OxfordAQA International A Level Chemistry
OxfordAQA International A Level Chemistry 3.1.2 Amount of Substance Exam Questions
Free OxfordAQA International A Level Chemistry revision for 3.1.2 Amount of Substance: what you need to know, common mistakes and exam-style questions with answers. Covers the mole, concentration, the ideal gas equation, empirical formulae, percentage yield, atom economy and titrations.
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About OxfordAQA International A Level Chemistry 3.1.2 Amount of Substance
Amount of substance is the quantitative core of the course and is examined in Unit 1, though its calculations appear in every unit. You need to define relative atomic and molecular mass, use the mole and the Avogadro constant, calculate concentrations in mol dm-3, and use the ideal gas equation pV = nRT in SI units. You also need to find empirical and molecular formulae, write balanced and ionic equations, and calculate percentage yield and atom economy. Required practical 1, making up a volumetric solution and carrying out an acid-base titration, belongs here. Examiners regularly set multi-step calculations where unit conversions cost marks.
What you need to know
- Relative masses Ar and Mr are measured relative to one twelfth of the mass of a carbon-12 atom. Relative formula mass is used for ionic compounds.
- The mole Amount in mol = mass / Mr. The Avogadro constant (6.022 x 10^23 mol-1) is the number of particles in one mole; its value is given when needed.
- Concentration Concentration in mol dm-3 = amount in mol / volume in dm3. Divide a volume in cm3 by 1000 to convert it to dm3.
- Ideal gas equation pV = nRT with p in Pa, V in m3, n in mol, R = 8.31 J K-1 mol-1 (given) and T in K. Convert cm3 to m3 by multiplying by 10^-6 and degrees Celsius to kelvin by adding 273.
- Formulae and equations Empirical formula is the simplest whole number ratio of atoms; molecular formula is the actual number. Ionic equations show only the species that react.
- Yield and atom economy Percentage yield = actual yield / theoretical yield x 100. Atom economy = Mr of desired product / sum of Mr of all reactants x 100; high atom economy means less waste.
Common mistakes in 3.1.2 Amount of Substance
- Using cm3 in pV = nRT. Volume must be in m3 and pressure in Pa.
- Mixing up percentage yield and atom economy. Yield depends on the experiment; atom economy depends only on the balanced equation.
- Rounding too early in multi-step calculations. Keep extra significant figures until the final answer.
- Using the wrong mole ratio from the equation in titration calculations.
Try three 3.1.2 Amount of Substance exam-style questions
1. 25.0 cm3 of sodium hydroxide was neutralised by 21.40 cm3 of 0.100 mol dm-3 hydrochloric acid. Calculate the concentration of the sodium hydroxide. (3 marks)
Show the mark scheme answer
Moles HCl = 0.100 x 21.40 / 1000 = 2.14 x 10^-3 mol; ratio 1:1 so moles NaOH = 2.14 x 10^-3 mol; concentration = 2.14 x 10^-3 / 0.0250 = 0.0856 mol dm-3.
2. Calculate the volume in cm3 occupied by 0.0200 mol of gas at 100 kPa and 298 K. R = 8.31 J K-1 mol-1. (3 marks)
Show the mark scheme answer
V = nRT / p = 0.0200 x 8.31 x 298 / 100 000; = 4.95 x 10^-4 m3; = 495 cm3.
3. A compound contains 85.7% carbon and 14.3% hydrogen by mass. 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 empirical formula CH2 (Mr 14); 56 / 14 = 4 so molecular formula C4H8.
3.1.2 Amount of Substance FAQs
What does OxfordAQA International A Level Chemistry 3.1.2 Amount of Substance cover?
It covers the mole, concentration, the ideal gas equation, empirical formulae, percentage yield, atom economy and titrations.
Which unit is 3.1.2 Amount of Substance examined in?
It is examined in Unit 1, the first International AS paper, which covers inorganic and physical chemistry. The Unit 5 practical and synoptic paper at A2 can also assess it.
What equations do I need for amount of substance?
Learn n = m / Mr, concentration = n / V, pV = nRT, percentage yield and atom economy. The values of the Avogadro constant and R are given, but you must use the correct units.
