Question

What should be the weight of phosphorus acid if $P = 31,{\text{ }}O = 16,{\text{ }}H = 1$
A.$82$
B.$41$
C.$20.6$
D. None of the above

Answer 1

Equivalent weight is the mass of one equivalent i.e. the mass of a given substance that combines with or displaces a fixed amount or quantity of another substance. For acids, the equivalent weight is equal to the ratio of molecular mass of the acid by its basicity whereas, for bases, the equivalent weight is equal to the ratio of molecular mass of base by its acidity.

Complete answer:
Phosphorous acid also known as phosphonic acid with chemical formula ${H_3}P{O_3}$ is a diprotic acid i.e. the acid readily ionizes into two protons.
According to the question for calculating the equivalent weight of phosphorus acid:
Step 1: Calculating the molecular mass:
Given in the question:
Molecular mass of hydrogen $= 1$ gram.
Molecular mass of phosphorus $= 31$ grams.
Molecular mass of oxygen $= 16$ grams.
So, the molecular mass of phosphorus acid $=3 \times$ molecular mass of hydrogen + molecular mass of phosphorus $+ 3 \times$ molecular mass of oxygen
The molecular mass of phosphorus acid $= 3 \times 1 + 31 + 3 \times 16 = 82$.
Step 2: Calculating the basicity:
Basicity of an acid is defined as the number of protons or ${H^ + }$ ions ionized by that acid. As phosphorus acid is diprotic acid i.e. it ionizes two protons only. Hence, the basicity of phosphorus acid is 2. Therefore, n or valency factor for phosphorus acid$= 2.$
Step 3: Calculating the equivalent weight:
$Equivalent{\text{ weight of acid = }}\dfrac{{Molecular{\text{ mass of acid}}}}{{Basicity{\text{ of the acid (n)}}}}$
So, equivalent weight of phosphorus acid$\Rightarrow \dfrac{{84}}{2} = 41$ grams.
Therefore, the equivalent weight of ${H_3}P{O_3} = 41$ grams.
So, the correct answer is option B.

Note:
Basicity is defined as the number of ${H^ + }$ ions or protons released or ionized by that acid whereas, acidity is defined as the number of $O{H^ - }$ or hydroxide ions are released or ionized by that base. But, for acid-base reactions, the equivalent weight of acid or base is that mass that reacts with one mole of hydrogen cation.