OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2018: Question 7

1 mark · Medium difficulty · Multiple Choice

Identify which statement about ammonium carbonate is not correct from a choice of four options.

Practise this question

Question

Multiple choice question 7 asking which statement about ammonium carbonate is not correct, with options A (reacts with barium nitrate to form a white precipitate), B (effervesces with dilute nitric acid), C (releases an alkaline gas with warm sodium hydroxide), and D (has the formula NH4CO3). An answer box is provided at the bottom left.
Question text

7 Which statement about ammonium carbonate is not correct?

A It reacts with Ba(NO3)2(aq) to form a white precipitate.

B It effervesces with dilute nitric acid.

C It release an alkaline gas with warm NaOH(aq).

D It has the formula NH4CO3.

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 7 showing the correct answer is D, worth 1 mark.

7 D 1 AO2.3

How to answer it

Analysing Ammonium Carbonate Properties

What this question tests

This question assesses your knowledge of qualitative analysis (testing for ions), formula construction using ionic charges, and standard chemical reactions of carbonates and ammonium compounds.

Question 7

Comprehensive Breakdown

✅ Correct Answer: D

Statement D is not correct because the chemical formula given is NH₄CO₃ . Since the ammonium ion has a charge of +1 ( NH₄⁺ ) and the carbonate ion has a charge of -2 ( CO₃²⁻ ), balancing the charges requires two ammonium ions for every one carbonate ion. The correct formula is (NH₄)₂CO₃ .

💡 Key Knowledge

  • Ionic Formulas: Always balance positive and neutral charges across ions (e.g., NH₄⁺ and CO₃²⁻ give (NH₄)₂CO₃ ).
  • Barium Precipitates: Ba²⁺(aq) reacts with CO₃²⁻(aq) to form a white precipitate of barium carbonate ( BaCO₃ ).
  • Acid Reactions: Carbonates react with dilute acids to produce effervescence due to carbon dioxide gas ( CO₂ ) evolution.
  • Ammonium Test: Warming ammonium compounds with NaOH(aq) releases ammonia gas ( NH₃ ), which is alkaline and turns damp red litmus paper blue.

🧠 Exam Technique

When faced with a "not correct" multiple-choice question, systematically evaluate each statement using established chemical tests and rules rather than guessing. Cross out statements you know to be true until you find the false statement.

❌ Common Errors

  • Failing to balance ionic charges properly when writing chemical formulas (forgetting the subscript 2 outside the bracket for ammonium).
  • Misremembering qualitative analysis observations, such as confusing the colours of barium precipitates or the gas evolved during alkali-ammonium reactions.
Mark Scheme Allocation: 1 mark awarded for selecting D (AO2.3 - Applying chemical knowledge to identify incorrect chemical statements).

Topics

Module 3: Periodic table and energy · Practical Activity Groups · PAG 4: Qualitative analysis of ions · 3.1 The periodic table

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.