Chemical and Ionic Equilibrium - Result Question 128

####71. How many gram-mole of $HCl$ will be required to prepare one litre of buffer solution (containing $NaCN$ and $HCl$ ) of $pH 8.5$ using $0.01 g$ formula weight of $NaCN$ ?

$$ K _{HCN}=4.1 \times 10^{-10} $$

$(1988,4 M)$

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Solution:

  1. $HCN$ for buffer will be formed by the reaction

$$ NaCN+HCl \longrightarrow NaCl+HCN $$

$mmol$ of $NaCN$ present initially $=\frac{0.01}{49} \times 1000=0.2$

Let $x mmol$ of $HCl$ is added so that $x mmol$ of $NaCN$ will be neutralised forming $x$ mmol of $HCN$.

$$ \begin{aligned} pH & =p K _a+\log \frac{[NaCN]}{[HCN]} \



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