Relative Atomic Mass & Relative Molecular Mass (WJEC GCSE Science (Double Award): Chemistry): Revision Note
Exam code: 3430
Ar & Mr
Relative Atomic Mass
The symbol for relative atomic mass is Ar
The relative atomic mass for each element can be found in the Periodic Table, along with the atomic number
Relative atomic mass is shown above the atomic symbol
It is always larger than the atomic number (except for hydrogen, where they are the same)
Key for chemical symbols on the Periodic Table
This key is given on the Periodic Table in exams and shows that the top number is the relative atomic mass.
Atoms are too small to accurately weigh but scientists needed a way to compare the masses of atoms
Carbon-12 is used as the standard atom and has a fixed mass of 12 units
The mass of all other atoms are compared against carbon-12
The relative atomic mass of carbon is 12
The relative atomic mass of magnesium is 24 which means that magnesium is twice as heavy as carbon
The relative atomic mass of hydrogen is 1 which means it has one-twelfth the mass of one carbon-12 atom
Examiner Tips and Tricks
Relative atomic mass (Ar) is the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of 12C
You are not required to know this definition but it can help with your understanding
Relative molecular (formula) mass
The symbol for the relative molecular 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 is used when referring to the total mass of an ionic compound
Relative Formula Mass Calculations Table
Substance | Atoms present | Calculation | Mr |
|---|---|---|---|
Hydrogen H2 | 2 x H | (2 x 1) | 2 |
Water H2O | (2 x H) + (1 x O) | (2 x 1) + (1 x 16) | 18 |
Potassium carbonate K2CO3 | (2 x K) + (1 x C) + (3 x O) | (2 x 39) + (1 x 12) + (3 x 16) | 138 |
Calcium hydroxide Ca(OH)2 | (1 x Ca) + (2 x O) + (2 x H) | (1 x 40) + (2 x 16) + (2 x 1) | 74 |
Ammonium sulfate (NH4)2SO4 | (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
Higher tier
Mass number is the sum of protons and neutrons present in the nucleus
Mass number is a whole number value
Relative atomic mass is the ratio of the mass of an element compared to 1/12th of the mass of a carbon atom
Relative atomic mass can be a decimal
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