Chemistry › Amount of substance
Amount of substance
Unit C02 · 5 lessons.
6.022 × 10²³ particles make a mole, and almost every calculation in the course is built from the steps here: moles from a mass, from a gas volume with pV = nRT, and from a concentration and a titre, then a mole ratio from the balanced equation, then back to the quantity asked for. Empirical formulae, percentage yield and atom economy are the same arithmetic pointed at different questions.
Marks are lost in the units rather than the chemistry. In pV = nRT the pressure must be in pascals and the volume in cubic metres, so a volume given in cm³ is multiplied by 10⁻⁶ and one in dm³ by 10⁻³, and a temperature in degrees Celsius has 273 added before anything else is done. Write each conversion as its own line and the method marks survive even when the arithmetic slips.
The papers that set it are AQA Papers 1, 2 and 3, OCR A Papers 1, 2 and 3, Edexcel Papers 1, 2 and 3 and Cambridge Papers 1 and 2.
Specification sections: AQA 3.1.2.1, 3.1.2.2, 3.1.2.3, 3.1.2.4, 3.1.2.5, 3.3.14 · OCR A 2.1.3 · Edexcel 1.6 to 1.7, 5.1 to 5.14, 18.18 · CAIE 2.1, 2.2, 2.3, 2.4, 4.1
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- The mole, the Avogadro constant and molar mass
The mole as the unit of amount of substance, the Avogadro constant, relative molecular mass and relative formula mass, molar mass and n = m/M, counting particles with n × L, moles of atoms and ions within a compound, the mass of a single atom or molecule, reacting masses from a balanced equation, and finding the water of crystallisation in a hydrated salt by heating to constant mass or from its percentage of water.
- Gases and the ideal gas equation
The ideal gas equation pV = nRT and the units it needs, converting cm³ and dm³ to m³, kPa to Pa and °C to K, rearranging it for n, V, p or T, absolute zero and the Kelvin scale, the kinetic theory assumptions for an ideal gas and when real gases are not ideal, the molar gas volume of 24.0 dm³ mol⁻¹ at room temperature and pressure, reacting volumes of gases and the formula of a hydrocarbon from its combustion volumes, measuring the molar volume, and finding the relative molecular mass of a volatile liquid with a gas syringe.
- Empirical and molecular formulae
The empirical formula as the simplest whole-number ratio of atoms, finding it from masses or from percentage composition by dividing by Ar and then by the smallest, ratios such as 1 : 1.5 that must be multiplied up rather than rounded, the molecular formula from Mr, including an Mr from pV = nRT, combustion analysis to find the masses of carbon and hydrogen from carbon dioxide and water and why combustion must be complete, and how empirical, molecular, structural and displayed formulae differ, with structural isomers.
- Concentration, volumetric solutions and titration calculations
Concentration in mol dm⁻³, g dm⁻³ and parts per million, n = cV with the volume in dm³, dilution, primary standards and making a standard solution in a volumetric flask, deducing a reacting ratio from a titration, indicators and the end point, rough and concordant titres and the mean titre, calculating a concentration from a titre with the mole ratio, back titrations, the uncertainty in a titre, and AQA required practical 1 in full with its errors.
- Reacting masses, percentage yield and atom economy
Reacting mass calculations from a balanced equation, the limiting reagent and reagents in excess, theoretical and percentage yield, where yield is lost in a preparation and over several steps, atom economy and why it is not the same as yield, why addition reactions have an atom economy of 100%, and the economic and environmental case for high atom economy, with the two industrial routes to ibuprofen, and why chemists aim for processes that need no solvent.
Key definitions for this unit
Practical work
The practical capabilities this unit's work belongs to, each with its techniques, its calculations and the errors that cost marks.
Worked in full, from the apparatus to the evaluation:
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