JEE Advanced 2019 Paper 2 · Chemistry · s-Block, p-Block Elements and Hydrogen · easy

The correct statement(s) about the oxoacids of phosphorus is(are)

One or more options correct

A. $\mathrm{H_3PO_3}$ is dibasic and acts as a reducing agent because of its P–H bond
B. $\mathrm{H_3PO_2}$ is dibasic because it contains two P–H bonds
C. On heating, $\mathrm{H_3PO_3}$ disproportionates to $\mathrm{H_3PO_4}$ and $\mathrm{PH_3}$ in the mole ratio $3:1$
D. $\mathrm{H_3PO_4}$ reduces $\mathrm{AgNO_3}$ to metallic silver because phosphorus can increase its oxidation state

Show answer and solution

Answer: A, C

\mathrm{H_3PO_3}

[

\mathrm{HPO(OH)_2}

] only the two P–OH hydrogens are ionisable, so it is dibasic, and the P–H bond (P in

+3

) makes it a reducing acid; (A) is correct. In

\mathrm{H_3PO_2}

[

\mathrm{H_2PO(OH)}

] there are two P–H bonds but only one P–OH, so it is monobasic; the P–H hydrogens do not ionise and (B) is wrong. Heating

\mathrm{H_3PO_3}

gives

4\mathrm{H_3PO_3}\to3\mathrm{H_3PO_4}+\mathrm{PH_3}

(P:

4=3+1

, H:

12=9+3

, O:

12=12

), a

3:1

ratio of acid to phosphine; (C) is correct. In

\mathrm{H_3PO_4}

phosphorus is already in its highest oxidation state

+5

, so it cannot reduce

\mathrm{Ag^+}

; (D) is wrong.

Original question written in the JEE Advanced 2019 Paper 2 pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake