CCEA GCSE Chemistry 1.7 Quantitative Chemistry Exam Questions

About CCEA GCSE Chemistry 1.7 Quantitative Chemistry

Quantitative Chemistry in Unit 1 of CCEA GCSE Chemistry introduces the mole and calculations based on chemical equations. You need to calculate relative formula mass, convert between mass and moles, use mole ratios to find reacting masses, identify limiting reactants and calculate percentage yield. You also find empirical and molecular formulae and the number of molecules of water of crystallisation in a hydrated salt, which is a prescribed practical. Unit 2 extends this with concentrations, titrations and gas volumes in 2.6. Examiners expect clear working with units and often set multi-step problems, so practising structured layouts is essential for gaining method marks.

What you need to know

• Relative formula mass Mr is the sum of the relative atomic masses of all the atoms in the formula. For example Mr of CaCO3 = 40 + 12 + (3 x 16) = 100.
• The mole One mole contains 6 x 10^23 particles (Avogadro constant). Moles = mass (g) / Mr. Mass = moles x Mr.
• Reacting masses Find moles of the known substance, use the balanced equation's mole ratio to find moles of the unknown, then convert to mass. In a limiting reactant problem, the reactant that is used up first limits the amount of product.
• Percentage yield Percentage yield = actual yield / theoretical yield x 100. Yields are below 100% because of incomplete reactions, side reactions and losses during filtering or transferring.
• Empirical and molecular formulae Empirical formula is the simplest whole number ratio of atoms. Divide each mass (or percentage) by Ar, then divide by the smallest answer. Molecular formula = empirical formula multiplied up so its Mr matches the given Mr.
• Prescribed practical: water of crystallisation Weigh hydrated crystals in a crucible, heat to drive off the water, cool and reweigh, then repeat until the mass is constant (heating to constant mass). Use the masses of water and anhydrous salt to find the mole ratio and the value of x in, for example, CuSO4.xH2O.

Common mistakes in 1.7 Quantitative Chemistry

• Using Ar instead of Mr for diatomic elements. Oxygen gas is O2 with Mr 32.
• Forgetting to use the mole ratio from the balanced equation. Always multiply or divide by the ratio before converting to mass.
• Rounding too early in multi-step calculations. Keep full values until the final answer.
• Not heating to constant mass in the water of crystallisation practical, which leaves some water in the solid and gives a value of x that is too low.
• Using the percentage by mass directly as the ratio for an empirical formula. Divide each by Ar first.

Try three 1.7 Quantitative Chemistry exam-style questions