Chemical and Ionic Equilibrium 2 Question 61
61. What is the $\mathrm{pH}$ of a $0.50 \mathrm{M}$ aqueous $\mathrm{NaCN}$ solution?
( $\mathrm{p} K_{b}$ of $\mathrm{CN}^{-}=4.70$ ).
(1996, 2M)
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Solution:
- $\mathrm{CN}^{-}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{HCN}+\mathrm{OH}^{-}$
$$ K_{h}=2 \times 10^{-5} $$
$$ \left[\mathrm{OH}^{-}\right]=\sqrt{K_{h} C}=\sqrt{2 \times 10^{-5} \times 0.5}=\sqrt{10^{-5}} $$
$$ \mathrm{pOH}=2.5 \text { and } \mathrm{pH}=11.5 $$
