Question

If $p{K}_b$ for $C{N}^{-}$ at $25^{o}C$ is $4.7$ , the $pH$ of $0.5M$ aqueous $NaCN$ solution is

• A. 10
• B. 11.5
• C. 11
• D. 12

Answer 1

Answer: (B)

$p{k}_b$ $ofC{N}^{-}=4.7$
$p{k}_a$ $of$ $HCN=14-4.7=9.3$
$NaCN$ is salt of weak acid $\left(HCN\right)$ and strong base $\left(NaOH\right)$ So
$pH=7+\frac{1}{2}p{k}_a+\frac{1}{2}logC$
$=7+\frac{1}{2}\times 9.3+\frac{1}{2}log\left(0.5\right)$
$\therefore pH=11.5$