gcsescience.com 6 gcsescience.com

Compounds.

To find the mass of one mole of a compound,

just
add up all the RAMs of the
elements of the
compound,

taking account of the
formula.

The number you get is called the
Relative Formula Mass.

It is the mass of one mole of the compound in grams.

The Relative Formula Mass can be
written as M_{r} or
RFM.

For example, the mass of one mole of carbon dioxide (CO_{2}) is

(1 x RAM
of carbon) + (2 x RAM of
oxygen)

= (1 x 12) + (2 x
16) = 44 g.

So, one mole of carbon dioxide has a mass of 44 g.

The Relative Formula Mass of
carbon dioxide is 44.

This may also be called the Relative
Molecular Mass (RMM),

since carbon dioxide is a molecule.

The mass of one
mole of calcium carbonate (CaCO_{3})
is

(1 x RAM
of calcium) + (1 x RAM of
carbon) + (3 x RAM of
oxygen)

= (1 x 40) + (1 x
12) + (3
x 16) = 100 g.

So, one mole of calcium carbonate has
a mass of 100
g.

The Relative Formula Mass of
calcium carbonate is 100.

If you are not sure why
CaCO_{3} means 1
calcium + 1 carbon +
3 oxygens, see balancing
equations.

The mass of one
mole of magnesium oxide (MgO) is

(1 x RAM
of magnesium) + (1 x RAM of
oxygen)

= (1 x 24) + (1 x
16) = 40 g.

So, one mole of magnesium oxide has a
mass of 40
g.

The Relative Formula Mass of
magnesium oxide is 40.

Links Moles Revision Quizzes Revision Questions

gcsescience.com The Periodic Table Index Moles Quiz gcsescience.com

**Home**
**GCSE Chemistry**
**GCSE Physics**

Copyright © 2015 Dr. Colin France. All Rights Reserved.