Starter for five Write the equation showing the relationship

between density, mass and volume. Write the equation showing the relationship

between density, molar mass and molar volume.

What is the relationship between all gases at the same temperature and pressure?

Starter for five answers

Density = mass ÷ volume

Density = molar mass ÷ molar volume

At the same temperature and pressure all

gases have the same molar volume.

Higher ChemistryUnit 1(e)

Gas densities and molar volumes

After today’s lesson you should be able to:

Use the two relationships learnt in the last lesson to calculate either the molar mass, molar volume or density of a gas given relevant information.

Use the molar mass calculated to identify the unknown gas.

Example 1A 25cm3 sample of a gas weighs 0.0221g and has a molarvolume of 29.4l. Calculate its molar mass.

Pick out information from question:v = 25cm3 = 0.025l, m = 0.0221g and mv = 29.4l

Apply to equations:density = mass ÷ volume = 0.0221 ÷ 0.025 = 0.884gl-1

molar mass = density x molar volume = 0.884 x 29.4 = 26.0g

mass

density volume

molar mass

densitymolar volume

Example 2A molar volume of CO2 at standard temperature and pressure is 22.4l. Calculate the mass of a 50cm3 of the gas under these conditions.

Pick out information from question:

v = 50cm3 = 0.050l and mv = 22.4l

Apply to equations:

density = molar mass ÷ molar volume = 44 ÷ 22.4 = 1.964gl-1

mass = density x volume = 1.964 x 0.05 = 0.098g

mass

density volume

molar mass

densitymolar volume

Example 3The molar mass of a gas is calculated to be 40g. Identify this gas.

First check the molar mass of the monatomic gases on the Periodic Table to see if any have this molar mass.

Argon has a molar mass of 40g therefore this gas is Argon.

If this was not the case you would then check the molar mass of the diatomic gases to see if they match.

Exercise

Complete Q8.11 – 8.25 on p74 - 76 of

‘Test your Higher Chemistry Calculations’.

5 – 10 of these questions will be sufficient.