Eduqas GCSE Chemistry
Eduqas GCSE Chemistry 3 Chemical Formulae, Equations and Amount of Substance Exam Questions
Free Eduqas GCSE Chemistry revision for 3 Chemical Formulae, Equations and Amount of Substance: what you need to know, common mistakes and exam-style questions with answers. Covers formulae and ions, balanced and ionic equations, relative formula mass, moles, empirical formulae, limiting reactants and molar gas volume.
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About Eduqas GCSE Chemistry 3 Chemical Formulae, Equations and Amount of Substance
This is the quantitative chemistry topic of Eduqas GCSE Chemistry. You learn to write formulae for compounds and ions, balance symbol equations with state symbols, calculate relative formula mass and percentage composition, and use the mole and the Avogadro constant. It then covers reacting mass calculations, empirical formulae, limiting reactants and the volume of one mole of gas. These skills are used across the whole course, especially in acids and titrations (topic 7) and yields and atom economy (topic 11). Common questions include reacting masses, finding an empirical formula from experimental data and working out which reactant is limiting.
What you need to know
- Formulae and ions Use ion charges to write formulae, for example Na+ and SO4 2- give Na2SO4; Ca2+ and OH- give Ca(OH)2. Learn common ions such as nitrate NO3-, carbonate CO3 2-, sulfate SO4 2-, hydroxide OH- and ammonium NH4+.
- Equations Balance equations by changing the numbers in front of formulae, never the formulae themselves. Include state symbols (s), (l), (g) and (aq). Mass is conserved because atoms are rearranged, not made or destroyed.
- Relative formula mass Mr is the sum of the relative atomic masses of all atoms in the formula. Percentage by mass of an element = (Ar x number of atoms / Mr) x 100.
- The mole One mole contains 6.02 x 10^23 particles (the Avogadro constant). Moles = mass / Mr. Use the mole ratio from the balanced equation to work out reacting masses.
- Empirical formula The empirical formula is the simplest whole number ratio of atoms. Divide each mass by Ar, then divide by the smallest answer to find the ratio.
- Limiting reactants and gas volume The limiting reactant is completely used up and determines the amount of product; the other reactant is in excess. One mole of any gas occupies 24 dm3 at room temperature and pressure.
Common mistakes in 3 Chemical Formulae, Equations and Amount of Substance
- Changing small numbers inside a formula to balance an equation. Only add numbers in front of formulae, for example 2H2O, not H2O2.
- Forgetting brackets in formulae such as Ca(OH)2. Without brackets, the 2 only applies to the hydrogen.
- Using the mass ratio instead of the mole ratio from the equation. Convert masses to moles first, then use the equation ratio.
- Thinking mass is lost when a gas is given off in an open container. Mass is conserved; the gas escapes so the measured mass falls.
- Rounding too early in empirical formula calculations. Keep values to at least 2 decimal places until the final ratio.
Try three 3 Chemical Formulae, Equations and Amount of Substance exam-style questions
1. Calculate the mass of calcium oxide formed when 25 g of calcium carbonate is heated. CaCO3 → CaO + CO2. (Ar: Ca = 40, C = 12, O = 16) (3 marks)
Show the mark scheme answer
Mr CaCO3 = 100 and moles = 25 / 100 = 0.25 mol; mole ratio 1:1 so 0.25 mol CaO; Mr CaO = 56 so mass = 0.25 x 56 = 14 g.
2. A compound contains 2.4 g of carbon and 0.6 g of hydrogen. Find its empirical formula. (Ar: C = 12, H = 1) (3 marks)
Show the mark scheme answer
C: 2.4 / 12 = 0.2; H: 0.6 / 1 = 0.6; ratio 1:3 so empirical formula CH3.
3. Calculate the volume of carbon dioxide, at room temperature and pressure, produced from 0.25 mol of calcium carbonate. (2 marks)
Show the mark scheme answer
0.25 mol CO2 produced (1:1 ratio); volume = 0.25 x 24 = 6.0 dm3.
3 Chemical Formulae, Equations and Amount of Substance FAQs
What does Eduqas GCSE Chemistry 3 Chemical Formulae, Equations and Amount of Substance cover?
It covers writing formulae and balanced equations, relative formula mass and percentage composition, moles and reacting masses, empirical formulae, limiting reactants and molar gas volume.
Which paper is 3 Chemical Formulae, Equations and Amount of Substance examined in?
It is examined in Component 1: Concepts in Chemistry (2 hours 15 minutes, 120 marks). Component 2: Applications in Chemistry can also test it through practical contexts and an unseen article.
What are the key formulas for Eduqas GCSE Chemistry 3 Chemical Formulae, Equations and Amount of Substance?
Moles = mass / Mr, percentage by mass = (Ar x number of atoms / Mr) x 100, and gas volume = moles x 24 dm3 at room temperature and pressure. You also need the Avogadro constant, 6.02 x 10^23.
