A student carries out an experiment to find the empirical formula of magnesium oxide.
The student heats a piece of magnesium ribbon in a crucible with a lid. The student lifts the lid slightly from time to time during heating. The table shows the student's results.
| Measurement | Mass / g |
|---|---|
| crucible and lid | 24.60 |
| crucible, lid and magnesium before heating | 24.84 |
| crucible, lid and magnesium oxide after heating | 25.00 |
Relative atomic masses: Mg = 24, O = 16
Give a reason why the student lifts the lid from time to time during heating.
Give a reason why the lid is kept on the crucible for most of the time.
Use the results in the table.
Calculate the mass of magnesium used.
Calculate the mass of oxygen that combined with the magnesium.
Determine the empirical formula of magnesium oxide. You must show your working.
Another student does the same experiment. Their mass of oxygen is lower than expected. Suggest one reason for this.
55 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.