Question

What is the approximate value of $pH$ of a $0.2M$ solution of ammonia? Given, $K_b=1.76\times {10}^{-5}$.

• A. $11.28$
• B. $9.82$
• C. $8.56$
• D. $13.47$

Answer 1

The correct option is A $11.28$

We know that for weak bases,
$pOH=\frac{1}{2}(p{K}_b-logC)$
and $pH+pOH=14$
so,
$pH=14-\frac{1}{2}(p{K}_b-logC)$
$\Rightarrow pH=14-\frac{1}{2}(-log{K}_b-logC)$
putting the values,
$\Rightarrow pH=14-\frac{1}{2}[-log(1.76\times {10}^{-5})-log(0.2\left)\right]$
$\Rightarrow pH=14-\frac{1}{2}(4.75+0.69)$
$\Rightarrow pH=14-\frac{1}{2}\left(5.44\right)$
$\Rightarrow pH=14-2.72=11.28$