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Molality

Calculate the...

Question

Calculate the $p H$ of $0.01$ M solution of $N H_{4} C N$ . The dissociation constants $K_{a}$ for $H C N = 6.2 \times 10^{- 10}$ and $K_{b}$ for $N H_{3} = 1.6 \times 10^{- 5}$ .

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Solution

$p H = 7 + \frac{1}{2 (p k a - p k b)}$
$p k a = - l o g k a$
$\Rightarrow - l o g (6.2 \times 10^{- 10})$
$\Rightarrow 10 - 0.79 = 9.21$
$p k b = - l o g k b$
$\Rightarrow - l o g (1.6 \times 10^{- 5})$
$\Rightarrow 5 - 0.20 = 4.8$
$7 + \frac{1}{2} [9.21 - 4.8]$
$7 + 1 / 2 [4.41] = 9.21$ .

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