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Finding the Relative Molecular Mass of a Gas

You can find the relative molecular mass of a gas by finding the mass of one mole of gas.The ideal gas equation can be rearranged to find the number of moles in a given mass of gas at a particular temperature and pressure and occupying a particular volume. The pressure is converted to 100,000Pa, the volume is converted to 100×10-6m3 and the temperature is converted to 293K by adding 273.
For example, if 0.084 moles of gas has a mass of 4.58g then 1 mole has a mass of (4.58/0.084) 54.5gSo the Mr is 54.5.

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Specification: AQA Chemistry (7404), AS-level, 3.1.2.1 Relative atomic mass and relative molecular mass. Topic: Amount of Substance.

Key terms in this lesson

Relative molecular mass
The weighted mean mass of a molecule compared to 1/12th the mass of a carbon-12 atom.
Mole
The amount of substance that contains as many particles as there are atoms in exactly 12g of carbon-12.
Ideal gas equation
The equation pV = nRT relating the pressure, volume, number of moles and temperature of an ideal gas.

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