First slide

Acidic buffer

Question

An acid-base indicator which is a weak acid has a $p K_{I n}$ value = 5.45. At what concentration ratio of sodium acetate to acetic acid would the indicator show a colour half-way between those of its acid and conjugate base forms ?
$[P^{K_{a}} of acetic acid = 4.75]$

Moderate

A

4 : 1
B

6 : 1
C

5 : 1
D

3 : 1

Solution

$P^{H} = P^{K_{a}} + \log \frac{[Salt form]}{[acidform]}$
$\to P^{H} = P^{K_{In}} = 5.45$
For a Buffer solution
$pH = {pK}_{a} + \log \frac{[{CH}_{3} COONa]}{[{CH}_{3} COOH]}$
$\Rightarrow 5.45 = 4.75 + \log \frac{[{CH}_{3} COONa]}{[{CH}_{3} COOH]}$
$0.7 = \log \frac{[{CH}_{3} COONa]}{[{CH}_{3} COOH]}$ ; Antilog 0.7= 5
$\Rightarrow \frac{5}{1} = \log \frac{[{CH}_{3} COONa]}{[{CH}_{3} COOH]}$

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