OCR A-Level Chemistry AS Breadth in chemistry (01), June 2019: Question 8

1 mark · Medium difficulty · Multiple Choice

Determine the correct chemical formula for copper(I) phosphate(V) given the formula of a phosphate(V) ion.

Practise this question

Question

Multiple-choice question 8. It states that a phosphate(V) ion has the formula PO4 3- and asks for the formula of copper(I) phosphate(V). Four options are provided: A, Cu(PO4)5; B, Cu5PO4; C, Cu(PO4)3; D, Cu3PO4. An answer box is provided at the bottom.
Question text

8 A phosphate(V) ion has the formula PO 3−.

What is the formula for copper(I) phosphate(V)?

A Cu(PO4)5

B Cu5PO4

C Cu(PO4)3

D Cu3PO4

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 8 showing the correct answer is D, worth 1 mark.

8 D 1 AO1.2

How to answer it

Determining Ionic Formulae

What this question tests

This question assesses your ability to construct the chemical formula of an ionic compound by combining individual ions with differing charges. You must successfully interpret Roman numerals for oxidation states, recall or deduce the charges on complex ions, and balance overall electrical charges to neutralise the compound.

Question 8 Multiple Choice

Constructing the Formula of Copper(I) Phosphate(V)

✅ Correct Answer: D

The correct formula is Cu₃PO₄ .

💡 Key Knowledge

  • Roman Numerals: Copper(I) tells you the copper ion has a charge of +1, written as Cu⁺ .
  • Complex Ions: The phosphate(V) ion is given in the stem as PO₄³⁻ , carrying a 3- charge.
  • Charge Neutrality: Ionic compounds have no overall charge, so total positive charges must equal total negative charges.

🧠 Exam Technique

Use the "cross-over" method for ionic charges: take the magnitude of the anion charge to be the subscript for the cation, and vice versa.

Cu⁺ (1+) and PO₄³⁻ (3-) crosses over to give Cu₃(PO₄)₁ , which simplifies to Cu₃PO₄ .

❌ Common Errors

  • Confusing Roman numeral (I) with the number of atoms rather than the oxidation state/ionic charge.
  • Inversely crossing charges to produce distractors like Cu(PO₄)₃ (Option C).
  • Misinterpreting Roman numeral (V) in phosphate(V) as the ionic charge instead of the phosphorus oxidation state (which is already accounted for in the given PO₄³⁻ formula).
Mark Scheme Allocation: 1 mark available for selecting option D (AO1.2 - Demonstrating knowledge and understanding of scientific ideas).

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 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.