Question

The ionisation constant of acetic acid is $2\times {10}^{-5}$. The pH of buffer containing acetic acid and sodium acetate is 4.7. The acid and salt ratio in the buffer is :

• A. one : one
• B. ten : one
• C. one : ten
• D. two : one

Answer 1

Answer: (A)

one : one

The expression for the pH of the acidic buffer solution is as given below.
$pH=p{K}_a+log\frac{\left[salt\right]}{\left[acid\right]}$

$p{K}_a=-log{K}_a=-log(2\times {10}^{-5})=4.7$.

Substitute values in the above equation.
$4.7=4.7+log\frac{\left[salt\right]}{\left[acid\right]}$

$log\frac{\left[salt\right]}{\left[acid\right]}$=0

$\therefore$ $\frac{\left[salt\right]}{\left[acid\right]}$=${10}^0$=1

Hence, $\frac{\left[salt\right]}{\left[acid\right]}=\frac{1}{1}=1:1$.