A 0.250 g sample of an unknown gas occupies 0.200 L at 27°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. 20.5 g/mol
Principle or equation
Ideal gas law: PV = nRT. Molar mass M = mass / n. Convert temperature to kelvin.
Why this answer is correct
T = 27 + 273 = 300 K. n = PV/(RT) = (1.50 atm × 0.200 L) / (0.0821 L·atm·mol⁻¹·K⁻¹ × 300 K) = 0.01218 mol. M = 0.250 g / 0.01218 mol = 20.5 g/mol.
Example
If 0.500 g of gas occupies 0.500 L at 1.00 atm and 273 K, n = 0.0223 mol, M = 22.4 g/mol.
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