles Here also it is 10 millimoles Right It means both will get neutralized completely And when this reaction takes place it forms a salt Which is CS3 COOH Na And water Since it gets neutralized completely complete neutralization So it is a case of salt hydrolysis We can apply the formula of pH For the salt of weak acid in a strong base Since the base is strong, so it is basic pH would be more than 7 so 7 plus half of pKa Plus log C pKa value for this is 5 minus log 2 that is 4.7 7 plus half of 4.7 plus log C The concentration of salt would be what? The one that reacts equal amount it will form That would be 10 1 5.7 5.7 by 2 2.35 2.85 Yeah, it's 10 by 10,000 correct So we can write 10 to the power minus 2 So 4.7 minus 2 2.7 2.7 by 2 Is 1.35 so answer would be 8.35 we get okay Similarly, this is a strong acid This is a weak base Acha 4.7 you can get like this See k value is what? 2 into 10 to the power minus 5 pKa is what? 5 minus log 2 Yes Log a into Log a into b is log a plus log b Got it It's 4.7 pKa b is also 4.7 only here Okay, so number of millimoles if you calculate here Because it is it is millimoles So if you convert this into moles 10 by 1000 you will need to write down 10 millimoles we have no Okay So number of millimoles here it is again 10 And here also it is 10 again So we'll write down the pH For acidic solution Which is 7 minus half of pKa What is the formula plus or minus c? pKb Plus log c Because base is weak so we have b over here So 7 minus half of pKb again 4.7 And log c is 10 millimoles so 10 to the power minus 2 So same thing 7 minus 1.35 That is 5.65 isn't it minus 2 4.7 minus 2 is 2.7 2.7 by 2 is 1.35 1.35 so 5.65 Is this right? Let's check the calculation once clear Try this one third one is 7 Because both are strong acid and a strong base Right with see it's not like only strong acid is strong base is the condition here. Obviously answer here is 7 But you need to calculate the number of millimoles here If it is equal Then only we have complete neutralization see this 10 millimoles Suppose you have 15 millimoles here, then the solution will be basic And then we can use the formula of mixture of acid and base Are you getting my point 5 millimoles extra So OH minus concentration would be 5 divided by 100 plus 100 And then POH and then pH Did you get it? In this one, tell me fourth one. Oh, it is there's a mistake in this It should be NH4 OH Not NH4CN Okay, NH4OH right on 11.7 you are getting It is weak base see this one is The weak base and this one is the weak acid Number of millimoles is same 10 and 10 we have here Okay, so we can write down pH is equals to 7 plus half of pKa minus pKb Right, so what is the value of pKa and pKb? pKa and pKb is same Yeah, pKa, kb value is will be given in the question. I don't what are you doing Anusha? pKa pKb value is equal no Yes When it is equal Then it should be seven only right? Why? These two are equal so it will be zero Kansas out And pH equals to seven Simple right? So these are the simplest questions I discussed but always you see whenever you have acid based neutralization First of all you count the number of millimoles or milliequivalent Right based on that you can no think about the formula that you should use the condition Okay The next topic we need to understand here is Write down the heading buffer solution BU develop ER definition write down a solution which Resist the change in pH the change in pH on addition of on addition of a small amount of acid or base Is called Is called a buffer solution These are the solution which resist the change in pH Okay, when you add a small amount of acid or base to it See first of all, it does not mean that the pH change is zero It resists the change in pH to a certain extent Okay Next time write down This does not mean This does not mean That the pH will not change But it means The pH Change Would be Would be lesser than Would be lesser than The change That Would have occurred That would have occurred Had it Not been Had it not been A buffer Solution Copy this down first Okay, so the solution is what These are the solution Which resists the change in pH When you add a small amount of acid or base into it Right, there will be a change in pH but that change will be too less Or lesser than the change If it was not a buffer solution Right, so if you have two solutions Right, one is buffer other one is not a buffer solution Add equal amount of acid or base to these two solutions Then you would observe that in the non-buffer solution The change in pH is more Than the buffer solution That is what the Meaning of buffer solution we have Okay, it's very important Like, you know There are many different solutions which act as a buffer solution Right, and it tries to maintain the pH If I take one example You have an acid, weak acid, correct And it's salt with a strong base So this combination of weak acid and the salt with a strong base Gives you a buffer solution Okay Buffer solutions are of three types In this also we have You know A pH calculation that we need to do Okay, so that's what I said That buffer solution or salt hydrolysis Whatever it is First of all you need to identify That What condition we have here Accordingly you can use the formula of pH Okay, just a second So Buffer solution you see We have three types of buffer solution The first one is Acidic buffer See in the In human body also if you see Blood has a certain pH value Correct If you, you know, eat something like Which increases the pH H plus concentration Or pH of the blood Right Then you will feel uneasy Because the pH of the blood has been increased Right So There is a mechanism By which, you know, the reaction that happens There in the body It tries to maintain the pH of the blood If it works well Then you won't feel that that uneasy You will be better in some time But if it does not go well Then again you have to take some medicine In order to control the pH of the blood So that everything is fine Right So basically we have a lot of examples Food preservative also we require a certain pH So we mix certain things So that it maintains the pH of the, you know Of the substance over there So that the food, you know, keeps fresh Or, you know, will be fresh for a certain period of time So we have a huge application of buffer solution I'm not talking about a specific kind of buffer solution Whether it is acidic or basic But application is huge Like few examples I have given you So the question is how this solution works Like how it maintains the pH of its own solution Again, I'm repeating myself But I say that it does not, you know Change the pH of the solution It does not mean the change is zero We'll have a change But that change is very less, very small Which won't affect much Okay Or we can say the change is lesser than the change Had it not been a buffer solution Okay, that's what the meaning These theoretical questions also sometimes they ask What is acidic buffer and how it is prepared Acidic buffer again the same example we have for buffer solution Acidic buffer is the solution of Copy this down It is the solution of Weak acid And it's Salt with strong base Could you give me one example of this One second guys Yeah Give me one example of this CS3-CONA, yes No, yes We can take CS3-COOH One second guys Okay So what I said here Solution of weak acid and it salts with strong base So you see here We have a weak acid CS3-COOH I'm taking And salt of this weak acid with a strong base NaOH layer So it forms what? CH3-COONA Plus H2O So when the reaction takes place Between a weak acid and a strong base Here like this If this is present in excess Then this would be the limiting reagent Right It gets consumed in the reaction completely When the reaction gets over Then in the reaction what all things are left We have CS3-COOH as left And CS3-COONA present So when this combination is there Like the combination of A weak acid And it's salt with strong base That is CS3-COONA This forms an acidic buffer Got it? That's why you see In salt hydrolysis also we have the same reaction But the condition is this too must be equal If it is not equal suppose We can write this is either it is excess Or NaVA Is more than NBVB If this is the condition we have NaVA is more than NBVB Correct? Then it is the condition of buffer solution Buffer solution forms in this case And hence we use the formula of buffer here Yeah we have the mechanism We'll discuss how it resist the change in pH We'll discuss that But first you understand this We'll discuss the mechanism Copy this down So now you understood the reason Why in the salt hydrolysis We were talking about equal equivalence Of this and this You understood it right? Clear? So you must keep that in mind Whenever you get this question First of all you calculate The number of equivalence of acid and base And accordingly you can use the formula So this is an example of acidic buffer CS3-COOH With CS3-COONA Okay Now how this buffer solution works You see There are two reactions here CS3-COONA It gives CS3-COO-NA plus Reaction one Other one is CS3-COOH Which gives CS3-COO-H So this is the reaction we have In the solution we have this ion present Now what happens You are adding a small amount of H plus Acid add We are adding acid It means H plus we are adding So what happens is The extra H plus that you are adding To maintain the pH This H plus It will consume So that the H plus concentration is maintained So what happens This extra H plus is taken up by CS3-COOH minus And then the second reaction The second reaction goes in backward direction Reaction two Goes in Backward direction Backward direction To maintain the concentration of H plus H plus Okay If you add a base OH minus you are adding OH minus you are adding Then this H plus Reaction two Goes in Forward direction To maintain the concentration of H plus So what happens is OH minus that you have added This OH minus will take this H plus Forms an dissociated water molecule And this reaction goes in Forward direction The H will be maintained H plus concentration will be maintained The use of this is what This minus When H plus it consumes No, it goes in backward direction Correct Further dissociation of this is resist by this Because these two has one common ion present So we have here a common ion effect H plus it consumes Forms a weak electrolyte CS3-COOH Which dissociation is further suppressed By the presence of this common ion And that is how the H plus Concentration is maintained Got it understood? Yeah, tell me guys Sometimes they ask theoretical questions also on this Like how this works Which reaction goes in which direction Okay that also you have to keep in mind Okay We have a question in this We call it Henderson's equations Right Okay There's two moments If I write down the expression for Ka here It is Concentration of CS3-COO-H plus Divided by CH3-COOH H plus concentration we need to find out So H plus equals to Ka Concentration of acid Divided by The concentration of the salt CS2-COO-H is nothing but the salt If you take log both sides Minus of log Minus of log H plus Is equals to minus of log Ka Plus Minus of log CH3-COOH CH3-COO-H Minus of log H plus is pH Minus of log Ka is Pa P Ka Plus this minus I'll take it up and then we'll revert this So log of Salt Concentration By acid concentration This one is Salt CS3-COO-H coming from the salt And this one is acid This equation we call it as Henderson's equation We'll discuss a bit more about it in the next class Acetic buffer But this is the equation of acidic buffer Understood? Yes guys, understood, clear? Any doubt in this? Okay So next class we'll continue with this acidic buffer Then we'll see basic buffer and salt buffer Okay I will share one PDF today You solve that, okay? Assignment Bye, take care Thank you