OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2020: Question 4

1 mark · Medium difficulty · Multiple Choice

Calculate the mass of calcium hydroxide that completely neutralises a given volume and concentration of phosphoric acid.

Practise this question

Question

Multiple choice question 4. Phosphoric acid is a tribasic acid. What is the mass of Ca(OH)2 that completely neutralises 100 cm3 of 0.100 mol dm-3 phosphoric acid? Four options are given: A 0.49 g, B 0.74 g, C 1.11 g, D 2.22 g. A box is provided for the answer.
Question text

4 Phosphoric acid is a tribasic acid.

What is the mass of Ca(OH) that completely neutralises 100 cm3 of 0.100 mol dm−3 phosphoric

acid?

A 0.49 g

B 0.74 g

C 1.11 g

D 2.22 g

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 4 showing the correct answer as C with 1 mark.

4 C 1 2.2

How to answer it

Neutralisation & Stoichiometry Calculation

What this question tests

This question assesses your ability to calculate moles from concentration and volume, interpret acid-base stoichiometry using acidity (tribasic acids), construct balanced equations, and convert moles into mass using molar mass.

Question 4 — Multiple Choice [1 Mark]

Calculating Mass in Neutralisation

✅ Correct Answer: C (1.11 g)

Option C is the correct choice because following the correct stoichiometric ratio yields exactly 0.0150 moles of calcium hydroxide, which corresponds to a mass of 1.11 g.

💡 Key Knowledge

  • Tribasic acid: Each mole of phosphoric acid (H₃PO₄) releases 3 moles of H⁺ ions in aqueous solution.
  • Base formula: Calcium hydroxide is Ca(OH)₂ , meaning each formula unit provides 2 moles of OH⁻ ions.
  • Mole equation: Moles = (Concentration × Volume) / 1000 when volume is in cm³.

🧠 Exam Technique

Don't guess multiple-choice calculations! Always write down a quick balanced equation to identify the reacting mole ratio before doing your math. Watch out for distractors created by missing the 2:3 ratio.

❌ Common Errors

  • Assuming a 1:1 molar ratio between the acid and base.
  • Forgetting to convert cm³ to dm³ when calculating moles.
  • Using the wrong molar mass for Ca(OH)₂ (forgetting to multiply O and H by 2: 40.1 + (16.0 + 1.0) × 2 = 74.1 g mol⁻¹).

📐 Step-by-Step Calculation

  1. Calculate moles of phosphoric acid (H₃PO₄):
    Moles = 0.100 mol dm⁻³ × (100 / 1000) dm³ = 0.0100 mol
  2. Determine the balanced equation and reacting ratio:
    2H₃PO₄ + 3Ca(OH)₂ → Ca₃(PO₄)₂ + 6H₂O
    Ratio: 2 moles of H₃PO₄ react with 3 moles of Ca(OH)₂.
  3. Calculate moles of calcium hydroxide needed:
    Moles of Ca(OH)₂ = 0.0100 mol × (3 / 2) = 0.0150 mol
  4. Convert moles to mass:
    Molar mass of Ca(OH)₂ = 40.1 + (2 × 16.0) + (2 × 1.0) = 74.1 g mol⁻¹
    Mass = Moles × Molar Mass = 0.0150 mol × 74.1 g mol⁻¹ = 1.1115 g (rounds to 1.11 g to 3 significant figures).

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), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.