Salt preparations
4 marks · NormalA student wants to prepare a pure, dry sample of barium sulfate in the laboratory. Barium sulfate is insoluble in water.
Name two suitable soluble solutions that could be mixed to prepare barium sulfate.
Describe the steps the student should follow to obtain pure, dry barium sulfate from the mixture after the reaction is complete.
Explain why filtration is necessary in this method.
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(a) Any pair from: sodium sulfate + barium chloride; potassium sulfate + barium chloride; ammonium sulfate + barium chloride. · must show Both reactants must be soluble. One provides Ba²⁺, the other SO₄²⁻.
(b) Mix the two solutions, filter the mixture to separate the solid barium sulfate from the solution, wash the residue with distilled water, then dry it in a warm oven. · must show mix solutions, filter, wash residue, dry solid
(c) To separate the insoluble barium sulfate from the solution. · must show separates solid from liquid
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Marking points
B1 (a) · 1 mark · Correctly names two suitable soluble compounds (e.g. sodium sulfate and barium chloride) that will react to form barium sulfate.
M1 (b) · 1 mark · States mixing solutions and filtering the mixture to separate the solid.
M2 (b) · 1 mark · Describes washing the residue with distilled water and drying it.
E1 (c) · 1 mark · Explains that filtration is for separating the insoluble solid from the liquid.
Worked solution
a) Suitable reactants are solutions of barium chloride and sodium sulfate (other combinations: ammonium sulfate, potassium sulfate, etc. with barium chloride).
b) Step 1: Mix the two solutions together to allow barium sulfate to form as a precipitate.
Step 2: Filter the mixture to separate the solid barium sulfate from the filtrate.
Step 3: Wash the residue with distilled water to remove impurities.
Step 4: Dry the solid barium sulfate in a warm oven to obtain a pure, dry sample.
c) Filtration is needed to separate the insoluble barium sulfate from the rest of the liquid mixture.