AQA A-Level Chemistry Paper 3, June 2018: Question 5

1 mark · Easy difficulty · Multiple Choice

Identify which formula can represent both the empirical and molecular formula of a stable compound.

Practise this question

Question

Question 05 asks 'Which can be both an empirical and molecular formula of a stable compound?' followed by four multiple-choice options: A is CH2O, B is P4O10, C is NH2, and D is CH3. The question is worth 1 mark.
Question text

05 Which can be both an empirical and molecular formula of a stable compound?

[1 mark]

A CH2O

B P4O10

C NH2

D CH3

Mark scheme

Show the mark scheme Mark scheme table for Question 05 showing the correct answer as option A under Marking Guidance.

Question Marking Guidance

05 A

How to answer it

Empirical vs Molecular Formulae of Stable Compounds

📋 What this question tests

This question assesses your fundamental understanding of formula definitions and molecular stability:

  • Empirical formula: The simplest whole number ratio of atoms of each element present in a compound.
  • Molecular formula: The actual number of atoms of each element present in one molecule of the compound.
  • Chemical stability: Recognizing whether a formula represents an existing, electronically neutral, stable molecule rather than a free radical, intermediate, or reducible formula.
Question 05 (1 Mark)

Which can be both an empirical and molecular formula of a stable compound?

Options: A: CH₂O  |  B: P₄O₁₀  |  C: NH₂  |  D: CH₃

✅ Correct Answer

A: CH₂O

  • Simplest ratio: The ratio of C : H : O is 1 : 2 : 1, which cannot be simplified further. Therefore, it is a valid empirical formula.
  • Stable molecule: It is also the exact molecular formula of methanal (formaldehyde, HCHO), which is a stable, neutral organic molecule with all octets satisfied.

💡 Key Knowledge: Evaluating Each Option

  • A (CH₂O): Simplest ratio is 1:2:1 (empirical). Corresponds to methanal (molecular). Meets both criteria.
  • B (P₄O₁₀): A stable molecule, but the ratio 4:10 simplifies to 2:5. Its empirical formula is P₂O₅, so P₄O₁₀ is not an empirical formula.
  • C (NH₂): Simplest ratio (empirical), but as a neutral molecular species it is an unstable radical (•NH₂). The stable compound with this ratio is hydrazine, which has molecular formula N₂H₄.
  • D (CH₃): Simplest ratio (empirical), but as a neutral species it is a methyl radical (•CH₃). The stable alkane is ethane, with molecular formula C₂H₆.

🧠 Exam Technique: Two-Step Elimination

Approach formula questions of this type using a rapid two-step filter:

  1. Test 1: Can the ratio be simplified?
    Look at the subscripts. If they share a common divisor > 1 (like 4 and 10 in P₄O₁₀), eliminate immediately—it cannot be an empirical formula.
  2. Test 2: Can it exist as a stable, uncharged molecule?
    Check normal valencies (C needs 4 bonds, N needs 3, O needs 2, H needs 1). Neutral CH₃ and NH₂ leave unpaired electrons (radicals), meaning they are reactive intermediates, not stable compounds.

❌ Common Traps & Misconceptions

  • Confusing P₄O₁₀ with P₂O₅: Students frequently see phosphorus(V) oxide written historically as P₂O₅ and mistakenly think P₄O₁₀ is already empirical.
  • Ignoring the word "stable": Candidates see that CH₃ or NH₂ cannot be simplified and pick them without realizing that neither exists as an isolated, stable neutral molecule under standard conditions.
  • Thinking CH₂O must be glucose: While glucose is C₆H₁₂O₆ (empirical formula CH₂O), remember that n = 1 gives methanal (HCHO), which is a real and stable compound in its own right.
Mark Scheme Breakdown: 1 mark for identifying option A. No partial marks awarded.

Topics

Physical Chemistry · 3.1.2 Amount of Substance

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.