Weak acids are partially ionised in aqueous solutions. The ionisation constants of some acids are given…
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}
$4 < 2 < 1 < 3$
$1 < 2 < 3 < 4$
$2 < 3 < 1 < 4$
$4 < 3 < 2 < 1$
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).