Question

Aqueous solution of $HN{{O}_{3}}$, $KOH$, $C{{H}_{3}}COOH$, $C{{H}_{3}}COONa$ of identical concentrations are provided. The pair(s) of solution which forms a buffer upon mixing are:
A. $HN{{O}_{3}}$ and $C{{H}_{3}}COOH$
B. $KOH$ $C{{H}_{3}}COONa$
C. $HN{{O}_{3}}$ and $C{{H}_{3}}COONa$
D. $C{{H}_{3}}COOH$ and $C{{H}_{3}}COONa$

Answer 1

Hint: As we know, the buffer solution is a mixture of a weak acid and a conjugate base and vice versa. A weak acid is one which doesn’t ionize fully when it is dissolved in water whereas a conjugate base is defined as in acid if the acidic part or ${{H}^{+}}ion$ is removed and the remaining part of the acid molecule is called conjugate base.

Complete step by step answer:
Buffer solution used as keeping $pH$ of solution at nearly constant by adding a little amount of strong acid or base.
When there is addition of some strong acid to an equilibrium solution (mixture of weak acid and its conjugate base) ${{H}^{+}}ions$ are added then we can observe shift in equilibrium towards left in accordance with Le Chatelier’s Principle
$HA{{H}^{+}}+{{A}^{-}}$
Likewise, when there is addition of some strong alkali to the mixture then ${{H}^{+}}ions$ concentration decreases.
$O{{H}^{-}}+HA{{H}_{2}}O+{{A}^{-}}$

As we know, there are three types of buffer solution which are as follows:
i) Acid Buffer: weak acid + its salt of strong base
ii) Basic Buffer: weak base + its salt of strong acid
iii) Neutral Buffer: Salt of weak acid and weak base
Now let us see which option can form buffer solution after mixing:
Option A: In option A it has a mixture of strong acid $HN{{O}_{3}}$and weak acid $C{{H}_{3}}COOH$. Therefore, it is not a buffer solution.
Option B: In option B it has a mixture of strong base $KOH$and salt of weak acid and strong base $C{{H}_{3}}COONa$. Therefore, it will also not give a buffer solution.
Option C: In option C, we can see here it has mixture of strong acid $HN{{O}_{3}}$ and $C{{H}_{3}}COONa$ its salt with a strong base. As we know, nitric acid is a strong acid, and buffers are made up of only weak acids and bases. Therefore, it will also not give a buffer solution.
$HN{{O}_{3}}+C{{H}_{3}}COONa\to C{{H}_{3}}COOH+NaN{{O}_{3}}$

Option D: As we already know that mixture of $C{{H}_{3}}COOH$ and $C{{H}_{3}}COONa$ is a buffer solution and it can only form when $C{{H}_{3}}COONa$ remains unreacted.
Hence, it is concluded that the correct option is only (D).

Note: Remember, don’t confuse a relationship between the strength of an acid (or base) and the strength of its conjugate (or conjugate acid). The stronger the acid, the weaker its conjugate base. The weaker acid, the stronger its conjugate base. The stronger the base, the weaker its conjugate acid.