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Using concentrations of solutions in mol/dm\\\\u00b3 (chemistry only) (HT only)

Practise 20 exam-style questions on Using concentrations of solutions in mol/dm\\\\u00b3 (chemistry only) (HT 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.

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20 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

Calculate the amount, in moles, of hydrochloric acid in 25.0 cm³ of 0.200 mol/dm³ hydrochloric acid.

Model answer

volume = 25.0 ÷ 1000 = 0.0250 dm³
amount = concentration × volume = 0.200 × 0.0250 = 0.00500 mol

Explanation

Amount in moles = concentration in mol/dm³ × volume in dm³. Always convert cm³ to dm³ by dividing by 1000 first; using 25.0 directly gives an answer 1000 times too big.

Mark scheme
  1. 1 mark

    volume = 25.0 ÷ 1000 = 0.0250 dm³

    allow 0.200 × 25.0 ÷ 1000

  2. 1 mark

    0.00500 (mol)

    allow 5 × 10⁻³; allow 0.005; correct answer with no working scores 2 marks

Question 2

2 marks

Solution A is 50.0 cm³ of 0.400 mol/dm³ copper(II) sulfate.
Solution B is 200 cm³ of 0.0800 mol/dm³ copper(II) sulfate.

Show by calculation which solution contains the greater amount of copper(II) sulfate in moles.

Model answer

A: moles = 0.400 × (50.0 ÷ 1000) = 0.0200 mol
B: moles = 0.0800 × (200 ÷ 1000) = 0.0160 mol
Solution A contains more copper(II) sulfate.

Explanation

The amount of solute depends on both the concentration and the volume. Solution B has the larger volume, but A is five times more concentrated, so A contains more moles. Do not judge by concentration or volume alone.

Mark scheme
  1. 1 mark

    A: 0.400 × 0.0500 = 0.0200 mol

  2. 1 mark

    B: 0.0800 × 0.200 = 0.0160 mol, so A contains more

    conclusion must be consistent with the calculated values

Question 3

3 marks

Explain how the concentration of a solution in mol/dm³ is related to the mass of solute dissolved and the volume of the solution.

Model answer

The mass of solute is converted into moles by dividing by its relative formula mass. The concentration is then the number of moles divided by the volume of the solution in dm³. So dissolving a greater mass of solute in the same volume gives a higher concentration, while dissolving the same mass in a larger volume gives a lower concentration.

Explanation

Concentration in mol/dm³ = (mass ÷ Mr) ÷ volume in dm³. Doubling the mass doubles the concentration; doubling the volume halves it. This relationship lets you calculate any one of mass, volume or concentration if you know the other two and the Mr.

Mark scheme
  1. 1 mark

    the mass of solute is converted to an amount in moles by dividing by its relative formula mass (Mr)

    allow moles = mass ÷ Mr

  2. 1 mark

    concentration = amount (moles) ÷ volume of solution in dm³

    allow concentration = mass ÷ (Mr × volume)

  3. 1 mark

    a greater mass of solute gives a higher concentration, and a larger volume of solution (for the same mass) gives a lower concentration

    allow concentration is proportional to mass and inversely proportional to volume

Question 4

4 marks

Magnesium reacts with hydrochloric acid:
Mg + 2HCl → MgCl₂ + H₂

Calculate the volume of 2.0 mol/dm³ hydrochloric acid, in cm³, needed to react exactly with 1.2 g of magnesium.
Relative atomic mass (Ar): Mg = 24

Model answer

moles of Mg = 1.2 ÷ 24 = 0.050 mol
From the equation, 1 mol Mg reacts with 2 mol HCl, so moles of HCl = 0.10 mol
volume = moles ÷ concentration = 0.10 ÷ 2.0 = 0.050 dm³
volume = 0.050 × 1000 = 50 cm³

Explanation

Rearranging concentration = moles ÷ volume gives volume = moles ÷ concentration. The answer comes out in dm³, so multiply by 1000 to give cm³, and remember the 1 : 2 ratio from the balanced equation.

Mark scheme
  1. 1 mark

    moles of Mg = 1.2 ÷ 24 = 0.050

  2. 1 mark

    moles of HCl = 2 × 0.050 = 0.10

    allow ecf

  3. 1 mark

    volume = 0.10 ÷ 2.0 = 0.050 dm³

    allow ecf

  4. 1 mark

    50 (cm³)

    allow ecf; 25 cm³ (ratio not used) scores 3 marks; correct answer with no working scores 4 marks

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