4CH1
Chemical Formulae, Equations and Calculations
Principles of Chemistry · 6 question types
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 (Mr) of a substance is the sum of the relative atomic masses of every atom in its chemical formula
- The relative atomic mass (Ar) 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), Mr = Ar
- For a compound, multiply each Ar by the number of atoms of that element in the formula, then add the products
| Substance | Atoms present | Calculation | Mr |
|---|---|---|---|
| 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 Mr on the reactant side (with each Mr multiplied by its coefficient) equals the total Mr on the product side
Worked example
Mr of a compound containing brackets
A student needs the relative formula mass of calcium hydroxide, Ca(OH)₂. Calculate it.
Solution:
- Identify all atoms: 1 Ca, and the bracket (OH)₂ means 2 O and 2 H
- Ar values: Ca = 40, O = 16, H = 1
- Mr = (1 × 40) + (2 × 16) + (2 × 1)
- Mr = 40 + 32 + 2 = 74