Formulae

O Level / IGCSE Chemistry · Stoichiometry. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

A formula shows which atoms are in a substance and how many of each. The molecular formula gives the actual number of each type of atom in one molecule. The empirical formula gives the simplest whole number ratio of the atoms or ions.

For an ionic compound, choose the numbers of each ion so that the positive and negative charges cancel. Use brackets when more than one of an ion made of several atoms is needed, as in Ca(OH)2.

A symbol equation must have the same number of each type of atom on both sides. Balance it by putting numbers in front of the formulae, then add state symbols. An ionic equation shows only the particles that actually change.

Rules to remember

Common mistake

Never change the small numbers inside a formula to make an equation balance, because that changes the substance. Only put numbers in front of the formulae.

Worked example

Magnesium ribbon reacts with dilute hydrochloric acid to form magnesium chloride solution and hydrogen. Write the balanced symbol equation with state symbols. (Mg2+, Cl−)

  1. Formula of magnesium chloride: one Mg2+ needs two Cl− for the charges to cancel, so it is MgCl2.
  2. Unbalanced: Mg + HCl → MgCl2 + H2. The right side has 2 Cl and 2 H, the left has 1 of each.
  3. Put 2 in front of HCl: Mg + 2HCl → MgCl2 + H2. Now each side has 1 Mg, 2 H and 2 Cl.
  4. Add state symbols: the metal is solid, the acid and the salt are in solution, and hydrogen is a gas.

Answer: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

Practice questions

Try each one, then open the answer.

1. What is the formula of aluminium sulfate? (aluminium ion Al3+, sulfate ion SO42−)

  1. A
    Al3(SO4)2
  2. B
    Al(SO4)3
  3. C
    Al2(SO4)2
  4. D
    Al2(SO4)3
Show answer

Answer: D. Two Al3+ ions (6+) balance three SO42− ions (6−), giving Al2(SO4)3. The ion is written in brackets when more than one is needed.

2. A hydrocarbon has the empirical formula C2H5 and a relative molecular mass of 58. What is its molecular formula? (H 1, C 12)

  1. A
    C2H5
  2. B
    C3H7
  3. C
    C4H10
  4. D
    C5H12
Show answer

Answer: C. The empirical formula mass of C2H5 is 24 + 5 = 29. 58 ÷ 29 = 2, so the molecular formula is C4H10 (butane).

3. Which state symbol should follow the formula of the water produced when hydrogen burns in oxygen at room temperature and pressure?

  1. A
    (l)
  2. B
    (s)
  3. C
    (g)
  4. D
    (aq)
Show answer

Answer: A. At room temperature water is a liquid: H2O(l). (aq) means dissolved in water.

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