 pH of weak acid solution and we are considering dissociation of water. Dissociation of water is considered here. So what happens in this you see? Okay, suppose we have a weak acid HA and it dissociates as H plus A minus and we have water also dissociating is H plus and OH minus. In this solution you see what all ions are present. We have H plus A minus and OH minus present. There are ions present in the solution. On charge balance, if you are balancing the charge, we need to write total positive charge equals to the total negative charge H plus equals to A minus and OH minus. Right, charge balance. If I write down the dissociation constant of an acid K A. K A equals to H plus look at this reaction product by reactant and if you find out A minus from this K A into concentration of HA by H plus. Similarly, the expression of KW if you see it is H plus and OH minus and the concentration of OH minus equals to KW by H plus concentration. Actually what we are trying to do, we have H plus already here. If A minus and OH minus is substitute in terms of H plus, then we can find out the total concentration of H plus and that is what we are doing. Now this A minus and OH minus will substitute here in this equation. So what we get here, you see H plus equals to K A into HA by H plus KW by H plus. So H plus concentration is square KW plus K A into concentration of HA, once again I will go back. Concentration of HA suppose if you are assuming C then H plus concentration would be K A into C plus KW root over of it. This is the formula we have of H plus. You can find out H plus from this and then further. Yes, you can find out. Finished. Can you guys? Okay. Now, see here we are assuming the mixture of one acid and water, only one acid. If you have more than one then what happens? Suppose we have two acid, if two acid A1 and A2 are mixing in water, okay. Then H plus concentration would be for one acid will write K A1 C1 for another one will write K A2 C2 and then KW. If it is three then we will have K A3 C3 also and so on it goes. Let me read out here. Next is if we are mixing in this only you write down the note. If the mixture of weak base and water, weak base and water and water dissociation we are considering, okay. Then we have exactly same formula but since we have base so we won't get H plus here but we get OH minus on the left side is equals to KB1 C1 plus KB2 C2 plus KW. Two acid, two base if you are mixing and this is the formula we have. No difference exactly same formula we have. Calculate the pH of a solution containing two acids. All the values are given K A1 equals to 10 to the power minus 8 molar. C1 is equals to 10 to the power minus T molar. This is not there. K is 10 to the power minus 8. K A2 is 10 to the power minus 7 molar and C2 is, why did I write this? C2 is 10 to the power minus 3 molar, okay. We need to consider the dissociation of water. I'll write down here. Consider dissociation of water. Tell me. Yes, what is the answer? Okay, so we have two acids so it gives H plus. H plus concentration would be K A1 C1, C2 and KW. K A1 C1 is 10 to the power minus 10. K A2 C2 is 10 to the power minus 10. 10 to the power minus 14. Temperature is not given. We are assuming 25 degree Celsius. So 10 to the power minus 10 we can take common here. So it is 10 to the power minus 5 outside this root 1 plus 1, 2, 2 plus 10 to the power minus 4 here. This value is very small. We'll just ignore this. So it is root 2 into 10 to the power minus 5, okay. So pH would be what? Minus log of H plus. So it is 5 minus log of, so 5 minus root 2 is what? 5 minus half of log 2. Log 2 is 0.30. So it is 0.15, 5 minus 0.15. 4.85 is the answer approximately we have. Tell me, is this right? Yes, any doubt in there? Okay. Next you see we have salt hydrolysis. What do you understand by this term? Salt hydrolysis. It is the hydrolysis of salt, right? When you dissolve salt in water, okay. So basically what happens if you look at this reaction here, we have a reaction in which an acid is reacting with base. It forms salt and water. Correct? So this reaction is neutralization reaction, right? This is neutralization reaction when acid and base are reacting. They both neutralize each other. Hydrolysis of salt is what? When the salt is dissolved in water. So basically, when you talk about hydrolysis of salt, it is the reverse of neutralization process. Okay, so this reaction is the reverse of neutralization reaction. And this reaction goes in backward direction actually. We are talking about this yellow wall reaction, salt in water. So what happens when you dissolve salt in water? So first of all, you see the salt hydrolysis, we are going to get a formula in this again, the formula of pH because all the solution we need to find out pH only, right? So salt hydrolysis is applicable in case of complete neutralization, right down. Complete neutralization means what? If you have this condition, number of equivalence of acid equals to the number of equivalence of base, okay. So if this condition is there, then salt hydrolysis formula will apply. Otherwise, we have a different, you know, case. What is that case? We'll discuss that later. But here, just to keep this in mind, that this is the condition for salt hydrolysis we have. If this condition is not there, then we have a different kind of solution it forms. And for that solution, how do we find out pH? We'll discuss that later. But the solution that we get here is generally we call it as buffer solution, okay. When you look at buffer solution, so in buffer solution and in salt hydrolysis, we have similar kind of reaction, similar kind of reaction. So what formula we need to apply when that you need to understand? When is the condition of salt hydrolysis? When is the condition of buffer solution that you can understand by this? When it is complete neutralization, buffer solution is not applicable. Keep that in mind, okay. This is one thing. So basically, suppose we have an acid HA and a base BOH. So when this to react, it forms BA salt and water H2O, right? Salt and water H2O. When this salt dissolved in water, when this salt dissolved in water, then what happens you see? Because it is salt, so it exists as B plus and A minus. So B plus plus H2O. It forms BOH plus H plus. A minus plus H2O gives HA plus OH minus. Any doubt in this? Tell me all of you. Yes, guys, tell me. No doubt. See, when you talk about the hydrolyzed solution, okay, hydrolyzed solution that you get means once the hydrolysis takes place, so you see depending upon the hydrolysis of ions, the solution can be either be acidic or basic or neutral. If suppose this reaction is taking place more than this one, then it will furnish more H plus. It gives acidic behavior to the solution. If this is taking place more, then it will furnish OH minus basic solution will be there. Okay, one second. Basic solution will be there. So that is what we need to understand that for a given condition, whether the solution will be acidic or basic or neutral. Okay? Why it is acidic or basic? You can understand it clearly. When salt gets hydrolyzed, it will produce H plus and OH minus. Now depending upon the concentration of these two, if this one is more acidic property, if this one is more basic property, if both are equal, neutral. So which one is more that depends upon the extent of hydrolysis of this ion of the salt. The ion which is more hydrolyzed, according to that will understand the acidic or basic behavior of the solution. Correct? So we have in this under salt hydrolysis, we have four possible cases. Like we are taking acid and base. So it can be strong, it can be weak. This also can be strong, can be weak. So there are four possible PNC, permutational combination we have over here. Strong, strong. Weak, weak. Weak is strong, strong weak. Okay? Four conditions of acid and base. So first condition you see hydrolysis of salt of heading right down all of you. Hydrolysis of salt of strong acid and strong base. Strong acid and light as SA. Strong base is SP. Hydrolysis of the salt of strong acid and strong base. This means we are taking a strong acid and base for this purpose. For example, suppose I'm taking HCl, strong acid and base NaOH. It gives what? It gives NaCl and OH and H2O. NaCl and H2O. Okay? Now this NaCl is the, you know, strong electrolyte basically. So it exists as Na plus and Cl minus. Now one thing is very important here. When hydrolysis of this salt, if you consider, it means the reaction of Na plus and Cl minus with H2O. This is what the reaction you're considering. But this forms, either it forms NaOH or it forms HCl, right? But both are strong acid and strong base. So both exist in the form of ions only. Na plus and Cl minus. Na plus and OH minus. H plus and Cl minus. So point here is what? Strong acid and strong base whenever you take, right? So the salt does not get hydrolyzed because they exist in the forms of ions itself, right? And we conclude here, one note you write down. Na plus is the cation of strong base. So cation of strong base, Cl minus is the anion of strong acid and anion of strong acid does not go under hydrolysis. Does not go under hydrolysis. So for this two, we do not have hydrolysis possible, right? And the reaction is said to be, the solution is said to be neutral. Its pH value is 7. So it is always a neutral solution. This is the condition here, clear? And out. Second case, we have hydrolysis of salt of weak acid and strong base, okay? So suppose I am taking a weak acid here, CH3COOH. When it gets hydrolyzed with H2O, what it forms, sorry, when it reacts with strong base, say NaOH. What is the reaction we have here? It forms salt and water. So salt is this CH3COONA and water is H plus OH minus H2O. Now when this salt, if you see, this dissociates like CS3COO minus and this one is Na plus. So Na plus is the cation of strong base. So for this, we have no hydrolysis. Only this will go under hydrolysis. So its reaction, what happens in this? We have CS3COO minus plus H2O hydrolysis reaction. It forms CS3COOH plus OH minus, right? Now you see, in the solution, we are getting OH minus ion. It means it is a basic solution, isn't it? Means the pH value for this solution should be less than 7. Oh, I am sorry. It is greater than 7. pH value should be greater than 7. POH is less than 7. Correct. So one thing you understood that acid is getting the, you know, weak acid. One more thing you can understand directly, because we have, we will see the derivation of this. How do we get the pH formula? Correct. But to memorize the formula, a few things you can think of. One thing directly you can think of here, that acid is weak, right? Acid is weak. Base is strong. So this is a strong base. So it will dominate and it gives the basic property to the solution. Directly you can understand here. If it is basic, then pH should be more than 7. Yes, like this you can memorize. You will see the derivation also over here. Okay. Now, suppose the salt that you have here, CS3CONA or CS3CO minus, having concentration C, right, water we are not considering, this is zero and zero initially. Remember, in order to find out pH, you can calculate H plus concentration. You can also calculate OH minus concentration, because once you know OH minus, you know pH and then we know, we know, when you know OH minus concentration, you know POH concentration, then you can find out pH and hence the, you know, we can calculate this. When this gets hydrolyzed, then what we'll write, it is C minus CH. This is CH and this is CH. H is the degree of hydrolysis. Degree of hydrolysis. You can consider this H as alpha only, right? Like we have H, alpha there, here we have H. Copy this down. Okay. If I write down the expression of KH here, the expression of KH would be, KH is similar to K, KC that we have, but this H stands for hydrolysis constant. It is like we have equilibrium constant. This is hydrolysis constant KH. So this would be the concentration of product that is CH3COOH into OH minus divided by CH3COOH. KH expression is this, where KH is the hydrolysis constant. Terms you must remember. Okay. Now you see the expression of KH equals to CH3COOH divided by CH3COO minus and on the top we have OH minus concentration. Okay. Now we are doing some change into this. What change I'll tell you. I'm multiplying H plus concentration in the numerator and denominator. Once you do this, you see easily you can observe what is this expression and this expression. This expression is one by KA, isn't it? And this one is nothing but KW. Yes, tell me. Should I go back? Okay. One second. Copy. Did you understand this? It is one by K. This is KW. So the expression of KH equals to KW by K. On this relation also they ask question in the exam. This is also a very important relation. First thing is this. Okay. Remember this K is that association constant of weak acid that is CH3COOH. Now I want you to write down the expression of KH again. We have KH is equals to CS3COOH into OH minus divided by concentration of CS3COO minus. In this only we have done some modification and we get this. CS3COOH, the concentration if you go back and see it is CH. OH minus also to CH is C into one minus H. C and C will get cancelled. So KH is equals to, we get CH square divided by one minus H. Okay. Again, we have assumption. If H is very small, we can take one minus H almost equals to one and hence KH equals to CH square. KH we know already. So H is equals to KH by C root over of it. KH is KW by K. Just now we did previous page you would see. So this also they asked sometimes in the exam the formula of H here. This is also very important. Once you know H because we need to find out this here you see. We need to find out the concentration of OH minus. See this concentration of this OH minus, the base right. This is CH. H we know already. So concentration of OH minus would be it is given as CH. So H is this formula we have. So when you solve this you'll get KW into C divided by KA. Okay. This is OH minus concentration. Now once you have this concentration you easily find out POH. Did I miss root? Yeah. We have a root over also. Okay. So pH calculation POH would be what minus log of OH minus which is KW into C divided by KA root over of it. Okay. And when you solve this you'll get POH equals to 7 minus half of pKa plus log C. POH is equals to 7 minus half of pKa plus log C. This is the another formula. And we know pH is 14 minus POH. pH is 14 minus POH. That would be 7 plus half of pKa plus log C. This is the formula. So you see pH we are getting greater than 7. POH we are getting less than 7 which means the solution is basic. And that is what our understanding since the base was strong and we were getting OH minus iron in the solution. So it should be basic only. Whatever formula I have written in this box all these formulas are important for Americans. Yes. Then then guys next one to third one is hydrolysis of hydrolysis of strong acid and weak base. Okay. What should be the nature of the solution here? Acidic. Why? Because the acid is strong here. So whatever is strong that will be the nature of the acid. You see I'm assuming a base here that is NH4 OH weak base. Acid is HCl gives NH4Cl and H2O. Okay. Salt is this which gives the iron which is NH4 plus and Cl minus. Dissolution of these ions in water. NH4 plus in water equals to you have NH4 OH plus. Okay. Cl minus won't go under hydrolysis because it is the anion of strong acid. Hence no hydrolysis. Okay. So hydrolysis of only this iron NH4 plus is possible. We are getting H plus here you see like I said we have H plus here. So solution is acidic in nature. And when it is acidic its pH value should be less than 0. Exactly similar kind of derivation we have here. It is C00 C minus CH CH and CH. So KH equals to NH4 OH. NH4 OH then we have H plus divided by NH4 plus concentration. Okay. So this would be equals to CH into CH C into 1 minus H. So basically it is CH square. So H. H is equals to KH by C root over of it. This is the first one. Copy this down. Remember H plus concentration is CH. Yes. Same thing. We are assuming the same thing. H is less than 1. Okay. Next we can write down since we have H plus concentration is C into H C into KH by C root over of it. So it is C KH. Okay. KH also we can find out KH equals to KW by KB here. If you remember in the previous one it was KW by KA. So whatever is weak. If acid is weak then we will have here KA. If base is weak we will have here KB. How do we get this? Same thing will multiply by OH minus in the numerator and denominator. Do you want me to do this again? Tell me once. You can write down the expression of KB. Okay. You can write down the expression of KB. The expression of KB would be for this reaction you see. We have NH4 plus plus H2O gives NH4 OH plus H plus. So KB for this would be NH4 OH H plus once again H plus divided by NH4 plus. So I will multiply here by OH minus in the numerator and OH minus in the denominator. So this OH minus and H plus in the numerator it is KW and this is 1 by KB or once again this is KH. I have written KB by mistake. Okay. So KH equals to you see we have KW in the top and KB in the bottom. That's what we got over here. Okay. So whatever is weak that will come down in that denominator. Last time it was weak acid so it is KW by KA. Here it is KW by KB. Now this KH you can substitute here and you will get the concentration of H plus. So concentration of H plus would be C into KW by KB root over then pH is equals to minus log of H plus and when you substitute the concentration of H plus you will get 7 minus half of PKB plus log C pH. Okay. So here also you see we are getting PKB here. The weak one will come over here. Last time it was PKA because acid was weak. If you want to calculate POH here will be 14 minus pH. So POH would be this is the formula of POH. Any doubt? You see we are getting pH less than 7 here hence it is acidic. Yes guys understood. Yes guys any doubt in this? Okay. This is quite easy. The formula is also you can you know logically you can compare and you can keep in mind but the thing is like what formula you have to use when that is important. Right. Yes. Once again this or this one minute. Okay. You need to analyze that. Like I said salt hydrolysis formula we can apply only when we have complete neutralization. Right. Complete neutralization. Okay. Once we finish this we have one more condition left into this that we will take after the break. Once we finish this you will see buffer solution and in buffer solution you will understand that both cases hydrolysis and buffer solution you will have the same reaction but same reaction we have and we are using different pH formula that you can understand once you keep this condition in mind. Like I said when there is no acid and base present it is only salt and water then only will apply the salt hydrolysis formula otherwise it may form buffer solution. We have few conditions for that that you need to keep in mind. Okay. Fine. We will take a break now. We will resume at 6.30. Okay. We will see the fourth you know condition in that and then we will see some questions. Take a break guys.