The relative formula mass (Mr) of a substance is the sum of the relative atomic masses (Ar) of all the atoms in its formula. It is the first step of nearly every amount calculation in relative masses and amounts, and it appears in Cambridge IGCSE Chemistry 0620 questions as a short mark or as the start of a longer one.
How do I build Mr step by step?
Here are the Ar values used on this page: H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, S = 32, Cl = 35.5, Ca = 40, Cu = 64. In an exam, use the values on the periodic table you are given.
- Write the formula and list each element.
- Count the atoms of each element. A small number after an element multiplies that element only. A small number after a closing bracket multiplies everything inside.
- Multiply each Ar by its count.
- Add the results.
- Check that the answer is larger than the largest single Ar in the formula and looks sensible.
Worked example
Find the Mr of calcium hydroxide, Ca(OH)₂.
Step 1, list and count: Ca appears once. The bracket contains one O and one H, and the 2 outside means two of each. So the count is Ca = 1, O = 2, H = 2.
Step 2, multiply: Ca: 1 × 40 = 40. O: 2 × 16 = 32. H: 2 × 1 = 2.
Step 3, add: 40 + 32 + 2 = 74.
Second route as a check: the bracket group OH has mass 16 + 1 = 17, and two of them give 34. Then 40 + 34 = 74. Both routes agree.
A plausible mistake
A student writes: Mr of Ca(OH)₂ = 40 + 16 + 1 × 2 = 58.
Mistaken answer: 58
The 2 was applied only to H, as if the formula were CaOH₂. The 2 sits outside the bracket, so it multiplies both O and H.
The correction is to rewrite the formula without brackets first: Ca(OH)₂ is CaO₂H₂. Then count: 40 + 32 + 2 = 74.
Check yourself
Use the Ar values above.
1. Find the Mr of magnesium chloride, MgCl₂.
Show answer
Mg: 24. Cl: 2 × 35.5 = 71. Total: 24 + 71 = 95.
2. Find the Mr of ammonium sulfate, (NH₄)₂SO₄.
Show answer
The bracket (NH₄) has N = 14 and H = 4, so 14 + 4 = 18, and two of them give 36. SO₄ is 32 + 64 = 96. Total: 36 + 96 = 132.
3. Find the Mr of hydrated copper(II) sulfate, CuSO₄·5H₂O.
Show answer
CuSO₄: 64 + 32 + 64 = 160. H₂O: 2 + 16 = 18, and 5 × 18 = 90. Total: 160 + 90 = 250.
Where this leads next
Once Mr is automatic, go to converting mass to amount using consistent units. The mole and equation-ratio tutor shows the Mr step inside a full calculation, and the equation balance reasoning trainer helps you practise counting atoms.
If you often lose a mark here through small counting slips, that pattern is worth having someone watch. It is the kind of habit we look at in online one-to-one Chemistry tuition.