Question

Addition of sodium hydroxide solution to a weak acid (HA) results in a buffer of pH 6. If ionisation constant of HA is $10^{-5}$, the ratio of salt to acid concentration in the buffer solution will be :

• A. 4 : 5
• B. 1 : 10
• C. 10 : 1
• D. 5 : 4

Answer 1

Answer: (C)

$K _{ a }=10^{-5}$
$pK _{ a }=-\log K _{ a }=-\log 10^{-5}=5$
$pH = pKa +\log \frac{[\text{salt} ]}{[ \text{acid} ]}$
$6=5+\log \frac{[ \text{salt} ]}{[ \text{acid} ]}$
$1=\log \frac{[ \text{salt} ]}{[ \text{acid} ]}$
$\frac{[\text{ salt} ]}{[\text{ acid} ]}=\frac{10}{1}$