A laboratory compares the magnesium content of three compounds. One compound is magnesium hydroxide, Mg(OH)2\mathrm{Mg(OH)_{2}}Mg(OH)2.
Relative atomic masses: H = 1, O = 16, Mg = 24
| Compound | Percentage by mass of magnesium (%) |
|---|---|
| magnesium hydroxide | to be calculated |
| magnesium carbonate | 28.6 |
| magnesium oxide | 60.0 |
How many oxygen atoms are shown in the formula Mg(OH)2\mathrm{Mg(OH)_{2}}Mg(OH)2?
Work out the Mr M_r\,Mr of Mg(OH)2\mathrm{Mg(OH)_{2}}Mg(OH)2 by adding together the relative atomic masses of every atom in the formula.
Find the value missing from the table for magnesium hydroxide. Divide the Ar A_r\,Ar of magnesium by your answer to (b), then multiply by 100.
Use your answer to (c) and the table. Give the name of the compound with the greatest percentage by mass of magnesium.
142 exam-style questions on Edexcel GCSE Chemistry 2.6 Calculations involving masses, covering 2.6.1 Relative formula mass and percentage composition, 2.6.2 Empirical and molecular formulae, 2.6.3 Conservation of mass, 2.6.4 Reacting masses and concentration in g dm⁻³, 2.6.5 The mole and the Avogadro constant, and 2.6.6 Limiting reactants and stoichiometry. Each one has a worked solution and a mark scheme showing where the marks go.