AQA A-Level Chemistry AS Paper 1, November 2021: Question 1

6 marks · Medium difficulty · State/Explain/Numerical

State features of the current atomic model missing from Rutherford's model, and calculate the relative atomic mass of tin from mass spectrometry data.

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Question

Question 01 features two parts. Part 01.1 shows Figure 1, a Rutherford atom model with a central positive nucleus and negative electrons in an orbit, and asks for two features of the current model not shown. Part 01.2 provides Table 1 listing m/z values (112, 114, 117, 120) and percentage abundances (22.41, 11.78, 34.97, to be determined) for tin, asking to identify the ion reaching the detector first and to calculate the relative atomic mass to 1 decimal place.
Question text

01 This question is about atomic structure.

01.1 Figure 1 is a model proposed by Rutherford to show the structure of an atom.

Figure 1

State two features of the current model that are not shown in the Rutherford model.

[2 marks]

Feature 1 of the current model

Feature 2 of the current model

01.2 A sample of tin is analysed in a time of flight mass spectrometer.

The sample is ionised by electron impact to form 1+ ions.

Table 1 shows data about the four peaks in this spectrum.

Table 1

m/z Percentage abundance

112 22.41

114 11.78

117 34.97

120 To be determined

Give the symbol, including mass number, of the ion that reaches the detector first.

Calculate the relative atomic mass of tin in this sample.

Give your answer to 1 decimal place.

[4 marks]

Symbol of ion

Relative atomic mass

Mark scheme

Show the mark scheme The mark scheme provides answers for questions 01.1 and 01.2. For 01.1, it awards marks for mentioning protons and neutrons in the nucleus and electrons in specific energy levels or orbitals. For 01.2, it awards marks for 112Sn+, finding the missing abundance as 30.84%, setting up the correct Ar calculation, and obtaining 116.5 to 1 decimal place.

Question Marking guidance Additional Comments/Guidelines Mark

Current model includes: neutrons and protons Rutherford model does not include neutrons and 1

protons

01.1 Current model shows electrons in different energy levels/orbitals Rutherford model does not show electrons in

different orbitals/energy levels

Allow 1st energy level only holds 2 electrons

M1: 112Sn+ 1

M2 missing abundance = 30.84% If M2 missing then allow M3 if denominator = 69.16 1

M3

01.2 1

(112 × 22.41) + (114 × 11.78) + (117 × 34.97) + (120 × 30.84)

RAM =

100 1

Allow M4 ecf

M4 RAM = 116.5 answer must be to 1dp

How to answer it

Atomic Structure and Mass Spectrometry Study Guide

What this question tests

This question assesses your knowledge of atomic models (comparing historical Rutherford models with modern quantum/sub-atomic structures) and your ability to interpret time-of-flight (TOF) mass spectrometry data to calculate relative atomic mass (Ar).

Question 01.1 [2 marks]

Comparing Atomic Models

State two features of the current model that are not shown in the Rutherford model.

✅ Correct Answers

  • The current model includes neutrons (alongside protons in the nucleus).
  • The current model shows electrons in specific energy levels / shells / sub-levels / orbitals (rather than arbitrary circular orbits/clouds).

💡 Key Knowledge

Rutherford's model identified a central, positive nucleus and orbiting electrons, but it pre-dated the discovery of the neutron (Chadwick, 1932) and Bohr's quantization of electron energy levels.

❌ Common Errors

Vague statements like "electrons move" or "it has a nucleus" do not score, as Rutherford's model already featured a nucleus and moving electrons. You must highlight structural features exclusive to modern atomic theory.

Question 01.2 [4 marks]

Time of Flight Mass Spectrometry & Relative Atomic Mass Calculation

A sample of tin is analysed in a time of flight mass spectrometer... Give the symbol, including mass number, of the ion that reaches the detector first, and calculate the relative atomic mass of tin to 1 decimal place.

✅ Correct Answers

  • Symbol of ion: 112Sn⁺ (or ¹¹²Sn⁺ )
  • Relative atomic mass (Ar): 116.5

🧠 Exam Technique & TOF Principles

In TOF mass spectrometry, all ions are accelerated to have the same kinetic energy. Therefore, lighter ions travel faster and reach the detector first. The ion with the lowest mass-to-charge (m/z) ratio is 112 .

📐 Step-by-Step Calculation Guide

  1. Step 1: Find the missing percentage abundance.
    Total percentage must equal 100%.
    100 - (22.41 + 11.78 + 34.97) = 100 - 69.16 = 30.84%
  2. Step 2: Set up the Relative Atomic Mass equation.
    Multiply each mass number by its percentage abundance, sum them together, and divide by 100.
    Ar = [ (112 × 22.41) + (114 × 1178) + (117 × 34.97) + (120 × 30.84) ] / 100
  3. Step 3: Evaluate and round correctly.g
    Ar = 116.524 / 100 = 116.524...
    Round to 1 decimal place as requested: 116.5

❌ Common Calculation Traps

  • Missing ionic charge: Writing just 112Sn instead of 112Sn⁺ loses the first mark, because the question specifically asks for the symbol of the ion.
  • Significant Figures: Failing to round to 1 decimal place penalises the final mark.
  • Arithmethic errors: Always double-check your subtraction for the missing abundance using a calculator. ECF (error carried forward) applies if M2 is wrong, provided the denominator matches your summed percentages.

Topics

Physical Chemistry · 3.1.1 Atomic Structure · 3.1.2 Amount of Substance

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.