Relative formula mass Cambridge IGCSE Chemistry revision
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In plain words
Atoms are far too light to weigh in grams, so chemists compare them with each other. Hydrogen, the lightest, counts as 1; carbon is 12 times heavier, so 12. To get the mass of a molecule, add up its atoms.
Three things to know
- Relative formula (or molecular) mass, Mr, is the sum of the relative atomic masses of all the atoms in the formula.
- CO₂: 12 + (2 × 16) = 44. For brackets, multiply everything inside: Ca(OH)₂ = 40 + 2 × (16 + 1) = 74.
- Percentage by mass of an element = (Ar × number of atoms ÷ Mr) × 100.
Worked example
Find the relative formula mass of calcium hydroxide, Ca(OH)₂. (Ca = 40, O = 16, H = 1)
- Work out the bracket first: O + H = 16 + 1 = 17.
- There are two of them: 2 × 17 = 34.
- Add the calcium: 40 + 34 = 74.
Tips and tricks
- A small number after a bracket multiplies everything inside the bracket.
- Relative masses have no units. Don't write "g".
It lands in your notebook with its questions as flashcards.
Relative formula mass: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is the Mr of methane, CH₄? (C = 12, H = 1)
12 + (4 × 1).
What is the Mr of sodium hydroxide, NaOH? (Na = 23, O = 16, H = 1)
23 + 16 + 1.
What is the percentage by mass of oxygen in magnesium oxide, MgO? (Mg = 24, O = 16)
16 ÷ 40 × 100.
What is the Mr of sulfuric acid, H₂SO₄? (H = 1, S = 32, O = 16)
2 + 32 + 64.
What is the Mr of magnesium hydroxide, Mg(OH)₂? (Mg = 24, O = 16, H = 1)
24 + 2 × (16 + 1).
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 7 marks. Write your answers on paper, then check them.
Relative formula mass
Cambridge IGCSE Chemistry 0620 · 7 marks · papermunch.org
Name ______________________________ Date ______________
Calculate the Mr of water, H₂O. (H = 1, O = 16)[1]
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18.
Calculate the Mr of calcium carbonate, CaCO₃. (Ca = 40, C = 12, O = 16)[2]
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100. 40 + 12 + (3 × 16).
Calculate the percentage by mass of carbon in carbon dioxide, CO₂. (C = 12, O = 16)[2]
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27% (27.3%). 12 ÷ 44 × 100.
Calculate the Mr of ammonium sulfate, (NH₄)₂SO₄. (N = 14, H = 1, S = 32, O = 16)[2]
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132. 2 × (14 + 4) + 32 + 64.
Answers: Relative formula mass
- 1. 18.
- 2. 100. 40 + 12 + (3 × 16).
- 3. 27% (27.3%). 12 ÷ 44 × 100.
- 4. 132. 2 × (14 + 4) + 32 + 64.