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Phosphine gas (PH3) decomposes to give phosphorus vapour and hydrogen gas according to the reaction: 4 PH3(g) → P4(s) + 6 H2(g). If 100 mL of phosphine is completely decomposed, what is the net change in volume of gas?

1. 50 mL decrease
2. 50 mL increase
3. 100 mL decrease
4. 100 mL increase

Correct answer: 50 mL increase

Solution

Reaction: 4 PH3(g) → P4(s) + 6 H2(g). Under conditions where P4 condenses to solid, only the gas volumes matter. From 100 mL PH3 (at constant T and P): H2 produced = (6/4)*100 = 150 mL. PH3 consumed = 100 mL (disappears). Net volume change = 150 - 100 = +50 mL increase.

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