Edexcel A-Level Chemistry Paper 3, June 2024: Question 1

Mark scheme

Show the mark scheme

Question

Answer Additional Guidance Mark

Number

1(a) An answer that makes reference to the following points: (2)

• (the relative isotopic mass is) the mass of an atom (of an Allow weight for mass

isotope) (1) Do not award reference to average/mean mass

• relative to 1/12 of the mass of a carbon-12 atom Accept divided by 1/12 of the mass of a carbon-12

or atom

relative to a carbon-12 atom which has a mass of

(exactly) 12 (1)

Allow reference to moles such as

The mass of one mole of atoms of an isotope relative

to 1/12 of the mass of one mole of carbon-12 atoms

Question

Answer Additional Guidance Mark

Number

1(b) An answer that makes reference to the following point: (1)

• five / 5

(Total for Question 1 = 3 marks)

How to answer it

Relative Isotopic Mass & Mass Spectra

What this question tests

This question assesses your foundational understanding of key mass spectrometry terminology, specifically distinguishing between isotopic and atomic mass definitions, as well as applying combinatorial logic to deduce the number of molecular ion peaks generated by isotopic combinations in diatomic molecules.

Question 1(a) — 2 Marks

Defining Relative Isotopic Mass

✅ Correct Answer

An isotope's mass compared to 1/12th the mass of a carbon-12 atom.

Mark breakdown:
• Mark 1: Mass of an atom of an isotope.
• Mark 2: Relative to 1/12 of the mass of a carbon-12 atom (or C-12 = exactly 12).

💡 Key Knowledge

Exact wording matters: Examiners are strict with definitions. You must refer to an atom of an isotope, not just an "element".
Standard reference: Carbon-12 is the universal international standard assigned a mass of exactly 12 units.

❌ Common Errors

The "Average" Trap: Using words like "average mass" or "mean mass" will instantly lose you the first mark. Relative isotopic mass refers to a single specific isotope, whereas relative atomic mass considers abundances.

Question 1(b) — 1 Mark

Deducing Molecular Ion Peaks in Mass Spectrometry

✅ Correct Answer

5 (or five peaks)

Mark breakdown:
• 1 mark for stating the correct total number of peaks (5).

📐 Working Out (Combinations)

The hydrogen gas sample consists of three isotopes: ¹H , ²H , and ³H . The molecular ions are diatomic ( H₂⁺ ), meaning each molecule contains two hydrogen atoms combined from these three isotopes.

Homonuclear combinations (same isotopes):
• ¹H-¹H = mass 2
• ²H-²H = mass 4
• ³H-³H = mass 6
Heteronuclear combinations (different isotopes):
• ¹H-¹H vs ¹H-²H = mass 3
• ²H-³H = mass 5

Listing the resulting masses gives: 2, 3, 4, 5, and 6. This yields exactly 5 distinct peaks on the mass spectrum.

🧠 Exam Technique

When tackling isotopic combinations, systematically list all possible pairs (permutations/combinations) to avoid missing any intermediate masses or double-counting.

Topics

Physical Chemistry · Topic 1: Atomic Structure and the Periodic Table