1.2.3Building Blocks for Understanding
Relative Formula Masses — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions
5 min read 9 parts Lesson 9 of 14
Part 1 of 7Two related quantities
Relative atomic mass (Ar)
- Relative atomic mass (Ar) The mass of an atom compared with one-twelfth of a carbon-12 atom, shown on the periodic table.
- For example Ar of Mg is 24 and of O is 16.
- Where an element has several isotopes, Ar is a weighted average, so it may not be a whole number (chlorine is about 35.5).
Relative formula mass (Mr)
- Relative formula mass (Mr) The sum of the Ar values of all atoms shown in the formula.
- It has no units because it is a ratio of masses.
- For a simple molecule it is often called the relative molecular mass.
Brackets and multipliers
- Brackets and multipliers A number outside a bracket multiplies everything inside it, so in Ca(OH)2 there are two O and two H atoms.
- A small number after a symbol multiplies only that symbol.
Part 2 of 7Finding Mr
1
Write the formula and list each element with the number of its atoms.
2
Look up each Ar on the periodic table.
3
Multiply the Ar by the number of atoms of that element.
4
Add the results together to give the Mr.
5
Example: CaCO3 gives 40 + 12 + (3 × 16) = 100.
Part 3 of 7Common examples
| Compound | Calculation | Mr |
|---|---|---|
| H2O | (2 × 1) + 16 | 18 |
| CO2 | 12 + (2 × 16) | 44 |
| MgO | 24 + 16 | 40 |
| NaCl | 23 + 35.5 | 58.5 |
| Ca(OH)2 | 40 + 2 × (16 + 1) | 74 |
| (NH4)2SO4 | 2 × (14 + 4) + 32 + (4 × 16) | 132 |
Part 4 of 7Percentage by mass of an element
1Total for the element
- Total for the element Ar multiplied by the number of its atoms in the formula.
2Divide by Mr
- Divide by Mr Compare that total with the Mr of the whole compound.
3Multiply by 100
- Multiply by 100 Percentage = (Ar × atoms ÷ Mr) × 100. For MgO, Mg = 24 ÷ 40 × 100 = 60%.
Part 5 of 7Worked example: nitrogen in ammonium nitrate
| Step | Working |
|---|---|
| Formula | NH4NO3 |
| Atoms | 2 N, 4 H, 3 O |
| Mr | (2 × 14) + (4 × 1) + (3 × 16) = 80 |
| Mass of nitrogen | 2 × 14 = 28 |
| Percentage of N | 28 ÷ 80 × 100 = 35% |
| Mass of N in 200 g | 35% of 200 = 70 g |
Part 6 of 7Why these calculations matter
Relative masses
Fertilisers
- Fertilisers Farmers compare the percentage of nitrogen in different fertilisers to choose the best value.
Ores
- Ores The percentage of metal in an ore shows how much metal could be extracted from it.
Later work
- Later work Mr is needed to convert between mass and moles in higher-tier calculations.
Part 7 of 7Exam technique for relative mass questions
Show every line
- Show every line Write out the Ar values you use and the sum you do, because marks are awarded for the method as well as the final answer.
- If a calculator slip occurs, a clear method can still earn most of the marks.
Use the data given
- Use the data given Questions often supply Ar values, and these may differ slightly from your periodic table, so always use the ones in the question.
- Check whether the answer should be left as a whole number or given to a stated number of significant figures.
Link to other topics
- Link to other topics Percentage by mass links to empirical formulae,
- and Mr links to reacting masses and yields,
- so accurate Mr values pay off throughout the course.
Key terms
| Term | Meaning |
|---|---|
| Relative atomic mass | The average mass of an atom of an element compared with one-twelfth of carbon-12. |
| Relative formula mass | The sum of the relative atomic masses in a chemical formula. |
| Percentage by mass | The proportion of a compound's mass made up by one element, shown as a percentage. |
| Formula | The symbols and numbers showing atoms in a substance. |
| Isotope | Atoms of the same element with different numbers of neutrons. |
Avoid these mistakes
1
Adding the atomic numbers rather than the relative atomic masses.
2
Forgetting to multiply everything in brackets by the number outside.
3
Giving Mr units such as grams; it is a ratio.
4
Dividing the element's mass by the wrong total when finding percentage.
5
Rounding chlorine's Ar to 35 or 36 when the question expects 35.5.
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