Aqueous solutions of $\ce{HNO3}$, $\ce{KOH}$, $\ce{CH3COOH}$, and $\ce{CH3COONa}$ of identical concentrations are provided. The pair (s) of solutions which form a buffer upon mixing is(are)?

A. $\ce{HNO3}$ and $\ce{CH3COOH}$
B. $\ce{KOH}$ and $\ce{CH3COONa}$
C. $\ce{HNO3}$ and $\ce{CH3COONa}$
D. $\ce{CH3COOH}$ and $\ce{CH3COONa}$

I think that option D is correct as it a solution of a weak base and its a salt; that's why it is a buffer solution. But how do I check the other options?

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    $\begingroup$ A screenshot or picture of an exercise is not searchable. Please consider rewriting it, so that it can be of help for future visitors. $\endgroup$ – Martin - マーチン May 18 '17 at 4:48

Well, it all boils down to the preparation of buffer solutions in the first place. One of the methods of preparation includes the mixing of a salt solution of an weak acid and a strong base with a solution of a strong acid, with the solution of the strong acid as a limiting reagent, which leads us to conclude positively for the third choice.

Alternatively, you can think which of the mixtures will resist the change in pH on mixing a strong acid or a strong base; for instance in the third choice, nitric acid being a strong acid will help to resist the change in pH when a strong base is added, and $\ce{CH3COONa}$ being a weak acid + strong base type of salt resists the change in pH that could've been caused due to the addition of a strong acid.

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  • $\begingroup$ Why option B is wrong $\endgroup$ – samsung May 18 '17 at 3:57
  • $\begingroup$ @samsung Neither of $\ce{KOH}$ and $\ce{CH3COONa}$ would be able to help when you add a strong base. $\endgroup$ – Berry Holmes May 18 '17 at 4:02

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