Relative Molecular Mass

Previous Page: Moles

A similar relative scale to the atomic mass scale could be developed for gases using masses of equal volumes of gases as a base for comparison. As gases exist as individual molecules, such a scale would be termed the relative molecular mass scale.

As before, a parallel scale measuring mass in grams will link to moles of gases.

For example:1 mole of hydrogen gas contains 6.02 x 1023 H2 molecules

1 mole of carbon dioxide gas contains 6.02 x 1023 CO2 molecules.

We shall see shortly that the relative molecular mass and relative atomic mass scale can in fact be thought of as a single scale. We will also be able to include masses of ions on such a scale.

Example 1:

How many moles of carbon dioxide gas will you have if you are told that you have 18.06 x 1023 molecules of the gas in your breath?


First we must remember that there are 6.02 x 1023 particles in 1 mole of any substance. So we can write this problem as a division:

We can also relate the number of moles of a substance to the mass of the substance using the relationship between the mole of a substance and the relative atomic (or molecular) mass.

Example 2:

How many moles of hydrogen gas will be produced from a reaction that evolved 40 grams of the gas? (Molecular mass of hydrogen gas, H2 = 2.0.)


Example 3:

An oxygen cylinder contains 6.0 moles of gas. If one mole of oxygen gas has a mass 32.0g, what will be the mass of the gas in the cylinder?


Mass of oxygen = 6.0 mol. x 32.0 g per mole

= 192.0 g

Defining and relating the terms

To formalise and extend our understanding of relative atomic mass and the mole we can give a definition based on our modern understanding of the atomic mass scale.

Now try these problems.

Next Page: Calculating Molecular Masses

Link to Index