Q37 / 155 · WBJEE 2021 · Chemistry

Aqueous solution of $\mathrm{HNO}_{3}, \mathrm{KOH}, \mathrm{CH}_{3} \mathrm{COOH}$ and $\mathrm{CH}_{3} \mathrm{COONa}$ of identical concentration are provided. The pair (s) of solutions which form a buffer upon mixing is (are)

  • A
    $\mathrm{HNO}_{3}$ and $\mathrm{CH}_{3} \mathrm{COOH}$
  • B
    $\mathrm{KOH}$ and $\mathrm{CH}_{3} \mathrm{COONa}$
  • Correct answer
    C
    $\mathrm{HNO}_{3}$ and $\mathrm{CH}_{3} \mathrm{COONa}$
  • Correct answer
    D
    $\mathrm{CH}_{3} \mathrm{COOH}$ and $\mathrm{CH}_{3} \mathrm{COONa}$

Solution

A Weak acid and its salt with a strong base forms a buffer.$\therefore$ (D) $\mathrm{HNO}_{3}$ being a strong acid, displaces weaker acid $\mathrm{CH}_{3} \mathrm{COOH}$. $\mathrm{HNO}_{3}+\mathrm{CH}_{3}-\mathrm{COO}^{-} \rightleftharpoons \mathrm{CH}_{3}-\mathrm{COOH}+\mathrm{NO}_{3}^{-}$ If $\left[\mathrm{CH}_{3} \mathrm{COO}^{-}\right]>\left[\mathrm{HNO}_{3}\right]$, then solution contains weak acid $\left(\mathrm{CH}_{3} \mathrm{COOH}\right)$ and its conjugate base $\left(\mathrm{CH}_{3} \mathrm{COO}^{-}\right) . \quad \therefore(\mathrm{C})$

Hence, the correct answer is options: 'C', 'D'.

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