The following pictures represent solutions that contain one or more of the compounds H2A, NaHA, and Na2A, where H2A is a weak diprotic acid. (Na+ ions and solvent water molecules have been omitted for clarity.)

(b) Which solution has the greatest buffer capacity?

Video transcript

Welcome back, everyone. We need to determine the solution with the greatest buffer capacity among the representations of solutions shown below containing one or more of the following components H two A K A H A and K two A where H two A is a weekday protic acid note that the K plus ions and water were omitted from the representations for clarity. So we should recognize that K H A and K two A are both salts or salt forms of an acid. And that's why the prompt mentions that our K plus ions and water are omitted because recall that our salts are going to dissociate into their ions. So we would form the K plus ions out of our Salts here. And so let's now focus on our week die protic acid H two a. So when we reacted with a strong base like hydroxide, the equilibrium That will form with our products being our conjugate base and conjugate acid will be the result of our acid H2 a donating a proton to our base hydroxide. And in that process, we will form our product being our conjugate base H A minus. So the conjugate base of our weak acid. And as a result, we will also form water as our conjugate acid of our base hydroxide. Now continuing the equilibrium, we will have our conjugate base H A. Now acting as an acid where we react with another molecule of hydroxide. And we will be in equilibrium with our products. When our acid donates a proton to hydroxide, we form the conjugate base A two minus and our second product, another molecule of water as the conjugate asset of hydroxide. Again. So recognize that the pattern here is that our conjugate base is the result of our weak acid donating a proton to the base. Next, we want to recall the definition of a buffer solution. We should recognize that a buffer solution will have the concentration of our conjugate base equal to the concentration of our weak acid in the solution. Or we could have the concentration of our conjugate acid equal the concentration of our weak base. In this case, we are worrying about the first scenario that we outlined, which is the conjugate base concentration being equivalent to our concentration of our weak acid H two a. So let's analyze our first depiction here in scenario I so or one. So looking at the molecules, we can see that we have just molecules of our weak acid age to A and so because we only see our weak acid, we can say therefore, this is not a buffer solution. It does not have a very high buffer capacity. So we can already rule out choice one moving on to image to we see that we have our molecules of our weak acid here. But we also have the A two minus molecules. And we should recognize that a two minus based on our equation is not the conjugate base of our weak acid H two A. And so therefore, it's not going to have buffer capacity. So this is not a buffer solution referred down to our equation below where we see that A two minus is the conjugate base of H A minus. So that's why we know what we wrote here is true. So let's rule out image two and move on to image three where we have molecules of H A minus and molecules of H two A. So we should be able to observe that H A minus Is indeed the conjugate base of our weak acid H two a. And so therefore, this does have the definition as a buffer solution. And so we would observe our capacity of the solution to be Extremely high as a buffer solution. Now let's move on to image four. So just to make sure it's all visible, gonna zoom out a little bit Okay. So we can fully see image four, hopefully All of the molecules are here and we see that we have molecules of a two and our H A minus molecules. Now we should recognize that A two minus is indeed the conjugate base of H A minus. And so therefore, this also matches the definition as being a buffer solution. Since we do have the equal concentrations of our conjugate base and weak acid H A, just like we did. In scenario three, we had three molecules of our conjugate base H A or rather four equivalent to our concentration of our weak acid H two A where we also had four molecules. So right now, we have ruled out choices one and two, but we're between choices three and four, we need the solution with the greatest buffer capacity. So we should note that our concentration of A two minus being our conjugate base of our concentration of the weak asset H A minus is greater. And let's just move this over. So we have enough room. Alright. So the concentration of our conjugate base A two minus to the concentration of our weak acid H A is greater because we have more molecules of these two compounds than the concentration of H A minus our conjugate base in depiction three, two. And we'll just move this over a little bit more to our concentration of our weak acid H two A, we know that it's greater because we see that for the concentration of H A as our conjugate base to the weak acid H two A, we have less molecules, we only have four molecules to the conjugate base to weak acid versus in depiction four, we have a lot more molecules. So a greater concentration of buffer capacity. And so we would confirm that based on this observation, therefore, solution for is the solution with the greatest buffer capacity. And so what's highlighted in yellow here is going to be our final answer to complete this example, which corresponds to choice D or choice D in the multiple choice. So I hope that everything I reviewed was clear. If you have any questions, please leave them down below. And I'll see everyone in the next practice video.