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ChemistryGCSEAQA & EdexcelYears 10–11

GCSE Chemistry Quantitative Chemistry Notes

Quantitative chemistry lets you predict exactly how much product a reaction gives. It rests on the mole, Mr and the conservation of mass.

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Key concepts

Mole
The amount of substance containing 6.02 × 10²³ particles. One mole has a mass equal to Mr in grams.
Relative formula mass (Mr)
Sum of the Ar values of all atoms in the formula.
Conservation of mass
Total mass of reactants = total mass of products in a closed system.
Percentage yield
Actual yield ÷ theoretical yield × 100. Real reactions never give 100% yield.
  • Mass appears to change in open systems when a gas is released or absorbed.
  • Yield is lost through incomplete reactions, side reactions and losses during purification.
  • Balanced equations tell you the mole ratio between reactants and products.

Key formulas

Moles from mass

moles = mass (g) ÷ Mr

Percentage yield

% yield = (actual yield ÷ theoretical yield) × 100

Concentration

concentration (g/dm³) = mass (g) ÷ volume (dm³)

Worked example

Calculate the mass of magnesium oxide made when 4.8 g of magnesium burns completely. (Ar: Mg = 24, O = 16)

  1. 1Moles of Mg = 4.8 ÷ 24 = 0.2 mol.
  2. 2Equation: 2Mg + O₂ → 2MgO. Ratio Mg : MgO = 1 : 1.
  3. 3Moles of MgO = 0.2 mol. Mr(MgO) = 24 + 16 = 40.
  4. 4Mass of MgO = 0.2 × 40 = 8.0 g.

Answer: 8.0 g of magnesium oxide

Common mistakes

✗ Using atomic number instead of Ar in mole calculations.

✓ Use the larger number (Ar) from the periodic table.

✗ Ignoring the mole ratio in the balanced equation.

✓ Always scale by the stoichiometric coefficients.

✗ Reporting >100% yield.

✓ Extra mass means impurities/water — the maximum real yield is 100%.

Quick check

Try these, then reveal the answers.

Moles in 22 g of CO₂ (Mr = 44)?Reveal answer

0.5 mol

State the law of conservation of mass.Reveal answer

Mass of reactants = mass of products in a closed system.

Calculate % yield if you make 3.6 g from a theoretical 4.0 g.Reveal answer

90%

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Exam tips

  • Set out working in steps: moles → ratio → moles → mass.
  • Watch units — concentration is g/dm³ (1 dm³ = 1000 cm³).
  • Round only at the end to the same significant figures as the data.

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Related topics

Isotopes & Relative Atomic MassChemical Changes

Key terms explained

Short, exam-ready definitions with a formula, a worked example and an examiner tip.

What is mole?Full glossary

Frequently asked questions

Why is the actual yield less than the theoretical yield?+

Reactions may be incomplete or reversible, side reactions can occur and some product is lost during filtering, evaporation or transfer.

What is a mole?+

A mole is 6.02 × 10²³ particles — the amount of substance whose mass in grams equals its Mr.

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