AQA GCSE Chemistry 4.3 Quantitative chemistry: a complete overview

What topic 4.3 actually demands

Quantitative chemistry is the calculation engine of AQA GCSE Chemistry. Topic 4.3 turns chemical equations into numbers, letting you predict how much product forms, how concentrated a solution is, and how efficient a process is. AQA tests fluent, well-shown calculation more than recall here, and the same moles skills reappear in chemical changes, energy changes and rates.

This guide walks through all five dot points of the topic in specification order, then sets out the exam patterns AQA repeats. Each dot point has a matching page with practice questions; this overview ties them together.

Conservation of mass and the mole

The topic opens with conservation of mass: no atoms are made or destroyed, so the mass of products equals the mass of reactants and equations must be balanced. Relative formula mass is the sum of the relative atomic masses in a formula. The mole is the unit for amount of substance, and one mole contains the Avogadro number of particles, $6.02 \times 10^{23}$, with a mass in grams equal to its $M_r$. Mass can appear to change when a gas enters or leaves an open container.

Amounts of substance and reacting masses

The core skill is the moles equation, $\text{moles} = \dfrac{\text{mass}}{M_r}$, and using the mole ratio from a balanced equation to relate substances. A reacting-mass calculation is three steps: mass to moles, mole ratio, moles back to mass. At Higher tier you also identify the limiting reactant, the one that runs out first and controls the product, and can deduce balancing numbers from masses.

Concentration of solutions

Concentration measures solute per volume. In g/dm$^3$ it is mass divided by volume; in mol/dm$^3$ (Higher) it is moles divided by volume, and you convert between them using the relative formula mass. Titration calculations use a known concentration and the reacting volumes to find an unknown concentration.

Yield and atom economy

Percentage yield compares the actual product with the theoretical maximum, and is below 100 percent because of losses, incomplete or reversible reactions, impurities and side reactions. Atom economy measures how much of the reactant mass ends up as useful product, so a high atom economy means less waste and a more sustainable process.

How topic 4.3 is examined

A typical AQA profile for this topic:

• Balancing and $M_r$. Balancing symbol equations and calculating relative formula masses.
• Moles calculations. Converting between mass, moles and particles, and reacting masses using mole ratios.
• Concentration and titration. g/dm$^3$ and mol/dm$^3$ work, with conversions and titration calculations at Higher.
• Efficiency. Percentage yield and atom economy calculations with explanation of their importance.

Check your knowledge

A mix of recall and calculation questions covering topic 4.3. Attempt them under timed conditions, then check against the solutions.

1. State the law of conservation of mass. (1 mark)
2. Calculate the relative formula mass of $CaCO_3$ ($A_r$: Ca = 40, C = 12, O = 16). (2 marks)
3. Calculate the number of moles in $44$ g of carbon dioxide ($M_r = 44$). (1 mark)
4. For $2Mg + O_2 \rightarrow 2MgO$, calculate the mass of $MgO$ from $24$ g of magnesium ($A_r$ Mg = 24, $M_r$ MgO = 40). (3 marks)
5. Calculate the concentration in g/dm$^3$ of $10$ g of solute in $500$ cm$^3$ of solution. (2 marks)
6. A reaction has a theoretical yield of $25$ g and an actual yield of $20$ g. Calculate the percentage yield. (2 marks)
7. Define atom economy. (1 mark)
8. Explain why the mass of a sealed flask does not change during a reaction. (2 marks)