Question

A buffer solution is prepared bymixing $50mL$ of $0.5$ molar ammonia solution, $40mL$ of $0.4$ molar ${NH}_{4}Cl$ solution and $10mL$ of distilled water. Calculate the pH of the buffer.
$K_b\left({NH}_3\right)=1.8\times {10}^{-8}$ at $298K$.

Answer 1

Total volume $=50+40+10=100mL$
Molarity of ${NH}_3=\frac{0.5\times 50}{100}=0.25M$
Molarity of ${NH}_{4}Cl=\frac{0.4\times 40}{100}=0.16M$

For basic buffer:

$p{OH}^{}=p{K}_b+\log \frac{\left[Salt\right]}{\left[Base\right]}$

$pOH=-log\left(K_b\right)+\log \frac{\left[Salt\right]}{\left[Base\right]}$

$pOH=-log(1.8\times {10}^{-8})+\log \frac{\left[0.16\right]}{\left[0.25\right]}$=$9.45$