Question

In the reaction, $2NaOH + {H_3}P{O_4} \to N{a_2}HP{O_4} + 2{H_2}O$, the equivalent weight of the acid is:
A.49
B.98
C.32.6
D.None of the above

Answer 1

Hint: We can calculate the equivalent weight of the acid by dividing the molar mass of the acid with the number of hydrogen ions given out by acids.

Complete step by step answer:
We can define equivalent weight as the mass of one equivalent, that is the mass of a given substance that could combine with or displace a fixed quantity of another substance.
When we react with sodium hydroxide phosphoric acid, disodium hydrogen phosphate and water are produced as products. We can write the chemical reaction as,
$2NaOH + {H_3}P{O_4} \to N{a_2}HP{O_4} + 2{H_2}O$
The reaction of sodium hydroxide with phosphoric acid is an acid-base reaction. Phosphoric acid acts as acid here.
We know that in case of acids, the equivalent weight would be equal to the ratio of the molar mass and number of hydrogen ions of the acid donated to the base.
We know that the molar mass of phosphoric acid is $98g/mol$.
The number of hydrogen ions given out by phosphoric acid to disodium hydrogen phosphate is two.
We can write the formula to calculate equivalent weight as,
Equivalent weight$ = \dfrac{{{\text{Molar mass}}}}{{{\text{Number of hydrogen ions donated}}}}$
Let us substitute the values of molar mass of phosphoric acid and the number of hydrogen ions given out.
Equivalent weight$ = \dfrac{{98g/mol}}{2}$
Equivalent weight = $49$
The equivalent weight of the acid is $49$.

Therefore, the option (A) is correct..

Note:
While calculating the equivalent weight from the molar mass and number of replaceable hydrogen atoms, we should not get confused between the basicity of the acid and the number of replaceable hydrogen atoms. Basicity of an acid is the number of hydrogen ions that can be formed by one molecule of acid. The basicity of phosphoric acid is three, but in this case only two hydrogen atoms are accepted by the base to form the product.