Detailed notes on formulae, equations and amount of substance including example tricky mole calculations. Notes written using Edexcel Chemistry textbooks, past papers and more. Written by a student with all A*s at GCSE, 3A* predictions at A Level and with an offer for Natural Sciences at Cambridge.
Topic 5: Formulae, Equations and Amounts of
Substance
1. Know that the mole (mol) is the unit for amount of a substance
A mole is an amount of substance containing a standard number of atoms, molecules or ions.
The symbol (n) is used for moles. One mole contains 6.02x1023 particles- these can be atoms,
molecules or ions. This is known as the Avogadro constant, L.
2. Be able to use the Avogadro constant, L, (6.02 × 1023 mol-1) in calculations
Number of particles you have
Moles=
Number of particles∈a mole
Example: How many moles of carbon are there in 1.5x24 C atoms?
n = 1.5x1024/6.02x1023 = 2.49 moles.
Example: How many atoms are there in one mole of oxygen?
Oxygen = O2
2(6.02x23) O atoms = 12.04x1023 atoms
3. Know that the molar mass of a substance is the mass per mole of the
substance in g mol-1
Molar mass (M) is the mass per mole of something. It is the same thing as relative molecular
mass (Mr), except is has a unit of grams per mole (g mol-1).
Mass
Moles=
Molar mass/ Mr
Example: Find the molar mass of CaCO3.
Mr = 40 + 12 + 3(16) = 100
M= 100 g mol-1
Example: How many Cl molecules are present in 71g of chlorine?
n = mMr = 71g35.5g mol-1 = 2 mol of Cl atoms
Hence 21 moles of Cl molecules 1 mol of Cl molecules
Example: How many atoms are in 8.5g of H2S?
Mols of H2S = mass/Mr = 8.5g/2(1) + 32 = 8.5g/34gmol -1= 0.25mol
Molecules of H2S in 8.5g = 0.25mol x 6.02x1023= 1.5x1023
There are three atoms in one molecule of H2S so 3(1.5x1023) = 4.5x1023 atoms
4. Know what is meant by the terms ‘empirical formula’ and ‘molecular
formula’
Empirical formula: The simplest whole number ratio of all atoms of each element in a
compound. (Used for ionic compounds as they are all giant e.g. NaCl, as well as giant covalent.)
Molecular formula: The actual number of atoms of each element in a molecule e.g. butane C 4H10.
(Used for simple covalent and molecular substances)
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