Chemistry / Unit 7 Chem 1262 worked example, 60 to 90 seconds
Determining Molar Mass from Measured Gas Properties
The idea
Determining an unknown gas's molar mass means measuring a sample's mass, pressure, volume, and temperature, then substituting into the rearranged ideal gas law to solve for molar mass directly.
In practice
A 0.494-g sample of vaporized chloroform fills a 129 cm³ flask at 99.6 °C and 742.1 mm Hg. Converting volume to 0.129 L, temperature to 372.75 K, and pressure to 0.9764 atm, then solving molar mass = (mass × R × temperature) / (pressure × volume), gives a molar mass near 119 g/mol, matching chloroform's known formula mass of 119.4 g/mol.
Where people slip
Forgetting to convert the flask's Celsius temperature and mm Hg pressure into kelvin and atmospheres before substituting produces a molar mass off by a large, unit-driven factor rather than a small rounding error near the true value.
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