Class XII students were asked to prepare one litre of buffer solution of $\mathrm{pH} \,8.26$ by their Chemistry teacher: The amount of ammonium chloride to be dissolved by the student in $0.2\, \mathrm{M}$ ammonia solution to make one litre of the buffer is :
(Given: $\mathrm{pK}_{\mathrm{b}}\left(\mathrm{NH}_{3}\right)=4.74$
Molar mass of $\mathrm{NH}_{3}=17 \mathrm{~g} \mathrm{~mol}^{-1}$
Molar mass of $\mathrm{NH}_{4} \mathrm{Cl}=53.5 \mathrm{~g} \mathrm{~mol}^{-1}$ )
Solution
For basic Buffer, $$\mathrm{pOH}=\mathrm{pK}_{\mathrm{b}}+\log \frac{[\text { salt }]}{[\text { Base }]}$$
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$\mathrm{pOH}=14-8.26=5.74$
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$5.74=4.74+\log \frac{\left[\mathrm{NH}_{4} \mathrm{Cl}\right]}{0.2}$
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$\left[\mathrm{NH}_{4} \mathrm{Cl}\right]=2 \mathrm{M}$
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Moles of $\mathrm{NH}_{4} \mathrm{Cl}=2 \times 1=2$ moles
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Weight of $\mathrm{NH}_{4} \mathrm{Cl}=2 \times 53.5=107 \mathrm{~g}$
About this question
Subject: Chemistry · Chapter: Equilibrium · Topic: Chemical Equilibrium and Kc, Kp
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