Question

Aqueous solution of $HN{O_3},KOH,C{H_3}COOH$ and $C{H_3}COONa$ of identical concentration are provided. The pair(s) of solutions which form a buffer upon mixing is (are):
(A) $HN{O_3}$ and $KN{O_3}$
(B) $KOH$ and $C{H_3}COONa$
(C) $HN{O_3}{\text{ and C}}{{\text{H}}_3}COONa$
(D) $C{H_3}COOH{\text{ and C}}{{\text{H}}_3}COONa$

Answer 1

Hint: Buffer solutions are either the mixture of weak acid and its salt with strong base or the mixture of a weak base and its salt with strong acids. The former is called acidic buffer and the later is called basic buffer solution.

Complete step by step solution:
We will know what buffer solutions are and get information about the requirements of a mixture to act as a buffer solution.
- Buffer solutions are the solutions that prevent the change in pH of the solution when we add a small amount of acid or base into it. Thus, they keep the pH of the solution constant up to an extent.
- The mixture of a weak acid and its salt formed with a strong base acts as an acidic buffer solution. That means the pH containing this buffer will be acidic i.e. less than 7.
- The mixture of a weak base and its salt formed with strong acid acts as a basic buffer solution. The solution containing this buffer solution has slightly basic pH.
- We know that $HN{O_3}$ is a strong acid as it ionizes completely in aqueous medium. Acetic acid ($C{H_3}COOH$) is a weak acid.
- $C{H_3}COONa$ is a salt of a weak acid (acetic acid) and strong base NaOH. KOH is a strong base.
Now, we will take a look at all the given options to see whether they are correct.
A) $HN{O_3}{\text{ and KN}}{{\text{O}}_3}$
This is a mixture of a strong acid and its salt with a strong base. So, this cannot act as a buffer solution.
B) KOH and $C{H_3}COONa$
This is a mixture of a strong base and a salt of weak acid and strong base. So, this solution also cannot act as a buffer solution.
C) $HN{O_3}$ and $C{H_3}COONa$
$HN{O_3}$ is a strong acid but the reaction between it and sodium acetate produces acetic acid.
\[HN{O_3} + C{H_3}COONa \to C{H_3}COOH + NaN{O_3}\]
Thus, acetic acid gets produced and sodium acetate is also present in the solution. This makes this solution an acidic buffer.
D) $C{H_3}COOH{\text{ and C}}{{\text{H}}_3}COONa$
This is a mixture of a weak acid and its salt with a strong base. Thus, this makes the solution an acidic buffer.

Thus, we can conclude that options (C) and (D) are correct.

Note: In the case of the mixture of $HN{O_3}$ and $C{H_3}COONa$ , the concentration of nitric acid needs to be less as compared to that of sodium acetate. If the concentration of nitric acid is more, then the solution cannot act as a buffer because the pH becomes very less.