OCR A-Level Chemistry AS Breadth in chemistry (01), June 2022: Question 8

1 mark · Medium difficulty · Multiple Choice

Calculate the volume of concentrated hydrochloric acid required to prepare a diluted solution of specified concentration and volume.

Practise this question

Question

Multiple choice question 8 asking for the volume of 18.0 mol dm-3 hydrochloric acid needed to dilute to 250.0 cm3 to prepare a 0.450 mol dm-3 solution. Four options are provided: A 4.50 cm3, B 6.25 cm3, C 10.0 cm3, and D 32.4 cm3, along with an answer box and [1] mark allocation.
Question text

8 Which volume of 18.0 mol dm–3 hydrochloric acid should be diluted to 250.0 cm3 to prepare a

0.450 mol dm–3 solution of hydrochloric acid?

A 4.50 cm3

B 6.25 cm3

C 10.0 cm3

D 32.4 cm3

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer for question 8 is option B, worth 1 mark.

8 B 1 AO2.4

How to answer it

Solution Dilution Calculation

What this question tests

This question assesses your ability to apply dilution formulas for solutions. You must calculate the required volume of a concentrated stock solution needed to prepare a specific volume and concentration of a diluted solution, demonstrating fluency with concentration units (mol dm⁻³) and volume conversions (cm³ to dm³).

Question 8

Exam Breakdown & Step-by-Step Solution

✅ Correct Answer

The correct option is B: 6.25 cm³

Mark Awarded: 1 / 1 (AO2.4 - Applying chemical knowledge to calculation)

💡 Key Knowledge

  • Dilution Formula: c₁V₁ = c₂V₂ (where moles of solute remain constant before and after dilution).
  • Alternatively, calculate moles required using moles = concentration × volume .
  • Volumes can stay in cm³ as long as both sides of the equation use the exact same unit.

📐 Step-by-Step Calculation

  1. Step 1: Identify the variables given in the question.
    c₁ = 18.0 mol dm⁻³
    c₂ = 0.450 mol dm⁻³
    V₂ = 250.0 cm³
  2. Step 2: Rearrange the dilution formula to solve for V₁ .
    V₁ = (c₂ × V₂) / c₁
  3. Step 3: Substitute the values into the equation.
    V₁ = (0.450 × 250.0) / 18.0
  4. Step 4: Calculate the final answer.
    V₁ = 112.5 / 18.0 = 6.25 cm³

🧠 Exam Technique

In multiple-choice questions involving calculations, double-check your rearrangement before punching numbers into your calculator. Check that your calculated volume makes logical sense: a concentrated acid diluted down to a much lower concentration will require a very small volume of the stock solution, ruling out larger distractors.

❌ Common Errors & Distractors

  • Unit conversion trap: Unnecessarily converting 250.0 cm³ into dm³ (0.250 dm³) and forgetting to convert back or mix-matching units can throw off powers of 10.
  • Inverted formula: Dividing c₁ by (c₂ × V₂) leads to distractor answers like option A ( 4.50 cm³ ).
  • Arithmetic mix-ups: Multiplying the wrong concentration pairs leads students straight to incorrect options like C ( 10.0 cm³ ) or D ( 32.4 cm³ ).

Topics

Module 2: Foundations in chemistry · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.