Calculate Relative Mass (Edexcel IGCSE Science (Double Award))
Revision Note
Written by: Stewart Hird
Reviewed by: Lucy Kirkham
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Relative formula (molecular) mass
How to calculate relative formula mass
The symbol for the relative atomic mass is Ar
This is calculated from the mass number and relative abundances of all the isotopes of a particular element
The symbol for the relative formula mass is Mr and it refers to 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 |
In accordance with the Law of Conservation of Mass, the sum of the relative formula masses of the reactants will be the same as the sum of the relative formula masses of the products
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
The relative atomic mass of every element is given on the Periodic Table. It is the larger of the two numbers.
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