Formula mass
Science, Chemistry. Usually met in years 10 and 11.
Work out relative formula mass from a formula and the percentage by mass of an element in a compound.
What you practise
- Adding up Ar values
- Formulae with brackets
- Percentage by mass
Moves from H₂O to formulae such as Al₂(SO₄)₃. Practice moves up a level when a round goes well and down when it does not.
Sample questions
Each sample comes with the method and the reason every wrong option is wrong. Real practice rounds are ten questions at your level.
Sample question, level 2 of 5
Work out the relative formula mass (Mr) of KOH. (Ar: K = 39, O = 16, H = 1)
- 57
- 66
- 28
- 56
- 55
Show the answer and method
Answer: D, 56
The method: Add the Ar of every atom in the formula, counting each as many times as its subscript says. KOH has Mr = 56.
Why the other options are wrong:
- A, 57: Close, but check each atom is counted as many times as its subscript says.
- B, 66: Check the number of atoms of each element.
- C, 28: The total was halved.
- E, 55: Close, but check each atom is counted as many times as its subscript says.
Sample question, level 4 of 5
What is the percentage by mass of carbon in CaCO₃? (Ar: Ca = 40, C = 12, O = 16)
- 12%
- 88%
- 6%
- 17%
- 14%
Show the answer and method
Answer: A, 12%
The method: Mr of CaCO₃ = 100. The element's mass in the formula is 12. Percentage = 12 ÷ 100 × 100 = 12%.
Why the other options are wrong:
- B, 88%: This is the percentage of everything else in the compound.
- C, 6%: The percentage was halved.
- D, 17%: Close, but check the division: mass of the element ÷ Mr × 100.
- E, 14%: Close, but check your working: this is a small slip away from the answer.
See where this fits: GCSE science with AQA and Key Stage 3.
Related topics: States of matter, Atoms, Moles.
