Question

Equivalent weight of an acid
(A) depends on the reaction involved with the base.
(B) Depends on the number of oxygen atoms present.
(C) is always constant.
(D) None.

Answer 1

Equivalent weight of an acid is the ratio of its molecular weight to its basicity. Basicity refers to the number of acidic protons present in the given molecule.
${\text{Equivalent weight = }}\dfrac{{{\text{Molecular weight }}}}{{{\text{ Basicity }}}}$

Complete step by step answer:
For an acid, the equivalent weight is the ratio of the molecular weight to its basicity. Basicity is the number of ionizable hydrogen atoms present in the acid molecule.

${\text{Equivalent weight = }}\dfrac{{{\text{Molecular weight }}}}{{{\text{ Basicity }}}}$

Consider three different acids, hydrochloric acid $\left( {{\text{HCl}}} \right)$ , sulphuric acid $\left( {{{\text{H}}_2}{\text{S}}{{\text{O}}_4}} \right)$ and phosphoric acid $\left( {{{\text{H}}_3}{\text{P}}{{\text{O}}_4}} \right)$. These acids have different basicity.
Hydrochloric acid is monobasic acid. It means that one molecule of hydrochloric acid contains one ionisable proton and reacts with one molecule of sodium hydroxide (a base) to form salt and water. Thus hydrochloric acid has basicity 1. So the equivalent weight of hydrochloric acid is equal to its molecular weight.
Sulphuric acid is dibasic acid. It means that one molecule of sulphuric acid contains two ionisable protons and reacts with two molecules of sodium hydroxide (a base) to form salt and water. Thus sulphuric acid has basicity 2. So the equivalent weight of sulphuric acid is equal to one half of its molecular weight.
Phosphoric acid is tribasic acid. It means that one molecule of phosphoric acid contains three ionisable protons and reacts with three molecules of sodium hydroxide (a base) to form salt and water. Thus phosphoric acid has basicity 3. So the equivalent weight of phosphoric acid is equal to one third of its molecular weight.

{{\text{H}}_2}{\text{S}}{{\text{O}}_4}{\text{ + 2 NaOH }} \to {\text{ N}}{{\text{a}}_2}{\text{S}}{{\text{O}}_4}{\text{ + 2 }}{{\text{H}}_2}{\text{O}} \\\ {{\text{H}}_3}{\text{P}}{{\text{O}}_4}{\text{ + 3 NaOH }} \to {\text{ N}}{{\text{a}}_3}{\text{P}}{{\text{O}}_4}{\text{ + 3 }}{{\text{H}}_2}{\text{O}} \\\\$$ Equivalent weight of an acid depends on the reaction involved with the base. The equivalent weight of an acid does not depend on the number of oxygen atoms present. Consider the oxyacids of chlorine $${\text{HClO, HCl}}{{\text{O}}_2}{\text{, HCl}}{{\text{O}}_3}{\text{ and HCl}}{{\text{O}}_4}$$ . They contain different numbers of oxygen atoms. They contain one ionizable hydrogen atom. So their equal weight is equal to their molecular weight. _**Hence, the option (A) is the correct option.**_ **Note:** Equivalent weight of acid is not a constant. Thus, carbonic acid may react with one molecule of sodium hydroxide to form sodium hydrogen carbonate and water. In this reaction, the equivalent weight of carbonic acid is equal to its molecular weight. Carbonic acid may react with two molecules of sodium hydroxide to form sodium carbonate and water. In this reaction, the equivalent weight of carbonic acid is equal to one half its molecular weight.