Starter for five

What is meant by the ‘density’ of a substance?

What units are used when measuring the density of a substance?

What state will a substance be in if it has a low density? Why?

Answers

The mass per volume e.g. the mass of 1cm3 of the substance.

gcm-3 or gl-1

a gas. The particles of a gas have lots of space between them so there will be only a small number of particles in the given volume compared to a solid or liquid whose particles have very little space between them.

Higher ChemistryUnit 1(e)

Gas densities and molar volumes

After today’s lesson you should be able to:state the equation showing the relationship

between density, mass and volume of a gas.state the equation showing the relationship

between density, molar mass and molar volume of a gas.

calculate the density or molar volume of a gas given relevant information.

give the relationship between all gases at the same temperature and pressure

Relationship between density, mass and volume

The units of density that are used to measure gases are gl-1.

grams = mass per l = volume

i.e. density = mass per volume = mass ÷ volume

This relationship can be represented by the following triangle:

mass (g)

density volume

(gl-1) (l)

Relationship between density, mass and volume for one mole of a gas

This relationship can be expanded further to calculate the molar volume of a gas i.e. the volume taken up by 1 mole of a gas.

molar mass(g)

density molar volume

(gl-1) (l)

Example 1

The density of methane, CH4, is 1.06gl-1.

Calculate the molar volume of this gas.

From the triangle:

Molar volume = molar mass ÷ density

= 16 ÷ 1.06

= 15.1l

Example 2

The molar volume of sulphur dioxide, SO2, is 26l. Calculate the density of this gas.

From the triangle:density = molar mass ÷ molar volume

= 64.1 ÷ 26 = 2.47gl-1

Example 3

The density of a gas is 0.8gl-1 and its molar volume is 35l.Calculate the molar mass of this gas.

From the triangle:Molar mass = density x molar volume

= 0.8 x 35 = 28g

Exercise

Complete Q8.1 – 8.10 on p71 of

‘Test your Higher Chemistry Calculations’

Relationship between all gases at the same temperature and pressure

At 0oC and 1 atmosphere pressure, nitrogen gas, N2, has a density of 1.25gl-1.

Its molar volume = molar mass ÷ density

= 28 ÷ 1.25

= 22.4l

Exercise

Using the table on p8 of your notes calculate the molar volume of oxygen, carbon monoxide and helium at the same temperature (0oC) and pressure (1atm).

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