Question

Find the pH of an aqueous solution of 1.0 M ammonium formate assuming complete dissociation. $({\text{pK}}_a{\text{of formic acid = 3.8 and pK}}_b\text{of ammonium hydroxide}=\mathrm{4.8.})$

• A. 7
• B. 5.3
• C. 6.5
• D. 8.1

Answer 1

The correct option is C 6.5

Ammonium formate is a salt of weak acid and weak base.

$HCOON{H}_4+HOH\leftrightharpoons HCOOH+N{H}_{4}OH$
$HCOOH\leftrightharpoons HCO{O}^{-}+H^{+}$
$N{H}_{4}OH\leftrightharpoons N{H}_4^{+}+O{H}^{-}$

For the hydrolysis of salt of weak acid and weak base, the pH of the solution is given by

$\text{pH}=\frac{1}{2}[{\text{log}}_{10}{\text{K}}_{\text{b}}-{\text{log}}_{10}{\text{K}}_{\text{a}}-{\text{log K}}_{\text{w}}]$

$=\frac{1}{2}[{\text{pK}}_{\text{a}}+{\text{pK}}_{\text{w}}-{\text{pK}}_{\text{b}}]$

$=\frac{1}{2}[3.8+14-4.8]$

= 6.5