Edexcel GCSE Combined Science: Chemistry: Calculations and Quantitative Chemistry

A student burned a strip of magnesium in a crucible. The magnesium reacted with oxygen from the air to form magnesium oxide, and the student found that the mass of solid had increased.

A second student carried out the thermal decomposition of calcium carbonate in an open crucible. It broke down to form calcium oxide and carbon dioxide, and this time the mass of solid decreased.

Explain, using the idea of conservation of mass, why the solid gained mass in the first experiment but lost mass in the second. (6 marks)

A sample of an oxide of copper was analysed and found to contain 8.0 g of copper combined with 1.0 g of oxygen.

(Relative atomic masses: Cu = 64, O = 16.)

Calculate the empirical formula of this copper oxide. Show your working. (4 marks)

Calcium carbonate decomposes when heated, according to the balanced equation:

CaCO₃ → CaO + CO₂

(Relative atomic masses: Ca = 40, C = 12, O = 16.)

(a) Calculate the relative formula mass (Mᵣ) of calcium carbonate, CaCO₃. (1 mark)

(b) Calculate the maximum mass of calcium oxide (CaO) that could be made by completely decomposing 25 g of calcium carbonate. Show your working. (2 marks)

A student dissolved 30 g of potassium nitrate in water and made the solution up to a total volume of 0.50 dm³.

(a) Calculate the concentration of the solution in g/dm³. Show your working. (2 marks)

(b) The student then took 100 cm³ of this solution. State the mass of potassium nitrate present in this 100 cm³ sample. (1 mark)

In an experiment to make copper sulfate, the maximum (theoretical) mass of product that could be made was 16 g. The student actually obtained 12 g of copper sulfate.

Calculate the percentage yield of this experiment. Show your working. (2 marks)

A balanced symbol equation must have the same number of atoms of each element on both sides.

State the law that explains why the total mass of the products of a reaction is equal to the total mass of the reactants. (1 mark)