Sulfates (chemistry only) - AQA GCSE Chemistry Exam Questions

Practise 18 exam-style questions on Sulfates (chemistry only) for AQA GCSE Chemistry. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.

18 questions
1–6 marks each
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18 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

A student acidifies a solution with dilute sulfuric acid, instead of dilute hydrochloric acid, before adding barium chloride solution.

Explain why this is a mistake.

Model answer

Sulfuric acid contains sulfate ions. These would react with barium chloride to form a white precipitate of barium sulfate even if the sample contained no sulfate ions, so the result would be meaningless.

Explanation

The acid used in a test must not contain the ion being tested for. That is why hydrochloric acid is used for the sulfate test and nitric acid is used for the halide test.

Mark scheme

1 mark
sulfuric acid contains / adds sulfate ions
1 mark
so a white precipitate (of barium sulfate) would form whether or not the sample contained sulfate ions
allow gives a false positive result

Question 2

2 marks

Soluble barium compounds are toxic. However, patients drink a suspension of barium sulfate (a 'barium meal') before some X-ray scans.

Suggest why barium sulfate is safe to swallow.

Model answer

Barium sulfate is insoluble in water, which is why it forms a precipitate in the sulfate test. Because it does not dissolve, the barium ions are not absorbed into the blood, and the barium sulfate passes out of the body.

Explanation

The toxicity of barium compounds depends on dissolved barium ions getting into the body. The insolubility that makes barium sulfate useful as a test precipitate also makes it safe in a barium meal.

Mark scheme

1 mark
barium sulfate is insoluble (in water)
1 mark
so (barium ions) are not absorbed into the blood / body
allow so it passes through the digestive system without dissolving

Question 3

3 marks

A student adds barium chloride solution to solution X without adding any acid first. A white precipitate forms. The student concludes: "Solution X contains sulfate ions."

Evaluate the student's conclusion and describe how the test should be done.

Model answer

The conclusion is not justified, because carbonate ions also form a white precipitate (barium carbonate) with barium chloride. The student should first add dilute hydrochloric acid, which removes any carbonate ions, and then add barium chloride solution. If a white precipitate still forms, solution X contains sulfate ions.

Explanation

Without the acid the test is not specific to sulfate. Acidifying first is what makes the white precipitate a reliable positive result.

Mark scheme

1 mark
not justified because carbonate ions (if present) would also form a white precipitate
allow barium carbonate is also white / insoluble
1 mark
add dilute hydrochloric acid before the barium chloride
1 mark
a white precipitate in the presence of the acid shows sulfate ions
allow if the precipitate still forms after acid is added, sulfate is present

Question 4

4 marks

Excess barium chloride solution is added to 40.0 cm³ of 0.150 mol/dm³ sodium sulfate solution.

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)

Relative atomic masses (Ar): O = 16, S = 32, Ba = 137

Calculate the mass of barium sulfate precipitate formed, giving the answer to 3 significant figures.

Model answer

moles of Na₂SO₄ = (40.0 ÷ 1000) × 0.150 = 0.00600 mol
1 mol Na₂SO₄ gives 1 mol BaSO₄, so moles of BaSO₄ = 0.00600 mol
Mr of BaSO₄ = 137 + 32 + (4 × 16) = 233
mass = 0.00600 × 233 = 1.398 g = 1.40 g (3 significant figures)

Explanation

The barium chloride is in excess, so the amount of sulfate fixes the amount of precipitate. Note that 1.40 has three significant figures; writing 1.4 would lose the final mark.

Mark scheme

1 mark
moles of Na₂SO₄ = 0.0400 × 0.150 = 0.006(00) mol
1 mark
moles of BaSO₄ = 0.006(00) mol and Mr of BaSO₄ = 233
allow ecf
1 mark
mass = 0.00600 × 233 = 1.398 g
allow ecf
1 mark
1.40 (g)
3 significant figures needed for this mark; allow ecf for an incorrect value correctly rounded; correct answer with no working scores 4 marks

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