Relative formula mass

Add relative atomic masses, taking care with subscripts and brackets.

The chemistry

Relative formula mass, Mr, is the sum of the relative atomic masses in one formula unit. It is a relative quantity and has no unit.

A subscript applies to the element immediately before it. A number outside brackets multiplies every atom inside those brackets.

Use the relative atomic masses stated in the question. An equation coefficient tells you an amount ratio; it does not change the Mr of a substance.

For Ca(OH)₂, use Ca = 40, O = 16 and H = 1.

One Ca, two O and two H give Mr = 40 + 2 × 16 + 2 × 1 = 74.

Specification references

AQA 4.3.1.2; Edexcel 1.43a

Relative formula mass.

Course

sodium carbonate. Relative atomic masses: Na = 23, C = 12, O = 16. Mr has no unit.

Calculate the relative formula mass, Mr, of Na2CO3.

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