Question

The equivalent mass of phosphoric acid (${ H }_{ 3 }PO_{ 4 }$) is 49. It behaves as __ acid
A- Monobasic
B- Dibasic
C- Tribasic
D- Tetrabasic

Answer 1

We know that the equivalent mass of acid is molecular weight divided by n factor of acid. N-factor of acid is the basicity of acid. We are provided with an equivalent weight of acid and we can calculate the molecular weight of acid. From both of these, we can get the basicity of acid.

Complete step by step answer:
-We know that the equivalent weight of any compound is the weight of a compound that involves one mole of charge transfer in a chemical reaction. For acids equivalent weight of acids is molecular weight divided by the basicity of acid. Given the equivalent weight of phosphoric acid is 49. Let us calculate the molecular weight of phosphoric acid.
-The molecular weight of P is 31.
-The molecular weight of O is 16.
-The molecular weight of H is 1.
-Molecular weight of { H }_{ 3 }PO_{ 4 }\quad =\quad 3\quad \times \quad 1\quad +\quad 31\quad +\quad 4\quad \times \quad 16$$$$$ -Molecular weight of { H }{ 3 }PO{ 4 }\quad =\quad 3\quad +\quad 31\quad +\quad 64$$$$$
-Equivalent weight of ${ H }_{ 3 }PO_{ 4 }\quad =\quad \dfrac { Molecular\quad weight\quad of\quad { H }_{ 3 }PO_{ 4 } }{ Basicity\quad of\quad { H }_{ 3 }PO_{ 4 } }$
$49\quad =\quad \dfrac { 98 }{ Basicity\quad of\quad { H }_{ 3 }PO_{ 4 }\quad }$
$Basicity\quad of\quad { H }_{ 3 }PO_{ 4 }\quad =\quad \dfrac { 98 }{ 49 } \quad =\quad 2$

-Therefore ${ H }_{ 3 }PO_{ 4 }$ is a Dibasic acid. Hence option B is correct.

Note: Equivalent weight will vary from reaction to reaction and it is not a fixed quantity for a compound or molecule. We know that phosphoric acid has 3 ionizable protons so we may think ${ H }_{ 3 }PO_{ 4 }$ as tribasic. To avoid such mistakes calculate the equivalent weight for the given reaction.