Question

pH of a solution is $4$ and it does not becomes $5$ when it is diluted $10$ times for:
A) $HCl$
B) $HN{O_3}$
C) $HCl{O_4}$
D) ${H_3}P{O_4}$

Answer 1

As we know that acid strength is defined as the capacity of an acid to dissociate into a proton and an anion and also a weak acid has the capacity to partially dissociate. Dilution actually makes an acidic solution alkaline and vice versa.

Complete solution:
As we know that dilution changes the pH levels of acid as well as bases and we also know that solutions with high concentrations of hydrogen ions possess a low pH whereas the pH is high for solutions having low hydrogen ion concentrations.
We are also aware that hydrochloric acid, nitric acid and perchloric acid are strong acids and possess only one hydrogen ion which can be used for dissociation but the orthophosphoric acid, ${H_3}P{O_4}$ possess three hydrogen ions for dissociation and thus it is considered as a weak acid.
So, we can say that the weak acid will have an increased degree of dissociation on dilution therefore the pH will not change. We also know that strong acids on dilution do not readily dissociate their hydrogen ions and that they are highly acidic and their pH levels can change if diluted.

Therefore, from the above explanation we can say that the correct answer is (D).

Note: Always remember that acidity comes from a strong acid only and acidity is defined as the number of replaceable hydroxide ions from the base in general or during the course of the reaction whereas the basicity is defined as the number of hydrogen ions given by the acids in general or during the course of a reaction.