Relative formula mass, percentage by mass and moles
Quantitative chemistry starts with adding up relative atomic masses. From there you can work out what percentage of a compound is one element, and (for Higher) how many moles you have.
Part 1 of 3: Learn it
In short
- Mr is the sum of the relative atomic masses of all the atoms in the formula.
- Percentage by mass = (Ar × number of atoms of that element ÷ Mr) × 100.
- Moles = mass ÷ Mr (Higher tier for AQA).
Where this is in your specification
Spec points: AQA 4.3.1.1 to 4.3.1.3 (moles: 4.3.2, Higher), Edexcel Topic 1, OCR C3.1
| Board | Topic: Quantitative chemistry and the mole |
|---|---|
| AQA 8462 | 4.3 (8462); 5.3 (8464) |
| Edexcel 1CH0 | Topics 1 and 5 |
| OCR J248 | C3.1 |
Relative formula mass
Look up each element's relative atomic mass (Ar) on the periodic table, multiply by how many of those atoms are in the formula, and add them up. Brackets multiply everything inside them.
Percentage by mass
Conservation of mass
No atoms are made or destroyed in a reaction, so the total mass of the products equals the total mass of the reactants. If a reaction seems to gain mass in an open container, a gas from the air (such as oxygen) has joined the product; if it seems to lose mass, a gas product has escaped.
Moles (Higher)
A mole is the amount of substance that contains 6.02 × 10²³ particles (the Avogadro constant). The mass of one mole of a substance in grams equals its Mr.
So 18 g of water (Mr 18) is 1 mole and 9 g is 0.5 mol.
Find the Mr of sulfuric acid, H₂SO₄ (H = 1, S = 32, O = 16).
Show the answer
2 + 32 + 64 = 98.
Part 2 of 3: See it worked
Worked examples
Example 1
Calculate the percentage by mass of calcium in calcium carbonate, CaCO₃ (Ar: Ca = 40, C = 12, O = 16).
- Mr = 40 + 12 + 3 × 16 = 100
- % Ca = (40 ÷ 100) × 100
- = 40%
Answer: 40%.
Example 2
How many moles are in 22 g of carbon dioxide, CO₂ (C = 12, O = 16)?
- Mr = 12 + 2 × 16 = 44
- moles = 22 ÷ 44
- = 0.5 mol
Answer: 0.5 mol.
Common mistakes
- Forgetting to multiply by the small number: O₂ counts as 2 × 16.
- Applying a bracket's number to only the first atom inside it.
- Using the atomic number instead of the relative atomic mass.
- Thinking mass is lost when a gas is given off. It has escaped, not disappeared.
What percentage of water, H₂O, is hydrogen by mass?
Show the answer
2 ÷ 18 × 100 = 11.1%.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
Find the Mr of sulfuric acid, H₂SO₄ (H = 1, S = 32, O = 16).
2 + 32 + 64 = 98.
What percentage of water, H₂O, is hydrogen by mass?
2 ÷ 18 × 100 = 11.1%.
What is the mass of 2 moles of sodium chloride (Mr 58.5)?
2 × 58.5 = 117 g.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Diese Lernzettel sind auf Englisch, weil sie britischen Prüfungslehrplänen folgen.
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