Question

The $pH$ of aqueous solution of ammonia is $11.5$. Find molarity of the solution.

[Given that : $K_{b\left({NH}_{4}OH\right)}=1.8\times {10}^{-5}$]

• A. $0.556M$
• B. $0.66M$
• C. $5.6M$
• D. $6.6M$

Answer 1

The correct option is A $0.556M$

Given

$pH=11.5$,

$\left[H^{+}\right]={10}^{-11.5}M$

$\left[{OH}^{-}\right]=\frac{K_w}{\left[H^{+}\right]}=\frac{{10}^{-14}}{{10}^{-11.5}}={10}^{-2.5}M$

$NHOH\stackrel{K_b}{\rightleftharpoons }\underset{{10}^{-2.5}}{{{NH}_4}^{+}}+\underset{{10}^{-2.5}}{{OH}^{-}}$

$K_b=\frac{\left[N{H}_4^{+}\right]\times \left[O{H}^{-}\right]}{\left[N{H}_{4}OH\right]}$

$\frac{{(10}^{-2.5}{)}^2}{C}=1.8\times {10}^{-5}$

$\therefore C=\frac{1}{1.8}=0.556M$