Class XII students were asked to prepare one litre of buffer solution of pH \,8.26 by their Chemistry teacher: The amount of ammonium chloride to be dissolved by the student in 0.2\, M ammonia solution to make one litre of the buffer is : (Given: pK_b(NH_3)=4.74 Molar mass of NH_3=17 ~g ~mol^-1 Molar mass of NH_4 Cl=53.5 ~g ~mol^-1 )

Ionic Equilibrium PYQ — JEE Main 2022 · 26 Jul · Shift 2 · Q3 with Solution

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Correct answer: C

Identify the buffer system

The buffer is made from weak base $\mathrm{NH_3}$ and its salt $\mathrm{NH_4Cl}$ .

For a basic buffer: $\mathrm{pOH} = \mathrm{p}K_b + \log \frac{[\text{salt}]}{[\text{base}]}$
Calculate pOH from given pH

Given: $\mathrm{pH} = 8.26$ So, $\mathrm{pOH} = 14 - 8.26 = 5.74$
Use buffer equation

Given: $\mathrm{p}K_b(\mathrm{NH_3}) = 4.74$

Therefore, $5.74 = 4.74 + \log \frac{[\mathrm{NH_4Cl}]}{[\mathrm{NH_3}] }$

$\log \frac{[\mathrm{NH_4Cl}]}{[\mathrm{NH_3}]} = 1$

$\frac{[\mathrm{NH_4Cl}]}{[\mathrm{NH_3}]} = 10$
Find concentration of ammonium chloride needed

The ammonia solution is $0.2\,\mathrm{M}$ .

So, $[\mathrm{NH_4Cl}] = 10 \times 0.2 = 2.0\,\mathrm{M}$

Since total volume is $1\,\mathrm{L}$ , moles of $\mathrm{NH_4Cl}$ required are: $n = 2.0 \times 1 = 2.0\,\mathrm{mol}$
Convert moles to mass

Molar mass of $\mathrm{NH_4Cl}$ is $53.5\,\mathrm{g\,mol^{-1}}$ .

Hence, $\text{mass} = 2.0 \times 53.5 = 107.0\,\mathrm{g}$
Match with options

$107.0\,\mathrm{g}$ corresponds to Option C.

C: $107.0\,\mathrm{g}$