JEE Main Chemistry Class 11 Some Basic Concepts of Chemistry Part 1 Questions

The neutralization reaction is H3PO3+2‌KOH→K2HPO3+2H2O.
Phosphorus acid is diprotic acid as it has two ionizable hydrogens. Thus, 1 mole of phosphorous acid will neutralize 2 moles of KOH.
The number of moles of phosphorus acid present in 20‌mL of 0.1M aqueous solution is 0.1×20×‌

1

1000

=0.002‌mol.
They will neutralize 2×0.002‌mol=0.004 moles of KOH.
The molarity of KOH solution is 0.1M.
The volume of KOH solution required will be ‌

0.004

0.1

=0.04L=40‌ml.