1.5.2Principles of Chemistry
Calculate Relative Mass — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 12 parts Lesson 23 of 50
Part 1 of 5Relative atomic mass and relative formula mass
Relative atomic mass, Ar
- Relative atomic mass, Ar The Ar of an element is its average atom mass relative to carbon-12.
- It is the larger of the two numbers in each box of the Periodic Table.
Relative formula mass, Mr
- Relative formula mass, Mr The Mr of a substance is the sum of the Ar values of all the atoms shown in its formula.
- For a substance made of molecules it is also called the relative molecular mass.
Non-whole numbers
- Non-whole numbers Use the exact values given in the question, such as 35.5 for chlorine and 63.5 for copper.
Part 2 of 5Method
1
Write down the formula and list each element in it.
2
Count how many atoms of each element there are, multiplying by the number outside any brackets.
3
Write the Ar of each element from the Periodic Table.
4
Multiply each Ar by its number of atoms and add all the results together.
5
Give the answer without units, because relative masses are ratios.
Part 3 of 5Worked examples
| Substance | Atom count | Calculation | Mr |
|---|---|---|---|
| Water, H2O | H: 2, O: 1 | (2 × 1) + 16 | 18 |
| Sodium chloride, NaCl | Na: 1, Cl: 1 | 23 + 35.5 | 58.5 |
| Copper oxide, CuO | Cu: 1, O: 1 | 63.5 + 16 | 79.5 |
| Calcium hydroxide, Ca(OH)2 | Ca: 1, O: 2, H: 2 | 40 + (2 × 16) + (2 × 1) | 74 |
| Magnesium nitrate, Mg(NO3)2 | Mg: 1, N: 2, O: 6 | 24 + (2 × 14) + (6 × 16) | 148 |
Part 4 of 5Dealing with brackets
Number outside bracket multiplies everything inside
Rule
- Rule In Ca(OH)2 the small 2 applies to both O and H, so there are 2 oxygen atoms and 2 hydrogen atoms.
Nested counts
- Nested counts In (NH4)2SO4 there are 2 nitrogen atoms and 2 × 4 = 8 hydrogen atoms, plus one sulfur and four oxygen atoms.
- Mr = (2 × 14) + (8 × 1) + 32 + (4 × 16) = 132.
Check yourself
- Check yourself Count the atoms of each element before adding anything, so none is missed.
Part 5 of 5Link to conservation of mass
Balanced equations
- Balanced equations In a balanced equation the total Mr of the reactants (including the big numbers) equals the total Mr of the products.
Example
- Example 2Mg + O2 → 2MgO: reactants 2 × 24 + 32 = 80 and products 2 × 40 = 80.
Key terms
| Term | Meaning |
|---|---|
| Relative atomic mass (Ar) | The average mass of an atom of an element compared with 1/12 of a carbon-12 atom. |
| Relative formula mass (Mr) | The sum of the relative atomic masses of the atoms in the formula. |
| Subscript | A small number after a symbol that shows how many atoms of that element are present. |
| Coefficient | The large number in front of a formula in an equation. |
Avoid these mistakes
1
Using the atomic number instead of the mass number (Ar). The Ar is the larger number in the Periodic Table box.
2
Ignoring the number outside a bracket, so only one of the atoms in the bracket is counted.
3
Rounding chlorine to 35 or copper to 64 when the question gives 35.5 and 63.5.
4
Adding units such as grams. Relative formula mass has no units.
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