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.
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)
- 1Moles of Mg = 4.8 ÷ 24 = 0.2 mol.
- 2Equation: 2Mg + O₂ → 2MgO. Ratio Mg : MgO = 1 : 1.
- 3Moles of MgO = 0.2 mol. Mr(MgO) = 24 + 16 = 40.
- 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%
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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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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