Question

In a mixture of weak acid and its salt, the ratio of concentration of acid to salt is increased ten fold. The pH of the solution is ________.

• A. decreases by 1
• B. decreases by 1/10
• C. increases by 1
• D. increases ten fold

Answer 1

Answer: (A)

decreases by 1

Assume in the original mixture, $\left[salt\right]=\left[acid\right]$.

$pH=p{K}_a+log\frac{\left[salt\right]}{\left[acid\right]}=p{K}_a$

Now, the ratio of concentration of acid to salt is increased ten fold.

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

Hence, $\frac{\left[salt\right]}{\left[acid\right]}=\frac{1}{10}$

$pH=p{K}_a+log\frac{1}{10]}=p{K}_a-1$

Thus pH of solution decreases by 1.