Question

What is the pH of a 0.5 M aqueous NaCN solution ? ${\text{pK}}_{\text{b}}\text{of}{\text{CN}}^{-}$ is 4.70.

• A. 5.5
• B. 14
• C. 11.5
• D. 7

Answer 1

The correct option is C 11.5

For salt of weak acid and strong base, pH is given by $\text{pH}=\frac{1}{2}{\text{pK}}_{\text{W}}+\frac{1}{2}{\text{pK}}_{\text{a}}+\frac{1}{2}\text{log}\text{C}$

${\text{pK}}_{\text{a}}+{\text{pK}}_b=14$

${\text{pK}}_{\text{a}}+4.7=14$
${\text{pK}}_{\text{a}}=9.3$
$K_w={10}^{-14}$
$\text{pH}=\frac{1}{2}\times 14+\frac{1}{2}\times 9.3+\frac{1}{2}\times {\text{log}}_{10}0.5$

$=7+4.65-0.15$

= 11.5