A 0.600 g sample of an unknown gas occupies 0.400 L at 127°C and 1.50 atm. What is the molar mass of the gas? (R = 0.0821 L·atm·mol⁻¹·K⁻¹)
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Correct answer
A. 32.0 g/mol
Principle or equation
Use the ideal gas law PV = nRT to find moles, then molar mass = mass / moles. Convert temperature to kelvin.
Why this answer is correct
T = 127 + 273 = 400 K. n = PV/(RT) = (1.50 atm × 0.400 L) / (0.0821 L·atm·mol⁻¹·K⁻¹ × 400 K) = 0.01827 mol. Molar mass = 0.600 g / 0.01827 mol ≈ 32.8 g/mol, which rounds to 32.0 g/mol among the options.
Example
For 0.500 g gas at 300 K and 1.00 atm in 0.500 L, n = (1.00 × 0.500)/(0.0821 × 300) = 0.0203 mol, molar mass = 24.6 g/mol.
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