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  • Class XII students were asked to prepare one litre of buffer solution of <img alt="\mathrm{\mathrm{pH}\; 8.26 \mathrm\; {by}}" src="/latex-image/?%5Cmathrm%7B%5Cmathrm%7BpH%7D%5C%3B%208.26%20%5Cmat

Class XII students were asked to prepare one litre of buffer solution of \mathrm{\mathrm{pH}\; 8.26 \mathrm\; {by}} their Chemistry teacher. The amount of ammonium chloride to be dissolved by the student in \mathrm{0.2 \mathrm\; {M}} ammonia solution to make one litre of the buffer is

\mathrm{(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})}

Option: 1

\mathrm{53.5 \mathrm{~g}}


Option: 2

\mathrm{72.3 \mathrm{~g}}


Option: 3

\mathrm{107.0 \mathrm{~g}}


Option: 4

\mathrm{126.0 \mathrm{~g}}


Answers (1)

best_answer

Need to prepare one litre of butter solutions of pH 8.26

So, Volume (V) = 11

and pH = 8.26

\Rightarrow \mathrm{POH=14-8.26=5.74}

and we know the formula,

\mathrm{P O H=p^{K_b}+\log \frac{[\text { Salt }]}{[\text { Base }]}}

\mathrm{\mathrm{POH}=\mathrm{P^{K_{b}}}+\log \frac{\left[\mathrm{NH}_4^{+}\right]}{\left[\mathrm{NH}_3\right]}}

So,

\mathrm{\begin{aligned} 5.74 &=4.74+\log \frac{\left[\mathrm{NH}_4^{+}\right]}{\left[\mathrm{NH}_3\right]} \\ 1 &=\log \frac{\left[\mathrm{NH}_4^{+}\right]}{0.2} \end{aligned}}

\mathrm{\begin{aligned} \log\; 10 &=\log \frac{\left[\mathrm{NH}_4^{+}\right]}{0.2} \\ 10 &=\frac{\left[\mathrm{NH}_4^{+}\right]}{0.2} \end{aligned}}

\left [ \mathrm{NH}_4^{+} \right ]=2

We know concentration \mathrm{=\frac{mole}{v}=\frac{\omega }{M.V}}

\begin{aligned} &\Rightarrow \quad\left[\mathrm{NH}_4^+\right]=\frac{\omega}{53.5 \times 1}\\ &\Rightarrow \quad 2 = \frac{\omega}{53.5}\\ &\Rightarrow \quad \omega=107.0 \mathrm{~g} \end{aligned}

Hence, Option (3) is correct.

Posted by

Suraj Bhandari

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