Question

The pH of an aqueous solution of $\text{NaCl}$ at ${85}^{\circ }\text{C}$ should be:
A.7
B.>7
C.<7
D.0

Answer 1

Hint: pH can be measured on the pH scale from 0(highly acidic) to 14(highly basic). Higher the hydrogen ion concentration of a substance, lower is the pH value.
For a neutral solution, pH value is 7. If the pH value is less than 7, it represents an acidic solution and if it is higher than 7, it represents a basic solution.

Complete step by step answer:
Sodium chloride is a salt of a strong acid which is hydrochloric acid and a strong base which is sodium hydroxide and is neutral in nature. The hydrolysis of a salt of a strong acid and a strong base is not possible.
Since the salt is neutral in nature, its pH value should be 7. But, the temperature is more than ${25}^{\circ }\text{C}$ .
Now, at ${25}^{\circ }\text{C}$ , the value of the ionic product of water is ${10}^{-14}$ . The ionic product of water is $\begin{array}{c}{\text{K}}_{\text{w}}\text{ = }\left[{\text{H}}^{\text{ + }}\right]\left[\text{O}{\text{H}}^{\text{ - }}\right]\\ \Rightarrow {10}^{-14}\text{ = }\left[{\text{H}}^{\text{ + }}\right]\left[\text{O}{\text{H}}^{\text{ - }}\right]\end{array}$
At ${25}^{\circ }\text{C}$ , $\left[{\text{H}}^{\text{ + }}\right]=\left[\text{O}{\text{H}}^{\text{ - }}\right]$ .
So, $\left[{\text{H}}^{\text{ + }}\right]={10}^{-7}$ . Thus, pH is 7 at ${25}^{\circ }\text{C}$ .
Since the degree of ionization is proportional to temperature, the value of the ionic product of water increases with increase in temperature. So, the ionic product will increase at a temperature more than ${25}^{\circ }\text{C}$ .
Therefore, at ${85}^{\circ }\text{C}$ , the ionic product will increase and so the hydrogen ion concentration will increase and thus, the pH will become less than 7.

So, the correct option is C.

Note:
pH has a great importance in biochemical reactions such as digestion of food. Human blood has a pH of $7.4$ . If it changes by $0.2$ pH units, then death results.
pH also has a great importance in agriculture. Soil is tested to determine whether acidic or basic fertilizer is required for a particular crop to be grown in that soil.