UPSC NDA II 2018 General Ability Solved Paper

Concept:An acid's basicity equals the number of replaceable hydrogen atoms per molecule.Explanation:Phosphoric acid ($\mathrm{H}_3\mathrm{PO}_4$) has the structure $\mathrm{O}=\mathrm{P}(\mathrm{OH})_3$.Here, three hydrogen atoms are bonded to oxygen atoms, making them acidic and replaceable.Thus, $\mathrm{H}_3\mathrm{PO}_4$ can donate three protons, so it is a tribasic acid.In contrast, hydrochloric acid ($\mathrm{HCl}$) and nitric acid ($\mathrm{HNO}_3$) each have only one acidic hydrogen — they are monobasic.Sulphuric acid ($\mathrm{H}_2\mathrm{SO}_4$) has two acidic hydrogens, so it is dibasic.Answer:Phosphoric acid ($\mathrm{H}_3\mathrm{PO}_4$).