Edexcel International A Level Chemistry
Edexcel International A Level Chemistry Formulae, Equations and Amount of Substance Exam Questions
Free Edexcel International A Level Chemistry exam questions for Formulae, Equations and Amount of Substance, with a mark scheme for every pack. Covers the mole, Avogadro constant, empirical and molecular formulae, reacting masses, gas volumes, concentrations, titrations, percentage yield and atom economy.
Formulae, Equations and Amount of Substance exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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A-Level Chemistry Flashcards for 2027 Onwards
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About Edexcel International A Level Chemistry Formulae, Equations and Amount of Substance
This is the core calculation topic of Unit 1 in Edexcel International A Level Chemistry and underpins almost every other topic in the course. You learn to use the mole and the Avogadro constant, write balanced and ionic equations with state symbols, and calculate reacting masses, gas volumes, solution concentrations and titration results. It also covers empirical and molecular formulae, percentage yield and atom economy, along with the ideal gas equation. Examiners test multi-step calculations, often hidden inside practical contexts, and award marks for clear working, correct units and appropriate significant figures, so careful layout matters as much as the chemistry.
What you need to know
- The mole One mole contains 6.02 x 10^23 particles (the Avogadro constant). Amount in mol = mass / molar mass. Relative molecular mass is the sum of relative atomic masses.
- Empirical and molecular formulae Divide mass or percentage of each element by its Ar, then find the simplest whole number ratio. The molecular formula is a whole number multiple of the empirical formula found using Mr.
- Gas volumes At room temperature and pressure one mole of any gas occupies 24.0 dm3. The ideal gas equation is pV = nRT, with p in Pa, V in m3 and T in K.
- Solutions Concentration in mol dm-3 = moles / volume in dm3. Convert cm3 to dm3 by dividing by 1000. Concentration in g dm-3 = mol dm-3 x molar mass. Small concentrations may be given in ppm.
- Titrations Use concordant titres (within 0.20 cm3) to calculate a mean titre, then use the mole ratio from the equation to find an unknown concentration. Know how to make a standard solution.
- Equations Balanced equations include state symbols. Ionic equations show only the species that change, for example H+(aq) + OH-(aq) gives H2O(l).
- Yield and atom economy Percentage yield = actual yield / theoretical yield x 100. Atom economy = molar mass of desired product / total molar mass of reactants x 100, and a high atom economy means less waste.
Common mistakes in Formulae, Equations and Amount of Substance
- Using cm3 in the concentration equation without dividing by 1000. Always convert to dm3 first.
- Forgetting to convert units in pV = nRT. Pressure must be in Pa, volume in m3 (cm3 x 10^-6) and temperature in kelvin.
- Rounding too early in multi-step calculations, which changes the final answer. Keep full values in the calculator and round only at the end.
- Ignoring the mole ratio in the equation, especially for diprotic acids such as sulfuric acid reacting with sodium hydroxide in a 1 : 2 ratio.
- Confusing percentage yield with atom economy. Yield uses experimental masses; atom economy is calculated from the balanced equation alone.
Try three Formulae, Equations and Amount of Substance exam-style questions
1. A hydrocarbon contains 85.7% carbon and 14.3% hydrogen by mass. Its relative molecular mass is 56. Find its empirical and molecular formula. (3 marks)
Show the mark scheme answer
C: 85.7 / 12.0 = 7.14, H: 14.3 / 1.0 = 14.3; ratio 1 : 2 so empirical formula CH2; Mr of CH2 = 14, 56 / 14 = 4, so molecular formula C4H8.
2. Calculate the volume of carbon dioxide, in dm3 at room temperature and pressure, produced when 2.50 g of calcium carbonate reacts with excess hydrochloric acid. (Mr CaCO3 = 100.1, molar gas volume = 24.0 dm3 mol-1) (3 marks)
Show the mark scheme answer
Moles CaCO3 = 2.50 / 100.1 = 0.0250 mol; 1 : 1 ratio so 0.0250 mol CO2; volume = 0.0250 x 24.0 = 0.599 dm3 (accept 0.600 dm3).
3. 25.0 cm3 of sodium hydroxide solution is neutralised by 20.00 cm3 of 0.100 mol dm-3 sulfuric acid. Calculate the concentration of the sodium hydroxide. (3 marks)
Show the mark scheme answer
Moles H2SO4 = 0.100 x 20.00 / 1000 = 0.00200 mol; moles NaOH = 2 x 0.00200 = 0.00400 mol; concentration = 0.00400 / 0.0250 = 0.160 mol dm-3.
Formulae, Equations and Amount of Substance FAQs
What does Edexcel International A Level Chemistry Formulae, equations and amount of substance cover?
It covers the mole and Avogadro constant, writing balanced and ionic equations, empirical and molecular formulae, reacting masses, gas volumes and the ideal gas equation, concentrations, titrations, percentage yield and atom economy.
Which unit is this topic examined in?
It is taught in Unit 1, but mole calculations appear in every unit, including the practical skills units.
Which equations do I need for amount of substance?
You need n = m / M, n = c x V, volume of gas = n x 24.0 dm3 at RTP, pV = nRT, percentage yield and atom economy, and you should be fluent at converting between cm3, dm3 and m3.
