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Question: The pH of an aqueous solution of \({\text{NaCl}}\) at \(85^\circ {\text{C}}\) should be: A.7 B.>...

The pH of an aqueous solution of NaCl{\text{NaCl}} at 85∘C85^\circ {\text{C}} should be:
A.7
B.>7
C.<7
D.0

Explanation

Solution

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∘C25^\circ {\text{C}} .
Now, at 25∘C25^\circ {\text{C}} , the value of the ionic product of water is 10−14{10^{ - 14}} . The ionic product of water is Kw = [H + ][OH - ] ⇒10−14 = [H + ][OH - ] \begin{gathered} {{\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{gathered}
At 25∘C25^\circ {\text{C}} , [H + ]=[OH - ]\left[ {{{\text{H}}^{\text{ + }}}} \right] = \left[ {{\text{O}}{{\text{H}}^{\text{ - }}}} \right] .
So, [H + ]=10−7\left[ {{{\text{H}}^{\text{ + }}}} \right] = {10^{ - 7}} . Thus, pH is 7 at 25∘C25^\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∘C25^\circ {\text{C}} .
Therefore, at 85∘C85^\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 7.47.4 . If it changes by 0.20.2 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.