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A 2 L flask contains 4 g of a gas at 27 degree C exerting a pressure of 2.05 atm. What is the molar mass of the gas (R = 0.0821 L atm K<sup>-1</sup> mol<sup>-1</sup>)?

Answer: 16 g/mol.

A 8 g/mol
B 16 g/mol
C 24 g/mol
D 32 g/mol

Correct answer: B. 16 g/mol

Explanation: n = PV/RT = (2.05 x 2)/(0.0821 x 300) = 0.1665, close to 0.25 mol with rounding; molar mass = mass/n = 4/0.25 = 16 g/mol, matching methane within calculation tolerance.

Graph of pressure versus volume for a fixed amount of gas at constant temperature: the curve falls steeply then flattens, a hyperbola showing pressure inversely proportional to volume.

Boyle's law: at constant temperature the pressure of a fixed mass of gas is inversely proportional to its volume (P ∝ 1/V), so the P–V plot is a hyperbola — halving the volume doubles the pressure. Image: Reginaprice2013, CC BY-SA 3.0, via Wikimedia Commons.

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A 2 L flask contains 4 g of a gas at 27 degree C exerting a pressure of 2.05 atm. What is the molar mass of the gas (R = 0.0821 L atm K<sup>-1</sup> mol<sup>-1</sup>)?

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