CCEA A Level Chemistry
CCEA A Level Chemistry 2.1 Formulae and Amounts of a Substance Exam Questions
Free CCEA A Level Chemistry revision for 2.1 Formulae and Amounts of a Substance: what you need to know, common mistakes and exam-style questions with answers. Covers empirical and molecular formulae, molar gas volume, reacting volumes of gases, percentage yield and atom economy.
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About CCEA A Level Chemistry 2.1 Formulae and Amounts of a Substance
Section 2.1 opens Unit AS 2 by extending the mole calculations from AS 1. You need to calculate empirical and molecular formulae from percentage or mass composition, use the molar gas volume to work out reacting volumes of gases, and calculate percentage yield and atom economy, including using a percentage yield to find how much reagent is needed. These calculations appear in almost every section of the course, from organic preparations to industrial processes in equilibrium. Accuracy, sensible significant figures and clear working are what examiners reward. Questions often combine several steps, for example finding a formula and then a mass of product.
What you need to know
- Empirical formula The simplest whole number ratio of atoms. Divide each mass or percentage by Ar, then divide by the smallest number; multiply up if needed to remove fractions such as 1.5 or 1.33.
- Molecular formula The actual number of atoms of each element in a molecule. Divide the Mr by the empirical formula mass and multiply the empirical formula by the result.
- Molar gas volume One mole of any gas occupies the same volume at the same temperature and pressure, 24 dm3 at room temperature and pressure. Moles of gas = volume / molar volume. For gases, volume ratios equal mole ratios.
- Percentage yield Percentage yield = actual yield / theoretical yield x 100. Yields are below 100% because of incomplete reactions, side reactions and losses during transfer and purification.
- Atom economy Atom economy = mass of desired product / total mass of all products x 100, using the balanced equation. Addition reactions have 100% atom economy.
Common mistakes in 2.1 Formulae and Amounts of a Substance
- Rounding a ratio such as 1.5 to 2 in an empirical formula. Multiply all values by 2 instead.
- Using the reactants' total mass incorrectly in atom economy. Use the total mass of all products (which equals total reactants) from the balanced equation, including coefficients.
- Confusing yield and atom economy. Yield measures practical efficiency; atom economy measures how much of the reactants end up in the desired product.
- Forgetting to use mole ratios for gas volume questions. For gases at the same conditions, volume ratio equals the ratio in the equation.
- Mixing up cm3 and dm3 with the molar volume. 24 dm3 is 24 000 cm3.
Try three 2.1 Formulae and Amounts of a Substance exam-style questions
1. A hydrocarbon contains 85.7% carbon. Its Mr is 56. Find its molecular formula. (3 marks)
Show the mark scheme answer
C: 85.7 / 12 = 7.14; H: 14.3 / 1 = 14.3; ratio 1 : 2 so CH2; 56 / 14 = 4 so C4H8.
2. Calculate the atom economy for making ethanol by fermentation: C6H12O6 gives 2C2H5OH + 2CO2. (2 marks)
Show the mark scheme answer
Mass ethanol = 2 x 46 = 92; total products = 92 + 88 = 180; 92 / 180 x 100 = 51.1%.
3. Calculate the volume of carbon dioxide at room temperature and pressure produced when 5.00 g of calcium carbonate reacts with excess acid. (2 marks)
Show the mark scheme answer
Moles CaCO3 = 5.00 / 100.1 = 0.0500; volume CO2 = 0.0500 x 24 = 1.20 dm3.
2.1 Formulae and Amounts of a Substance FAQs
What does CCEA A Level Chemistry 2.1 Formulae and Amounts of a Substance cover?
It covers empirical and molecular formulae, molar gas volume and reacting volumes, percentage yield and atom economy.
Which unit is 2.1 examined in?
It is examined in Unit AS 2: Further Physical and Inorganic Chemistry and an Introduction to Organic Chemistry, a 1 hour 30 minute paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit AS 3.
What equations do I need for 2.1?
Moles = mass / Mr, moles of gas = volume / molar volume, percentage yield = actual / theoretical x 100, and atom economy = mass of desired product / total mass of products x 100.
