Edexcel A Level Chemistry
Edexcel A Level Chemistry Topic 5: Formulae, Equations and Amounts of Substance Exam Questions
Free Edexcel A Level Chemistry exam questions for Topic 5: Formulae, Equations and Amounts of Substance, with a mark scheme for every pack. Covers the mole, empirical and molecular formulae, reacting masses, gas volumes, concentrations, titrations, yield, atom economy and measurement uncertainty.
Topic 5: Formulae, Equations and Amounts 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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About Edexcel A Level Chemistry Topic 5: Formulae, Equations and Amounts of Substance
Topic 5 of Edexcel A Level Chemistry develops the quantitative skills used in every physical chemistry topic. You work with the mole and the Avogadro constant, calculate empirical and molecular formulae, and carry out calculations using reacting masses, gas volumes (with the molar volume and the ideal gas equation), solution concentrations and titration results. Percentage yield, atom economy and the uncertainty in measurements are also included, and the topic contains core practicals such as measuring the molar volume of a gas and preparing a standard solution for titration. It is assessed in Papers 1 and 2 and heavily in Paper 3. Examiners expect clear working and correct significant figures.
What you need to know
- The mole Amount in moles = mass/molar mass. The Avogadro constant is 6.02 x 10^23 mol-1. Concentration in mol dm-3 = moles/volume in dm3, and ppm is used for very dilute solutions.
- Gas volumes Molar volume is 24.0 dm3 mol-1 at room temperature and pressure. The ideal gas equation pV = nRT uses Pa, m3 and K.
- Formulae and equations Find empirical formulae from masses or percentages, and molecular formulae from Mr. Write balanced equations, including ionic equations with state symbols.
- Titrations Use concordant titres (within 0.20 cm3) to calculate a mean. Calculate moles of the standard solution, use the mole ratio, then scale for any dilution.
- Yield and atom economy Percentage yield = actual/theoretical x 100. Atom economy = molar mass of desired product/total molar mass of reactants x 100, and high atom economy reduces waste.
- Uncertainty Percentage uncertainty = uncertainty/measured value x 100. For a burette reading of 0.05 cm3, a titre involves two readings, so its uncertainty is 0.10 cm3.
Common mistakes in Topic 5: Formulae, Equations and Amounts of Substance
- Using concordant results incorrectly by averaging all titres including the rough one.
- Forgetting to scale up when a 25.0 cm3 portion is taken from a 250 cm3 volumetric flask.
- Using 24.0 dm3 mol-1 for gases that are not at room temperature and pressure.
- Confusing atom economy with percentage yield; atom economy depends only on the balanced equation.
- Giving answers to too many significant figures; match the least precise data given.
Try three Topic 5: Formulae, Equations and Amounts of Substance exam-style questions
1. 25.0 cm3 of sodium hydroxide solution was neutralised by 21.40 cm3 of 0.100 mol dm-3 hydrochloric acid. Calculate the concentration of the sodium hydroxide. (2 marks)
Show the mark scheme answer
Moles HCl = 0.100 x 0.02140 = 2.14 x 10^-3 mol = moles NaOH; concentration = 2.14 x 10^-3/0.0250 = 0.0856 mol dm-3.
2. Calculate the atom economy for making ethanol by fermentation: C6H12O6 → 2C2H5OH + 2CO2. (2 marks)
Show the mark scheme answer
Mass of desired product = 2 x 46.0 = 92.0; atom economy = 92.0/180.0 x 100 = 51.1%.
3. A burette has an uncertainty of 0.05 cm3 per reading. Calculate the percentage uncertainty in a titre of 21.40 cm3. (1 mark)
Show the mark scheme answer
0.10/21.40 x 100 = 0.47%.
Topic 5: Formulae, Equations and Amounts of Substance FAQs
What does Edexcel A Level Chemistry Topic 5 cover?
It covers moles, formulae, equations, reacting masses, gas volumes, concentrations, titrations, yield, atom economy and measurement uncertainty.
Which paper is Topic 5 examined in?
Topic 5 is assessed in Papers 1 and 2, and the calculation and practical skills appear throughout Paper 3.
Which equations do I need for Topic 5?
n = m/M, c = n/V, molar gas volume 24.0 dm3 mol-1, pV = nRT, percentage yield, atom economy and percentage uncertainty.
