Relative Formula Mass of a compound

Last updated on March 10th, 2022 at 04:57 pm

Relative atomic masses can be used to compare the masses of different atoms. Relative atomic masses can also be used to compare the masses of molecules in different compounds. The relative mass of a compound is called the Relative Formula Mass (symbol M_r).

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How to Calculate relative formula masses

The relative formula mass of a compound is obtained by adding up the relative atomic masses of all the atoms in its formula.

For example:
1)
The relative formula mass of water, H₂O, is:
M_r(H₂O) = 2 × relative atomic mass of hydrogen + relative atomic mass of oxygen = 2×1 + 16 = 18

2)

The relative formula mass of sulfuric acid, H₂SO₄, is:

M_r(H₂SO₄) = 2 × relative atomic mass of hydrogen + relative atomic mass of sulphur +4 x relative atomic mass of oxygen = 2×1 + 32 + 4×16 = 98

3)

The relative formula mass of aluminium oxide, Al₂O₃, is:

M_r(Al₂O₃) = 2 × relative atomic mass of Aluminium + 3 x relative atomic mass of oxygen = 2×27 + 3×16 = 102

Relative formula masses and moles

The relative formula mass of a compound in grams is sometimes called one mole in the same way that the relative atomic mass of an element in grams is also called a mole. So:

1 mole of water is 18 g
0.1 mole of water is 0.1 × 18 = 1.8 g
1 mole of aluminium oxide is 102 g
3 moles of aluminium oxide is 3 × 102 = 306 g