JEE Advanced 2022 Paper 1, Chemistry Section 1 Q5: Group 15 Elements: The Nitrogen Family

Molar mass of P${}_4$ = 124 g/mol. Moles = $\frac{1.24}{124}=0.01$.

Step 1 — formation of phosphine:

${\text{P}}_4+3\text{NaOH}+3{\text{H}}_2\text{O}\to {\text{PH}}_3+3{\text{NaH}}_2{\text{PO}}_2$

1 mol P${}_4$ yields 1 mol PH${}_3$. So PH${}_3$ formed = 0.01 mol.

Step 2 — reaction with CuSO${}_4$:

$2{\text{PH}}_3+3{\text{CuSO}}_4\to {\text{Cu}}_3{\text{P}}_2+3{\text{H}}_2{\text{SO}}_4$

CuSO${}_4$ needed = $\frac{3}{2}\times 0.01=0.015$ mol.

Molar mass of CuSO${}_4$ = 63 + 32 + 64 = 159 g/mol.

Mass = $0.015\times 159=2.385\approx 2.38$ g.