Relative Formula Mass (Mr) — Chemical Formulae, Equations and Calculations | IGCSE Chemistry | Examo

Relative Formula Mass (Mr) — Chemical Formulae, Equations and Calculations | IGCSE Chemistry | Examo

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 17% of your exam marks.

stable

Very High

Stable17%

Highest-frequency topic: moles, percentage yield and titration calculations appear on nearly every paper.

The relative formula mass (M_r) of a substance is the sum of the relative atomic masses of every atom in its chemical formula
The relative atomic mass (A_r) of each element is read from the periodic table (it's the larger of the two numbers in each box)
For an element written as just its symbol (e.g. Fe), M_r = A_r
For a compound, multiply each A_r by the number of atoms of that element in the formula, then add the products

Substance	Atoms present	Calculation	M_r
H₂O	2 H + 1 O	(2 × 1) + 16	18
CO₂	1 C + 2 O	12 + (2 × 16)	44
CaCO₃	1 Ca + 1 C + 3 O	40 + 12 + (3 × 16)	100
H₂SO₄	2 H + 1 S + 4 O	(2 × 1) + 32 + (4 × 16)	98
Mg(NO₃)₂	1 Mg + 2 N + 6 O	24 + (2 × 14) + (6 × 16)	148

By the law of conservation of mass, the total M_r on the reactant side (with each M_r multiplied by its coefficient) equals the total M_r on the product side