Formulas

AS Level Chemistry · Atoms, molecules and 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

The formula of an ionic compound must have no overall charge, so the positive and negative charges balance. The charge on a simple ion comes from its group. A Roman numeral gives the oxidation number of the metal, so iron(III) is Fe3+.

The empirical formula is the simplest whole-number ratio of atoms of each element. The molecular formula is the actual number of atoms of each element in one molecule. It is a whole-number multiple of the empirical formula.

An ionic equation shows only the particles that change. Spectator ions are left out. A hydrated salt contains water of crystallisation; an anhydrous salt contains none.

Rules to remember

Common mistake

Students divide the percentage by the proton number, or round a ratio such as 1.5 to 2. Divide by Ar, and multiply a ratio like 1 : 1.5 by 2 to get whole numbers.

Worked example

A hydrocarbon contains 85.7% carbon and 14.3% hydrogen by mass. Its Mr is 56. What is its molecular formula? (Ar: C 12.0, H 1.0)

  1. Moles: C = 85.7 ÷ 12.0 = 7.14; H = 14.3 ÷ 1.0 = 14.3.
  2. Divide by the smallest: C = 1, H = 14.3 ÷ 7.14 = 2.00. Empirical formula is CH2.
  3. Empirical formula mass = 12.0 + 2.0 = 14.0, and 56 ÷ 14.0 = 4.
  4. Molecular formula = 4 × CH2.

Answer: C4H8.

Practice questions

Try each one, then open the answer.

1. 2.50 g of hydrated copper(II) sulfate, CuSO4·xH2O, is heated to constant mass, leaving 1.60 g of anhydrous CuSO4 (Mr = 159.6). What is x?

  1. A
    5
  2. B
    7
  3. C
    4
  4. D
    3
Show answer

Answer: A. Moles CuSO4 = 1.60/159.6 = 0.0100; water lost = 2.50 − 1.60 = 0.90 g = 0.90/18.0 = 0.0500 mol. Ratio water : salt = 0.0500/0.0100 = 5, so x = 5.

2. What is the formula of iron(III) sulfate?

  1. A
    FeSO4
  2. B
    Fe(SO4)3
  3. C
    Fe3(SO4)2
  4. D
    Fe2(SO4)3
Show answer

Answer: D. Iron(III) is Fe3+ and sulfate is SO42−. Two Fe3+ (6+) balance three SO42− (6−). FeSO4 is iron(II) sulfate.

3. Which equation, with state symbols, correctly shows the reaction of marble chips with dilute hydrochloric acid?

  1. A
    CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
  2. B
    CaCO3(aq) + 2HCl(aq) → CaCl2(s) + H2O(l) + CO2(g)
  3. C
    CaCO3(s) + HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
  4. D
    CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(g) + CO2(aq)
Show answer

Answer: A. Marble (CaCO3) is an insoluble solid, calcium chloride is soluble, water is a liquid and carbon dioxide escapes as a gas. Two HCl are needed to balance the chlorine.

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