Salt preparations
5 marks · HardYou are asked to prepare a pure, dry sample of magnesium sulfate in the laboratory using insoluble magnesium carbonate and dilute sulfuric acid.
Explain why magnesium carbonate and sulfuric acid can be used to prepare magnesium sulfate by this method.
Describe the steps you would take to obtain a pure, dry sample of magnesium sulfate from these reactants.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Magnesium carbonate is insoluble in water and reacts with sulfuric acid to form soluble magnesium sulfate and water (plus carbon dioxide), allowing separation of the desired salt from excess base by filtration.
(b) Add excess magnesium carbonate to dilute sulfuric acid until no more reacts. Filter to remove unreacted solid, then heat the filtrate to evaporate water, allow to crystallise, filter the crystals, and dry them between filter papers.
Also accept: Alternative wording for steps, so long as method gives pure, dry salt
Common mistakes: Using soluble bases instead of insoluble ones or failing to wash or dry crystals
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Identifies that magnesium carbonate is insoluble and suitable for this reaction.
M2 (a) · 1 mark · States that magnesium sulfate formed is soluble and can be separated from unreacted magnesium carbonate by filtration.
M3 (b) · 1 mark · Describes adding excess magnesium carbonate to sulfuric acid and stirring/heating to complete reaction until no more dissolves.
M4 (b) · 1 mark · Filters the mixture to remove unreacted magnesium carbonate.
M5 (b) · 1 mark · Describes evaporating water from the filtrate and collecting/drying magnesium sulfate crystals appropriately (crystallisation and drying with filter paper or in a warm place).
Worked solution
a) Magnesium carbonate is insoluble in water but reacts with sulfuric acid to form soluble magnesium sulfate. This allows the excess solid to be removed by filtration.
b) Add excess magnesium carbonate to dilute sulfuric acid and stir. When no more reacts (no more bubbles), filter to remove unreacted solid. Heat the filtrate to evaporate some water, allow it to cool and form crystals, then filter and dry the magnesium sulfate crystals between filter papers.