OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2017: Question 11

1 mark · Medium difficulty · Multiple Choice

Identify the product formed when aqueous Cr3+ ions react with an excess of aqueous sodium hydroxide.

Practise this question

Question

Multiple choice question 11 asks which product is formed when aqueous Cr3+ ions are reacted with an excess of aqueous sodium hydroxide. Four options are given: A, Cr(OH)6 3-; B, Cr(OH)3; C, [Cr(OH)4(H2O)2]-; D, [Cr(OH)4]3-. An empty answer box and the mark allocation [1] are shown at the bottom right.
Question text

11 Aqueous Cr3+ ions are reacted with an excess of aqueous sodium hydroxide.

Which product is formed?

A Cr(OH) 3–

B Cr(OH)3

C [Cr(OH) (H O) ]–

42 2

D [Cr(OH) ]3–

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table shows question number 11 with the correct answer A and 1 mark.

11 A 1

How to answer it

Reactions of Aqueous Chromium(III) Ions with Excess Sodium Hydroxide

What this question tests

This question assesses your knowledge of transition metal chemistry, specifically the characteristic reactions of hexaaquachromium(III) ions ( [Cr(H₂O)₆]³⁺ ) with aqueous sodium hydroxide. It targets your understanding of amphoteric behavior, ligand substitution, precipitation reactions, and coordination numbers in complex ions.

Question 11 (Multiple Choice)

Aqueous Cr³⁺ ions reacted with excess aqueous sodium hydroxide

✅ Correct Answer: A

Cr(OH)₆³⁻ (often written as [Cr(OH)₆]³⁻ )

When an excess of OH⁻ is added to aqueous Cr³⁺ , the grey-green precipitate Cr(OH)₃(H₂O)₃ redissolves to form the hexahydroxochromate(III) complex ion with an octahedral geometry and a coordination number of 6.

💡 Key Knowledge

  • Starting Species: Aqueous Cr³⁺ exists as the complex ion [Cr(H₂O)₆]³⁺ .
  • Amphoteric Nature: Chromium(III) hydroxide is amphoteric—it dissolves in both excess acid and excess alkali.
  • Coordination Number Change: Unlike aluminum, which changes coordination number from 6 to 4 in excess alkali ( [Al(OH)₄]⁻ ), chromium(III) maintains a coordination number of 6, forming [Cr(OH)₆]³⁻ .

🧠 Exam Technique

Watch out for common coordination number traps! Many students automatically assume coordination numbers drop to 4 for all amphoteric hydroxides in excess alkali. Memorize the specific coordination numbers for Cr ( CN = 6 ) vs Al ( CN = 4 ).

❌ Common Errors

  • Choosing D ( [Cr(OH)₄]³⁻ ) by incorrectly applying the aluminum tetrahedro pattern to chromium.
  • Choosing B ( Cr(OH)₃ ), which represents the initial neutral precipitate formed with limited OH⁻ , not the product in excess.
  • Omitting or miscounting water ligands in transitional intermediate formulas (option C).
Mark Allocation: [1 mark] awarded for selecting option A.

Topics

Module 5: Physical chemistry and transition elements · 5.3 Transition elements

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.