Worksheet: Ideal Gas Molar Mass

In this worksheet, we will practice using the ideal gas law to calculate the molar mass of an ideal gas from its density, temperature, and pressure.


At a temperature of 500C and a pressure of 0.932 bar, sulfur vapor has a density of 3.71 g/dm3. What molecular formula for sulfur is compatible with this set of conditions?

  • A S 8
  • B S 2
  • C S 3
  • D ( S ) 2 3


The density of a certain gaseous fluoride of phosphorus is 3.88 g/L standard temperature and pressure (0.0C and 1.00 bar).

What is the molar mass of this fluoride?

What is the molecular formula of this fluoride?

  • A P F 2
  • B P F 3
  • C P F 5
  • D P F 4
  • E P F


What is the molar mass of an ideal gas if 0.281 g of the gas occupies a volume of 125 mL at a temperature of 126C and a pressure of 777 torr?


The approximate molar mass of a volatile liquid can be determined by:

  1. Heating a sample of the liquid in a flask, with a tiny hole at the top, which converts the liquid into gas that may escape through the hole.
  2. Removing the flask from heat at the instant when the last bit of liquid becomes gas, at which time the flask will be filled with only gaseous sample at ambient pressure.
  3. Sealing the flask and permitting the gaseous sample to condense to liquid, and then weighing the flask to determine the sample’s mass.

What of the following assumptions is not necessary in order to calculate a good estimate of the molar mass from the mass of the vapor?

  • AThat the vapor is an ideal gas
  • BThat the vapor pressure of the substance at room temperature is not significant
  • CThat the liquid has a high melting point
  • DThat air has been completely displaced from the flask, at the point where the liquid has fully evaporated

Using this procedure a sample of chloroform gas weighing 0.494 g is collected in a flask with a volume of 129 cm3 at 99.6C when the atmospheric pressure is 742.1 mmHg. What is the approximate molar mass of chloroform?

  • A 26 g/mol
  • B 56 g/mol
  • C 120 g/mol
  • D 104 g/mol
  • E 132 g/mol

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