Atoms are far too light to weigh one at a time, so chemists compare them with one standard atom, carbon-12. Add up the relative atomic masses in a formula and you have the relative formula mass, the number every calculation in this topic starts from.
Relative atomic mass Ar is the average mass of the isotopes of an element compared with 1/12 of the mass of an atom of carbon-12. Relative molecular mass Mr is the sum of the Ar values of all the atoms in a formula; the same sum is used for ionic compounds (relative formula mass). Relative masses are ratios, so they have no units.
Load an element into the simulator and look at its isotopes and their abundances. Before the display shows it, estimate the average: it is always pulled towards the most common isotope.
Add the Ar of every atom in the formula. A number after a bracket multiplies everything inside the bracket. Use the Ar values: H 1, C 12, N 14, O 16, Mg 24, S 32, Ca 40, Cu 64.
| Substance | Working | Mr |
|---|---|---|
| water, H2O | 2 × 1 + 16 | 18 |
| carbon dioxide, CO2 | 12 + 2 × 16 | 44 |
| calcium carbonate, CaCO3 | 40 + 12 + 3 × 16 | 100 |
| calcium hydroxide, Ca(OH)2 | 40 + 2 × (16 + 1) | 74 |
| magnesium nitrate, Mg(NO3)2 | 24 + 2 × (14 + 3 × 16) | 148 |
| ammonium sulfate, (NH4)2SO4 | 2 × (14 + 4 × 1) + 32 + 4 × 16 | 132 |
| hydrated copper(II) sulfate, CuSO4·5H2O | 64 + 32 + 4 × 16 + 5 × 18 | 250 |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Relative atomic mass is measured compared with:
What is the Mr of carbon dioxide, CO2? (Ar: C 12, O 16)
What is the relative formula mass of calcium carbonate, CaCO3? (Ar: C 12, O 16, Ca 40)
Why is the Ar of chlorine 35.5 and not a whole number?
The percentage of an element in a compound is the mass of that element in one formula, divided by Mr, times 100. Farmers use this to compare fertilisers by their nitrogen content.
Count every atom of the element. Ammonium nitrate, NH4NO3, has one N in the ammonium ion and one in the nitrate ion, so the nitrogen mass is 28, not 14. Using 14 gives 17.5%, exactly half the right answer.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
What is the relative formula mass of ammonium sulfate, (NH4)2SO4? (Ar: H 1, N 14, O 16, S 32)
What is the percentage by mass of nitrogen in ammonium nitrate, NH4NO3? (Ar: H 1, N 14, O 16)
What is the percentage by mass of water in hydrated copper(II) sulfate, CuSO4·5H2O? (Ar: H 1, O 16, S 32, Cu 64)
Which fertiliser contains the greatest percentage by mass of nitrogen? (Ar: H 1, C 12, N 14, O 16, S 32, K 39)
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