OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2017: Question 5

1 mark · Medium difficulty · Multiple Choice

Calculate the volume of hydrochloric acid required to react completely with a given amount of calcium oxide.

Practise this question

Question

Multiple-choice question 5 asks for the volume in cm3 of 2.00 mol dm-3 HCl required to react completely with 0.0200 mol of calcium oxide. Four options are provided: A (15), B (20), C (30), and D (60), along with a box for the answer and a mark allocation of [1].
Question text

5 0.0200 mol of calcium oxide is reacted completely with 2.00 mol dm–3 HCl.

What is the volume, in cm3, of 2.00 mol dm–3 HCl required for this reaction?

A 15

B 20

C 30

D 60

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme for question 5 indicates the correct answer is B, awarding 1 mark.

5 B 1 ALLOW 20 in the box

How to answer it

Difficulty: Easy

Calculating Solution Volume from Moles

What this question tests

This question assesses your ability to write a balanced neutralisation equation, apply stoichiometric molar ratios, and convert between moles, concentration, and volume in cubic centimetres (cm³).

Question 5 Solution Guide

Multiple Choice Part (Option B)

✅ Correct Answer

B (20)

The required volume of hydrochloric acid is 20 cm³ .

💡 Key Knowledge

  • Metal oxides react with acids to form a salt and water: CaO + 2HCl → CaCl₂ + H₂O
  • The stoichiometric ratio of calcium oxide to hydrochloric acid is 1 : 2 .
  • Concentration formula: Moles = Concentration × Volume (in dm³)

🧠 Exam Technique

Always write out the balanced chemical equation first to secure the correct molar ratio. Watch out for unit conversions between dm³ and cm³ (multiply by 1000 at the final step).

❌ Common Errors

  • 1:1 ratio trap: Forgetting the coefficient of 2 in front of HCl , leading to a calculated volume of 10 cm³ (Option A).
  • Unit conversion omission: Forgetting to multiply by 1000 , leaving the volume in dm³ ( 0.020 dm³ ).

📐 Step-by-Step Calculation

  1. Step 1: Write the balanced equation
    CaO (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l)
  2. Step 2: Determine moles of HCl required using the 1:2 ratio
    Moles of CaO = 0.0200 mol
    Moles of HCl = 0.0200 × 2 = 0.0400 mol
  3. Step 3: Rearrange the concentration formula to find volume in dm³
    Volume (dm³) = Moles / Concentration = 0.0400 / 2.00 = 0.0200 dm³
  4. Step 4: Convert volume into cm³
    Volume (cm³) = 0.0200 × 1000 = 20 cm³
Mark Scheme Note: Award 1 mark for selecting option B.

Topics

Module 2: Foundations in chemistry · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.