Consider a buffer solution that contains equal concentrations of H2PO4- and HPO42-. Will the pH increase, decrease, or remain the same when each of the following substances is added? (a) Na2HPO4 (b) HBr (c) KOH (d) KI (e) H3PO4 (f) Na3PO4

Video transcript

hi everyone for this problem. It reads determine what happens to the ph increases, decreases or remains the same of a buffer solution containing equal concentrations of the following on the addition of each of the following a through f. Okay so we want to determine what's going to happen to the P. H. For each. So let's go ahead and get started. And we're also told it is a buffer solution. So that means we're working with a weak acid and its conjugate base for a. When we disassociate this into its ions, this becomes three K. Plus and The an eye on C6 H three minus. And this an ion can react with hydro ni. Um Okay and so let's go ahead and write out that equation. So for a R when it reacts with hydro ni um actually this becomes so the first two those are our are ions or what they dissociate into. And when this an ion reacts with hydro ni um it forms the following. Okay, and so we can write a before equilibrium reaction for this reacting with water. So let's go ahead and do that down here. So we have the following, an ion is going to react with water and this is going to produce an equilibrium of hydro ni um ions plus. Okay. And so after the addition of a to the solution, what this is then going to become is okay, so well we'll go out right on the side, this is before equilibrium and this is after the addition of answer choice. A Okay so when we do this this is going to decrease the concentration of hydro knee um from the buffer equilibrium. So by decreasing the concentration of hydro ni um from the buffer equilibrium this is going to result in an increase in P H. Okay, so for a our answer is it's going to result in an increase N P H. So that is for a okay, so let's go ahead and move on to be okay, so for b this is a neutral salt and the addition of a neutral salt to a buffer solution does not change the P. H since it's neutral. Okay, so for B this answer is going to be it remains the same. Okay, so for B it's going to remain the same because it is a neutral salt. Okay, For c what we have here is this can be added to the solution and it could be a source of c six H 707 minus. Okay, so what we can do is we can create a reaction with this reacting with water. So let's go ahead and write that down. So what this looks like when it reacts with water. Mhm. So consequently this an ion will add to the buffer system. So let's go ahead and write what the buffer equilibrium is. Okay and up here, I meant to write buffer equilibrium and not before equilibrium. So this should say buffer equilibrium. So let's go ahead and write out the buffer equilibrium. And what that is going to be is oops that's not clear. 8707 -. Okay so this is our buffer equilibrium and the addition of C. Will cause the equilibrium to shift to the right. So if the equilibrium shifts to the right this is going to increase the hydro knee um ion concentration and this is going to result in a decrease in P. H. So for see our answer is going to be decreases or decrease N. P. H. Okay so let's keep going for d. This is a strong acid. Okay so the conjugate base component of the buffer is going to react with the strong acid and the addition of a strong acid increases the weak acid concentration and decreases the ph so then we can write a reaction to show that and that is going to be the strong acid plus water yields hydro ni um plus the conjugate base. Okay and so when we add and this is from the hydrochloric acid at equilibrium we have. Okay and as we said the addition of the strong acid increases the weak acid concentration and decreases the P. H. So our answer for D. Is decrease N. P. H. Okay for e let's take a look at E. For e. This is a source of Okay and so when we add this this is what we're going to be adding to the buffer system. Okay so the addition of E will cause the equilibrium to shift to the left and this is going to consume or decrease the hydro ni um ion concentration which is going to result in an increase in P. H. Okay. So let's go ahead and write that. So the buffer equilibrium is going to be Yeah. aN:aN:000NaN Okay. Yeah. Mhm. Okay. So as we said, this is going to result in an increase and ph Yes. Okay. And lastly we have f and F is a strong base. Okay. Okay. So the weak acid component of the buffer will react with the strong base. Okay. And let's go ahead and write that the weak acid component of the buffer reacting with the strong base. This now becomes mm hmm. And this is from the strong base. Okay, So the addition of a strong base increases the conjugate base concentration and increases the ph so our answer for F is going to be increase N P. H. And that is the final answer for F. So we went through them all and these are the final answers. That is it for this problem? I hope this was helpful