Relative Formula Mass (Oxford AQA IGCSE Combined Science Double Award)
Revision Note
Written by: Alexandra Brennan
Reviewed by: Stewart Hird
Relative Formula Mass
The symbol for the relative formula mass and relative molecular mass is Mr
Relative formula mass is used when referring to the total mass of an ionic compound
It is the total mass of the molecule
To calculate the Mr of a substance, you have to add up the relative atomic masses of all the atoms present in the formula
Relative Formula Mass Calculations Table
Substance | Atoms present | Calculation | Mr |
|---|---|---|---|
Hydrogen | 2 x H | (2 x 1) | 2 |
Water | (2 x H) + (1 x O) | (2 x 1) + (1 x 16) | 18 |
Potassium carbonate | (2 x K) + (1 x C) + (3 x O) | (2 x 39) + (1 x 12) + (3 x 16) | 138 |
Calcium hydroxide | (1 x Ca) + (2 x O) + (2 x H) | (1 x 40) + (2 x 16) + (2 x 1) | 74 |
Ammonium sulfate | (2 x N) + (8 x H) + (1 x S) + (4 x O) | (2 x 14) + (8 x 1) + (1 x 32) + (4 x 16) | 132 |
Worked Example
Calculate the relative formula mass of:
Sodium chloride, NaCl
Copper oxide, CuO
Magnesium nitrate, Mg(NO3)2
Answers:
Sodium chloride
NaCl = 23 + 35.5 = 58.5
Copper oxide
CuO = 63.5 + 16 = 79.5
Magnesium nitrate
Mg(NO3)2 = 24 + (14 x 1 x 2) + (16 x 3 x 2) = 148
Examiner Tips and Tricks
It is worth showing your working out for these calculations in exams- for more complicated exam questions it can be easy to miscalculate the final value, and you might score one mark for showing your working out.
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