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A triatomic volatile substance X3 has a molecular mass such that 48 mg of it produces a vapour that displaces 11.2 mL of air at 1 atm and 273 K. Find the atomic weight of element X.

1. 96
2. 48
3. 32

Correct answer: 32

Solution

At STP, 11.2 mL of gas corresponds to 11.2/22400 = 5 * 10⁻⁴ mol. Molar mass of X3 = 48 mg / (5*10⁻⁴ mol) = 96 g/mol. Atomic weight of X = 96/3 = 32.

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