Q30 / 160 · AP EAMCET 2020 (22 Sep Shift 2) · Chemistry

Weak acids are partially ionised in aqueous solutions. The ionisation constants of some acids are given below. Arrange these acids in increasing order of their acid strength \begin{array}{cll} \hline Index & \multicolumn{1}{c}{ Acid } & \multicolumn{1}{c}{\begin{array}{c} Ionisation \\ constant \end{array}} \\ \hline 1 & Formic acid (\mathrm{HCOOH}) & 1.8 \times 10^{-4} \\ \hline 2 & Hypochlorous acid (\mathrm{HClO}) & 3.0 \times 10^{-8} \\ \hline 3 & Nitrous acid \left(\mathrm{HNO}_2\right) & 4.5 \times 10^{-4} \\ \hline 4 & Hydrocyanic acid (\mathrm{HCN}) & 4.9 \times 10^{-10} \end{array}

  • Correct answer
    A
    $4 < 2 < 1 < 3$
  • B
    $1 < 2 < 3 < 4$
  • C
    $2 < 3 < 1 < 4$
  • D
    $4 < 3 < 2 < 1$

Solution

The relative strengths of acids may be determined by measuring their equilibrium constants in aqueous solutions. Stronger acids ionise to a greater extent. The larger the $K_a$, of an acid, the larger the concentration of $\mathrm{H}_3 \mathrm{O}^{+}$and $A^{-}$ relative to the concentration of the non-ionised acid, $\mathrm{H} A$. Thus, a stronger acid has a larger ionisation constant than does a weaker acid. The ionisation constants increase as the strengths of the acids increase and hence, the correct option is (1).

Hence, the correct answer is option: 'A'.

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