JEE Advanced 2022 Paper 1 · Q05 · p-Block Elements (Groups 15-18)
Dissolving 1.24 g of white phosphorous in boiling NaOH solution in an inert atmosphere gives a gas Q. The amount of CuSO$_4$ (in g) required to completely consume the gas Q is _______. [Given: Atomic mass of H = 1, O = 16, Na = 23, P = 31, S = 32, Cu = 63]
Reveal answer + step-by-step solution
Correct answer:2.38
Solution
Molar mass of P$_4$ = 4 $\times$ 31 = 124 g mol$^{-1}$. Moles of P$_4$ = 1.24/124 = 0.01 mol.
White phosphorous reacts with hot NaOH in an inert atmosphere (the classic phosphine preparation): P$_4$ + 3 NaOH + 3 H$_2$O $\rightarrow$ 3 NaH$_2$PO$_2$ + PH$_3$$\uparrow$. Moles of PH$_3$ (gas Q) = 0.01 mol.
Phosphine reacts with CuSO$_4$ in solution (a common qualitative test): 2 PH$_3$ + 3 CuSO$_4$ $\rightarrow$ Cu$_3$P$_2$$\downarrow$ + 3 H$_2$SO$_4$.
Moles of CuSO$_4$ needed = (3/2) $\times$ 0.01 = 0.015 mol.
Molar mass CuSO$_4$ = 63 + 32 + 64 = 159 g mol$^{-1}$.
Mass of CuSO$_4$ = 0.015 $\times$ 159 = 2.385 $\approx$ 2.38 g.
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