IB Chemistry home > Syllabus 2016 > Practical Scheme of Work > The molar mass of a gas

The ideal gas law gives the relationship between the number of moles of a gas and the pressure, volume and temperature.

PV = nRT

This can be used to find the moles of volatile substance and, if the mass is known, the relative molecular mass of the substance.

The gas syringe

A gas syringe is a piece of equipment that can be used in an oven to measure the volume of a volatile substance at different temperatures.


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Procedure

Part 1 - preparing the syringe

Accurately weigh the gas syringe

Introduce a small portion of the liquid through the rubber septum using a microsyringe and needle

Reweigh the gas syringe and compound

Part 2 - the oven

Place the gas syringe in a suitably calibrated oven and allow to reach the desired temperature

Read the volume obtained on the syringe.

Repeat the procedure to reduce random error


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Recording the data

Ensure that all of the data is recorded with suitable units and inaccuracies.

The following shows example raw data and analysis.

Raw data

Mass of gas syringe: 58.63 g ± 0.01

Mass of gas syringe + compound: 58.94 g ± 0.01

Temperature of oven: 373K ± 0.5

Pressure: 100.5 kPa ± 0.5

Volume obtained: 57.0 ± 0.5 cm3 = 0.057 dm3

Data analysis

Mass of compound used = 58.94 - 58.63 = 0.31 g ± 0.02

Using the ideal gas equation: PV = nRT therefore n = PV/RT

number of moles = (100.5 x 0.057) / (8.314 x 373) = 5.7285/3101.122 = 0.001847 mol

As Mr = mass/mol = 0.31 / 0.001847 = 167.8

Treatment of errors and inaccuracies

Take a percentage error of each of the absolute inaccuracies and sum them:

Gas volume inaccuracy = 100 x 0.5/57 = 0.88%

Temperature: 100 x 0.5/373 = 0.13%

Pressure: 100 x 0.5/100.5 = 0.5%

Mass: 100 x 0.02/0.31 = 6.5%

Accumulated percentage error = 8.01%

This value can then be applied to the answer obtained

Relative mass of gas = 167.8 ± 13.4


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