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

Mark scheme

Show the mark scheme

Question

Acceptable Answer Additional Guidance Mark

Number

1(a) An answer that makes reference to the following (3)

points: Number of Number of Number of

Species

protons neutrons electrons

• first row correct (1) 32S 16 16 16

• second row correct (1) 33S 16 17 16

34S2− 16 18 18

• third row correct (1)

If no marks are scored, allow 1 mark for each correct

column

Question

Acceptable Answer Additional Guidance Mark

Number

1(b)(i) Example of calculation (1)

• calculation of missing value

(100 − 95.02 – 0.75 – 0.02 =) 4.21 (%)

Question

Acceptable Answer Additional Guidance Mark

Number

1(b)(ii) Example of calculation (2)

• correct expression for calculation of RAM = ((95.02 × 32) + (0.75 × 33) + (4.21 × 34) + (36 × 0.02))

RAM (1) 100

(= 32.0925)

Allow TE on 1(b)(i).

• value given to 2 decimal places (1) = 32.09

Allow units of g mol−1 / g mol−

Allow units of g/mol

Do not award any other unit

Correct answer with no working scores 2

(Total for Question 1 = 6 marks)

How to answer it

Atomic Structure and Relative Atomic Mass of Sulfur

📋 What this question tests

This question assesses fundamental GCSE-to-A-Level transition skills in Topic 1 (Atomic Structure and the Periodic Table):

Deducing subatomic particles (protons, neutrons, electrons) for neutral isotopes and negative ions (anions).
Locating the atomic number from the Periodic Table using element symbols.
Calculating missing isotopic percentage abundances from total sample percentage (100%).
Calculating relative atomic mass (Aᵣ) using a weighted average equation and applying correct rounding (two decimal places).

Part (a) — Subatomic Particles in Isotopes and Ions

Total: 3 Marks

✅ Completed Table (Correct Answer)

Species	Number of protons	Number of neutrons	Number of electrons
³²S	16	16	16
³³S	16	17	16
³⁴S²⁻	16	18	18

Mark Scheme Breakdown:
• 1 mark for Row 1 completely correct
• 1 mark for Row 2 completely correct
• 1 mark for Row 3 completely correct
Special rule: If 0 marks scored by row, allow 1 mark for each completely correct column.

💡 Key Knowledge

Atomic Number (Z): Look up Sulfur (S) on the Periodic Table: Z = 16 . Every sulfur species must have 16 protons.
Mass Number (A): The superscript is the nucleon number (protons + neutrons).
Neutrons = Mass Number − Protons
Neutral Atom: Electrons = Protons .
Anion (²⁻): A 2− charge indicates the gain of 2 extra electrons:
Electrons = 16 + 2 = 18 .

🧠 Exam Technique

Always fill the proton column first. Since all three species are sulfur, this column is immediately 16 for all rows. Then subtract 16 from the top mass number to get neutrons. Finally, check charges to write down the electrons.

❌ Common Errors

Charge subtraction error: Subtracting 2 instead of adding 2 for S²⁻, writing 14 electrons instead of 18.
Confusing mass and atomic number: Mistaking the mass number (e.g. 32) for the atomic number.

Part (b)(i) — Missing Isotope Percentage Abundance

Total: 1 Mark

✅ Correct Answer

4.21 (%)

Mark Scheme: 1 mark for correct calculation of the missing value.

📐 Step-by-Step Working

Recognise that the sum of all isotopic abundances in a natural sample must equal 100%:
% ³⁴S = 100 − (95.02 + 0.75 + 0.02)
Sum known abundances:
95.02 + 0.75 + 0.02 = 95.79%
Subtract from 100:
100 − 95.79 = 4.21%

Part (b)(ii) — Calculating Relative Atomic Mass (Aᵣ)

Total: 2 Marks

✅ Correct Answer

32.09

Mark Scheme Breakdown:
• Mark 1: Correct expression for calculation of RAM using all four isotopes.
• Mark 2: Final answer given strictly to two decimal places (32.09).
Note: Transfer of Error (TE) allowed from part (b)(i). Correct answer alone with no working scores 2/2. Aᵣ has no mandatory unit, but g mol⁻¹ is accepted; other units penalised.

📐 Step-by-Step Calculation

State the formula:
Aᵣ = Σ (isotopic mass × % abundance) ÷ 100
Substitute values:
Aᵣ = [(32 × 95.02) + (33 × 0.75) + (34 × 4.21) + (36 × 0.02)] ÷ 100
Calculate numerator:
Aᵣ = [3040.64 + 24.75 + 143.14 + 0.72] ÷ 100
Aᵣ = 3209.25 ÷ 100 = 32.0925
Round as requested:
Question specifies two decimal places:
32.0925 → 32.09

🧠 Exam Technique: Sanity Check

Always sense-check your calculated Aᵣ against two things:

The dominant isotope: ³²S is 95.02% abundant, so your final value must be just slightly above 32 (32.09 makes complete sense).
Periodic Table value: Sulfur on the Edexcel data booklet has Aᵣ = 32.1. Your calculated 32.09 rounds cleanly to 32.1.

❌ Common Errors to Avoid

Ignoring decimal places instruction: Writing 32.1 (1 d.p.) or 32.093 (3 d.p.) loses the second mark immediately.
Omitting the 4th isotope: Forgetting the tiny 0.02% of ³⁶S. All four isotopes must appear in the expression for Mark 1.
Adding incorrect units: Relative atomic mass is dimensionless. Do not write "g" or "amu". (Only no unit or g mol⁻¹ is accepted).

Topics

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