Edexcel International A Level Chemistry

Topic 1: Formulae, Equations and Amount of Substance

The toolkit for the whole course. Every calculation you will ever do in chemistry runs through one quantity, the amount of substance in moles.

The one idea

You cannot count atoms, but you can weigh them. The mole is the exchange rate between the mass on a balance and the number of particles in a reaction. Once you have an amount in moles, the balanced equation tells you the amount of everything else.

Almost every question in this topic is the same three moves:

1. Into moles from whatever you were given (a mass, a solution, a gas volume).
2. Across the equation using the mole ratio.
3. Out of moles into whatever you were asked for.

How to use these notes

• Read the pages in order the first time. Later pages assume the earlier ones.
• Each page opens with its one idea in a sentence. If you can explain that sentence to someone else, you know the page.
• Coloured boxes mean the same thing on every page: definitions you may be asked to quote, key ideas, common mistakes, exam technique, and beyond the spec (interesting, but you will not be examined on it).
• Each page ends with questions and fully worked answers. Cover the answers, try the questions, then mark yourself.
• Use the topic on a page the night before an exam.

Relative atomic masses used in these notes are the values printed on the Edexcel IAL periodic table, to one decimal place: H 1.0, C 12.0, N 14.0, O 16.0, Na 23.0, Mg 24.3, Al 27.0, S 32.1, Cl 35.5, K 39.1, Ca 40.1, Fe 55.8, Cu 63.5, Zn 65.4, Br 79.9, Ag 107.9, Ba 137.3.

Pages

1. Particles and formulae

   A formula is a count of atoms. Before you can calculate anything, you need to know what kind of particle a substance is made of and what its formula is counting.

   8 min 1.1

2. Relative mass and the mole

   Relative masses compare atoms with carbon-12. The mole turns those relative masses into grams, so that weighing becomes a way of counting.

   14 min 1.4(i)1.4(ii)1.21.4(iii)

3. Writing equations

   A balanced equation is a recipe in moles. It must balance in atoms and, for ionic equations, in charge, and it must say what state each substance is in.

   12 min 1.3

4. Masses from equations

   Masses are not in the ratio of the coefficients; moles are. Go into moles, use the equation, come back out, and the same idea turns analysis data into a formula.

   15 min 1.71.6

5. Solutions and parts per million

   Concentration is just amount per volume. Convert the volume to dm³ first, and a solution becomes another door into the mole.

   11 min 1.51.4(iv)

6. Gas volumes

   At the same temperature and pressure, a mole of any gas takes up the same volume. Use 24.0 dm³ at room conditions, and pV = nRT in SI units for anything else.

   15 min 1.8(i)1.8(ii)1.8(iii)

7. Core Practical 1: molar volume of a gas

   React known masses of calcium carbonate with excess acid, collect the carbon dioxide, and the gradient of volume against moles is the molar volume.

   13 min 1.111.10

8. Yield and atom economy

   Percentage yield measures how well an experiment went. Atom economy measures how much of the reactants could ever end up in the product, and it comes from the equation alone.

   10 min 1.9

9. Formulae and equations by experiment

   To test a formula or an equation, predict what each candidate would give, measure what actually happens, and compare in moles. Then judge how far to trust the measurement.

   12 min 1.101.6

10. Test-tube reactions

    Every observation is evidence for a species in the ionic equation. A colour fades because an ion is used up, a precipitate appears because an insoluble solid forms, and fizzing means a gas.

    14 min 1.12(i)1.12(ii)1.12(iii)

Also in this topic: Topic 1 on a page

Spec checklist

Every point you can be examined on in this topic, and where it is explained. Tick them off as you go.