When a formula contains BRACKETS, multiply everything inside the brackets by the number outside.
Examples:
Ca(OH)₂:
Ca: 40
O: 16 × 2 = 32 (there are 2 OH groups, each with 1 O)
H: 1 × 2 = 2 (each OH group has 1 H, × 2 groups)
Mr = 40 + 32 + 2 = 74
Mg(NO₃)₂:
Mg: 24
N: 14 × 2 = 28
O: 16 × 6 = 96 (each NO₃ has 3 O, × 2 groups)
Mr = 24 + 28 + 96 = 148
Al₂(SO₄)₃:
Al: 27 × 2 = 54
S: 32 × 3 = 96
O: 16 × 12 = 192 (each SO₄ has 4 O, × 3 groups)
Mr = 54 + 96 + 192 = 342
Using Mr in Mass Calculations
Mr allows us to calculate MASSES in reactions from a balanced equation.
FUNDAMENTAL PRINCIPLE:
The RATIO of masses of reactants and products in a reaction equals the RATIO of their Mr values (multiplied by the coefficients in the balanced equation).
Example:
2Mg + O₂ → 2MgO
Mr: Mg = 24, O₂ = 32, MgO = 40
Ratio of masses: 2 × 24 : 32 : 2 × 40 = 48 : 32 : 80
So: 48 g of Mg reacts with 32 g of O₂ to produce 80 g of MgO.
OR: to find mass of MgO from 12 g of Mg:
Scale factor = 12 ÷ 48 = 0.25
Mass of MgO = 80 × 0.25 = 20 g
This is the foundation for all quantitative chemistry calculations.
⚠️ Common Mistake
When a formula has BRACKETS with a subscript, MULTIPLY all atoms inside the bracket by the subscript. Ca(OH)₂ has 2 oxygen atoms and 2 hydrogen atoms — not 1 of each. Write out the count carefully: Ca(OH)₂ = Ca + 2O + 2H = 40 + 32 + 2 = 74.
📐 Variables
MrRelative formula mass (Mr) is measured in ()
ArRelative atomic mass (Ar) is measured in ()
📐 Key Equations
Mr = sum of all Ar values in the formula
📌 Key Note
Mr = sum of all Ar values in the formula. Use Ar from periodic table. Brackets: multiply atoms inside by the subscript outside. Mr is used to calculate mass ratios in reactions. Mr has no units.
🎯 Matching Activity — Calculate the Mr
Match each formula to its correct relative formula mass. — drag the symbols on the right to match the component names on the left.
H₂O
Drop here
CO₂
Drop here
NaCl
Drop here
CaCO₃
Drop here
Ca(OH)₂
Drop here
Mr = 74 — (1 × 40) + (2 × 16) + (2 × 1)
Mr = 58.5 — (1 × 23) + (1 × 35.5)
Mr = 100 — (1 × 40) + (1 × 12) + (3 × 16)
Mr = 18 — (2 × 1) + (1 × 16)
Mr = 44 — (1 × 12) + (2 × 16)
⚽ FIFA Worked Examples
Mr Calculation — H₂SO₄
Calculate the relative formula mass of sulfuric acid (H₂SO₄). Ar: H=1, S=32, O=16.
F
Mr = sum of all Ar values × number of atoms
I
H: 1 × 2 = 2. S: 32 × 1 = 32. O: 16 × 4 = 64
F
Mr = 2 + 32 + 64
A
Mr = 98
⭐ Higher Tier Only
Calculate percentage mass of an element in a compound: (Ar × number of atoms ÷ Mr) × 100. Calculate empirical formula from percentage composition. Convert empirical to molecular formula using Mr. These skills are foundations for mole calculations.
🎯 Test Yourself
Question 1 of 2
1. What is the relative formula mass of calcium carbonate (CaCO₃)? Ar: Ca=40, C=12, O=16.
2. What is the Mr of Mg(NO₃)₂? Ar: Mg=24, N=14, O=16.
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