OCR A-Level Chemistry AS Breadth in chemistry (01), June 2022: Question 15

1 mark · Easy difficulty · Multiple Choice

Determine the number of hydrogen atoms in one molecule of the given organic compound containing a carbonyl group, an alcohol group, and an alkene group.

Practise this question

Question

Multiple choice question 15 asking for the number of hydrogen atoms in one molecule of a displayed chemical structure. The structure shows a carbon chain with a double-bonded oxygen (carbonyl), a methyl branch, a hydroxyl group (OH), and a terminal alkene double bond. Four multiple choice options are listed: A (8), B (10), C (12), and D (14), alongside an answer box and mark allocation of [1].
Question text

15 What is the number of hydrogen atoms in one molecule of the compound below?

O OH

A 8

B 10

C 12

D 14

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 15 showing the correct answer is C, with 1 mark awarded.

15 C 1 AO1.2 ALLOW 12

How to answer it

Counting Hydrogen Atoms from Skeletal Formulae

What this question tests

This question assesses your ability to interpret organic structural representations—specifically skeletal and partial-structural formulae—and apply your foundational knowledge of carbon's tetravalency to deduce the molecular formula and count specific atoms (hydrogen) within a molecule.

Question 15 (Multiple Choice)

Exam Breakdown

✅ Correct Answer

C (12)

The compound has a molecular formula of C₇H₁₂O₂. Therefore, there are 12 hydrogen atoms in one molecule.

💡 Key Knowledge

  • Skeletal Formulae: Carbon-carbon bonds are represented by lines. Carbon atoms are implied at line terminals and vertices. Hydrogen atoms attached to carbons are not drawn.
  • Tetravalency: Carbon always forms 4 covalent bonds. To find missing hydrogens on any carbon, subtract the number of drawn bonds from 4.
  • Heteroatoms: Functional group hydrogens (like the -OH alcohol proton) are explicitly written out.

🧠 Exam Technique

  • Take your time translating the skeletal structure systematically from left to right.
  • Use a tally or write out the partial molecular formula fragment by fragment to avoid miscounting under timed conditions.

❌ Common Errors

  • Forgetting the -OH hydrogen: Students often count carbons and aliphatic hydrogens from the chain but omit the hydrogen part of the hydroxyl group.
  • Misinterpreting junctions: Overlooking the CH group at a bend or miscounting terminal methyl ( -CH₃ ) hydrogens.

📐 Step-by-Step Breakdown of the Structure

  1. Terminal Methyl group (left): CH₃- = 3 H atoms
  2. Carbonyl carbon (C=O): Bonded to the left carbon, the CH, and double-bonded to O. It has 4 bonds already, so it contributes 0 H atoms.
  3. CH group (with hanging methyl branch): Attached to the main chain carbon, the carbonyl carbon, and the downward methyl group ( -CH₃ ). That makes 3 bonds, leaving space for 1 H atom.
  4. Downward Methyl branch: -CH₃ = 3 H atoms.
  5. CH-OH group: Bonded to the adjacent CH, the terminal alkene carbon, and the -OH group. That's 3 bonds, leaving 1 H atom on the carbon, plus 1 H atom on the -OH group. (Total = 2 H).
  6. Terminal Alkene group ( -CH=CH₂ ): The CH has 2 bonds shown, leaving 1 H. The =CH₂ carbon has a double bond, leaving 2 H atoms. (Total = 3 H).
  7. Summing them up: 3 (left methyl) + 1 (backbone CH) + 3 (down methyl) + 1 (CH-OH carbon) + 1 (OH group) + 1 (alkene CH) + 2 (alkene CH₂) = 12 Hydrogen atoms.
Mark Scheme Note: 1 mark awarded for selecting C. AO1.2 (Demonstrating knowledge and understanding of scientific ideas).

Topics

Module 4: Core organic chemistry · 4.1 Basic concepts and hydrocarbons · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.