The question from 2017 USNCO is as below:
What is the $\mathrm{pH}$ of a $\pu{0.25 M}$ solution of $\ce{NaCN}$? (The $\mathrm{p}K_\mathrm{a}$ of $\ce{HCN}$ is $9.21$.)
(A) $4.91$
(B) $8.61$
(C) $11.30$
(D) $13.40$
The answer sheet states (C).
My understandings/reasoning:
- salt hydrolysis: solution of salt derived from strong base and weak acid is basic;
- $\ce{NaCN}$ is derived from the base $\ce{NaOH}$ (strong) and acid $\ce{HCN}$ (weak according to given $\mathrm{p}K_\mathrm{a}$);
- Solution is basic, so (A) is incorrect.
However, I am unable to proceed from here.