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Acidic Buffer Action

The ionizatio...

Question

The ionization constant of a certain weak is $10^{- 4} .$ What be the [salt] to [acid] ratio if we have to prepare a buffer with pH = 5 using the acid and one of the salts?

1 : 10

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10 : 1

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5 : 4

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4 : 5

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Solution

The correct option is B 10 : 1
Given, $K_{a} = 10^{- 4}, p H = 5$
According to Henderson-Hasselbalch equation,
$p H = p K_{b} + l o g \frac{[s a l t]}{[a c i d]}$
$p H = - l o g K_{b} + l o g \frac{[s a l t]}{[a c i d]}$
$5 = - l o g 10^{- 4} + l o g \frac{[s a l t]}{[a c i d]}$
$5 = 4 + l o g \frac{[s a l t]}{[a c i d]}$
$l o g \frac{[s a l t]}{[a c i d]} = 1$
$\frac{[s a l t]}{[a c i d]} = a n t i l o g 1 = 10$
Hence, the [salt] to [acid] ratio is 10:1

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