JEE Main · 2023 · 25 Jan 2023 · Shift 1 · Equilibrium

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 1 0^{- 3}

(Nearest integer)
[Given :

pK_{b} (NH_{3}) = 4.745

lo g 2 = 0.301

lo g 3 = 0.477

T = 298 K

]

Answer: 9079

Solution

The litre of buffer starts with

0.1

mol of ammonia and

0.1

mol of ammonium ion. The

0.02

mol of added hydrogen chloride turns an equal amount of ammonia into ammonium, leaving

0.08

mol of ammonia and

0.12

mol of ammonium. For this base buffer

pOH = p K_{b} + lo g \frac{[ N H _{4}^{+} ]}{[ N H _{3} ]} = 4.745 + lo g \frac{0.12}{0.08} = 4.745 + 0.176 = 4.921

. The pH is then

14 - 4.921 = 9.079

, which in the required form is

9079 \times 1 0^{- 3}

JEE Main · 2023 · 25 Jan 2023 · Shift 1 · Integer answer