Question

Calculate change in pH of 1 litre buffer solution. Containing 0.10 mole each of $N{H}_3$ and $N{H}_{4}Cl$ Cl upon adding of (a) 0.02 mole of dissolved gaseous HCl, Assuming volume to be constant,
$K_b\left[N{H}_3\right]=1.8\times {10}^{-5}$

• A. 0.167 unit
• B. 0.176 unit
• C. 0.175 unit
• D. 0.177 unit

Answer 1

The correct option is B 0.176 unit

$pOHofN{H}_{3}andN{H}_{4}Clbuffer=-log{K}_b+log\frac{\left[Salt\right]}{\left[base\right]}$
$=-log1.8\times {10}^{-5}+log\frac{0.1}{0.1}=4.75$
$pH=(14-4.75)=9.25$
$N{H}_3+HCl\to N{H}_{4}Cl$
$\left[Salt\right]=(0.1-0.02)=0.12M$
$\left[Base\right]=(0.1-0.02)=0.08M$
$pOH=-log{K}_b+log\frac{0.12}{0.08}=4.75+0.176=4.926$
$pH=(14-9.926)=9.074$
$\Delta pH=(9.25-9.074)=0.176unit.$