A litre of buffer solution contains 0.1 mole of each of and . On the addition of 0.02 mole of HCl by dissolving gaseous HCl, the pH of the solution is found to be ________ (Nearest integer) [Given: = 4.745 log 2 = 0.301

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Solution

Q.
A litre of buffer solution contains 0.1 mole of each of ${NH}_{3}$ and ${NH}_{4} Cl$ . On the addition of 0.02 mole of HCl by dissolving gaseous HCl, the pH of the solution is found to be ________ $\times 10^{- 3}$ (Nearest integer)

[Given: ${pK}_{b} ({NH}_{3})$ = 4.745

log 2 = 0.301

log 3 = 0.477

T = 298 K] [2023]

Ans.
(9079)

$\begin{matrix} {NH}_{3} & + & {NH}_{4} Cl \\ 0.1 mole & 0.1 mole \\ {NH}_{3} & + & HCl & \to & {NH}_{4} Cl \\ 0.1 mole & 0.02 mole & 0.1 mole \\ 0.08 mole & __ & 0.12 mole \end{matrix}$

$pOH = p K_{b} + \log \frac{[{NH}_{4} Cl]}{[{NH}_{3}]}$

$= 4.745 + \log (\frac{0.12}{0.08})$

$= 4.745 + \log (\frac{3}{2})$

$= 4.745 + [0.48 - 0.30]$

$= 4.745 + 0.18 = 4.921$

$pH = 14 - 4.921$

$= 9079 \times 10^{- 3}$

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