Q24 / 90 · JEE Main 2012 (12 May Online) · Chemistry

Given (i) $\operatorname{HCN}(a q)+\mathrm{H}_2 \mathrm{O}(b) \rightleftharpoons \mathrm{H}_3 \mathrm{O}^{+}(a q)+\mathrm{CN}^{-}(a q)$ $K_{\mathrm{a}}=6.2 \times 10^{-10}$ (ii) $\mathrm{CN}^{-}(a q)+\mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{HCN}(a q)+\mathrm{OH}^{-}(a q)$ $K_{\mathrm{b}}=1.6 \times 10^{-5}$. These equilibria show the following order of the relative base strength,

  • A
    $\mathrm{OH}^{-}>\mathrm{H}_2 \mathrm{O}>\mathrm{CN}^{-}$
  • Correct answer
    B
    $\mathrm{OH}^{-}>\mathrm{CN}^{-}>\mathrm{H}_2 \mathrm{O}$
  • C
    $\mathrm{H}_2 \mathrm{O}>\mathrm{CN}^{-}>\mathrm{OH}$
  • D
    $\mathrm{CN}^{-}>\mathrm{H}_2 \mathrm{O}>\mathrm{OH}^{-}$

Solution

The more is the value of equilibrium constant, the more is the completion of reaction or more is the concentration of products i.e. the order of relative strength would be $$ \mathrm{OH}^{-}>\mathrm{CN}^{-}>\mathrm{H}_2 \mathrm{O} $$

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

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