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.

top

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

top

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

top