Only litre buffer solution was prepared by adding 0.10 mol each of and in deionised water. The change in pH on addition of 0.05 mol of HCl to the above solution is ____ , (Nearest integer) (Given : of = 4.745 and 3 = 0.477)

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Solution

Q.
Only litre buffer solution was prepared by adding 0.10 mol each of ${NH}_{3}$ and ${NH}_{4} Cl$ in deionised water. The change in pH on addition of 0.05 mol of HCl to the above solution is ____ $\times 10^{- 2}$ , (Nearest integer)

(Given : $p K_{b}$ of ${NH}_{3}$ = 4.745 and $\log_{10}$ 3 = 0.477) [2025]

Ans.
(48)

Before adding HCl:

$p O H = p K_{b} + \log \frac{[N H_{4}^{+}]}{[N H_{3}]}$

$p O H = 4.745 + \log \frac{0.1}{0.1} = 4.745$

After adding 0.05 mol HCl:

$\begin{matrix} N H_{3} & + & H C l & \to & N H_{4} C l \\ t=0 (mol) & 0.1 & 0.05 & 0.1 \\ t =∞ (mol) & 0.1 - 0.05 & 0 & 0.1 + 0.05 \\ = 0.05 & = 0.15 \end{matrix}$

$p O H_{new} = p K_{b} + \log \frac{{[N H_{4}^{+}]}_{new}}{{[N H_{3}]}_{_{new}}}$

$p O H_{new} = p K_{b} + \log \frac{0.15}{0.05}$

$p O H_{new} = 4.745 + \log 3 = 4.745 + 0.477$

Change in pH = Change in pOH

$= p O H_{new} - p O H$

$= (4.745 + 0.477) - 4.745 = 0.477 = 47.7 \times 10^{- 2}$

(Nearest integer = 48)

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