OCR A-Level Chemistry AS Breadth in chemistry (01), November 2020: Question 14

1 mark · Medium difficulty · Multiple Choice

Identify the correct statement regarding the relationship between the two given organic structures, involving empirical formula, relative molecular mass, structural isomers, and functional groups.

Practise this question

Question

Question 14 asks which statement is correct for two displayed chemical structures shown side-by-side. The left structure shows a branched alkene with a terminal C=C bond, and the right structure shows the same molecule drawn flipped horizontally. Four multiple-choice options are listed below: A, They have the same empirical formula; B, They have different relative molecular masses; C, They are structural isomers; D, They have different functional groups. An answer box is provided at the bottom.
Question text

14 Which statement is correct for the two structures below?

A They have the same empirical formula.

B They have different relative molecular masses.

C They are structural isomers.

D They have different functional groups.

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme showing question number 14 with the correct answer A, worth 1 mark.

14 A 1 1.2

How to answer it

Comparing Organic Structures and Isomerism

What this question tests

This question tests your ability to interpret skeletal formulas, recognize molecular formulas, understand the relationship between identical molecules drawn in different orientations (stereochemical/rotational symmetry), and distinguish between empirical formulas, molecular masses, structural isomers, and functional groups.

Question 14

Identifying the Relationship Between Two Skeletal Structures

✅ Correct Answer: A

The correct option is A: They have the same empirical formula.

In fact, both structures represent the exact same molecule (2,4,4-trimethylpent-1-ene) drawn flipped horizontally. Because they are the exact same compound, they share the same molecular formula, empirical formula, relative molecular mass, and functional groups.

💡 Key Knowledge

  • Skeletal Formula: Carbon-carbon bonds are represented by lines; carbon and hydrogen atoms attached to carbons are implied.
  • Identical Molecules: Rotating or flipping a 2D skeletal drawing on the page does not change the identity of the molecule if the connectivity remains identical.
  • Empirical Formula: The simplest whole-number ratio of atoms of each element in a compound.

🧠 Exam Technique

  • Check connectivity first: Trace the longest carbon chain from left-to-right on the first structure, then right-to-left on the second structure.
  • Recognise mirror images or flipped structures: Structure 2 is simply Structure 1 reflected along a vertical axis.
  • Eliminate incorrect statements systematically.

❌ Common Errors

  • Mistaking identicals for isomers: Many students automatically assume that two structures shown side-by-side must be structural isomers (Option C). Always check numbering from both ends!
  • Miscounting carbon or hydrogen atoms when attempting to derive molecular formulas under time pressure.
Examiner Commentary: This question successfully discriminated between candidates who understand IUPAC naming/chain tracing and those who rely on visual assumptions. Option C was a very common distractor chosen by students who failed to notice that the second structure is just a reversed view of the first.

Topics

Module 4: Core organic chemistry · 4.1 Basic concepts and hydrocarbons

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.