Edexcel A-Level Chemistry AS Paper 1, November 2020: Question 4

12 marks · Medium difficulty · Calculations

Calculate the relative atomic mass of potassium from isotopic data and explain the principles and operation of a mass spectrometer including ion pathways and vacuum requirements.

Practise this question

Question

An exam question about isotopes containing a table of potassium isotopes (39K, 40K, 41K) with their relative isotopic masses and percentage abundances. Part (a) asks students to define relative isotopic mass and relative atomic mass, define isotopes using potassium examples, and calculate the relative atomic mass of potassium to 4 significant figures. Part (b) provides a simplified, incompletely labelled diagram of a mass spectrometer showing a vaporized sample entering an ionization/acceleration region, moving into a curved flight tube marked with letter X and three distinct paths A, B, and C towards a detector. Questions (b) ask to name the feature responsible for ion behavior at region X, explain the three ion pathways A, B, and C, and give a reason why the mass spectrometer must be operated under vacuum.
Question text

4 This question is about isotopes.

(a) The table shows data for some isotopes of potassium.

Isotope Relative isotopic mass Abundance %

39K 38.9637 93.218

40K 39.9340 0.012

41K 40.9618 6.770

(i) State what is meant by the terms ‘relative isotopic mass’ and ‘relative atomic mass’.

(3)

(ii) State what is meant by the term ‘isotopes’. Illustrate your answer by referring to

the isotopes of potassium.

(2)

(iii) Use the data in the table to calculate the relative atomic mass of potassium.

Give your answer to 4 significant figures.

(2)

(b) The relative isotopic abundances of an element can be measured using a mass spectrometer.

A simplified and incompletely labelled diagram of a mass spectrometer is shown.

+

A

8 X

vaporised *P62306A0824*

sample

B

C

to vacuum

pump

detector

(i) Name the feature of the mass spectrometer responsible for the behaviour of

the ions in the region indicated by the arrow X.

(1)

(ii) Explain the three ion pathways, A, B and C, shown in the region indicated by

the arrow X.

(3)

(iii) Give a reason why the mass spectrometer must be operated under vacuum.

(1)

(Total for Question 4 = 12 marks)

Mark scheme

Show the mark scheme A detailed mark scheme providing acceptable answers and additional guidance for all parts of Question 4. Part (a)(i) requires mentioning mass of an atom relative to 1/12th of a C-12 atom. Part (a)(ii) requires same number of protons and different neutrons, with specific examples. Part (a)(iii) shows the calculation yielding 39.10. Part (b)(i) accepts magnetic field or electromagnet. Part (b)(ii) explains pathways A, B, and C based on mass/charge ratio and deflection. Part (b)(iii) accepts preventing collisions with air molecules.

How to answer it

Isotopes and Mass Spectrometry

What this question tests

This question assesses your core understanding of atomic structure definitions (relative isotopic mass, relative atomic mass, and isotopes), your ability to perform weighted mean mass calculations using high-precision data to a specified number of significant figures, and your functional knowledge of how a mass spectrometer operates (deflection and vacuum conditions).

Question 4 (a) (i) - Definitions

Defining Isotopic and Atomic Mass

✅ Correct Answer

  • Relative isotopic mass: The mass of an atom of that isotope relative to 1/12th the mass of an atom of carbon-12.
  • Relative atomic mass: The weighted average (or mean) mass of an atom relative to 1/12th the mass of an atom of carbon-12.

❌ Common Errors

  • Using the word "average" when defining relative isotopic mass (isotopes have a single fixed mass relative to C-12, they are not averages).
  • Omitting reference to "atom" (penalised once only).
  • Forgetting to mention the standard reference standard: 1/12th the mass of carbon-12.
🎯 3 Marks available: 1 mark for each precise definition point including the C-12 standard.
Question 4 (a) (ii) - Isotopes

Understanding Isotopes

💡 Key Knowledge

Isotopes are atoms of the same element with the same number of protons (atomic number) but different numbers of neutrons (mass number).

🧠 Exam Technique: Contextualizing

To score the second mark, you must apply the definition to the provided potassium data. For example: "Potassium-39 has 19 protons and 20 neutrons, whereas Potassium-40 has 19 protons and 21 neutrons."

🎯 2 Marks available: 1 for general definition (protons/neutrons), 1 for referencing specific potassium isotope numbers.
Question 4 (a) (iii) - Calculation

Calculating Relative Atomic Mass

📐 Step-by-Step Calculation

  1. Multiply each isotopic mass by its percentage abundance:
    (38.9637 × 93.218) = 3632.0917
    (39.9340 × 0.012) = 0.479208
    (40.9618 × 6.770) = 277.301386
  2. Sum the values:
    3632.0917 + 0.479208 + 277.301386 = 3909.872294
  3. Divide by 100:
    3909.872294 / 100 = 39.09872294
  4. Round to 4 significant figures:
    39.10

❌ Calculation Traps

  • Using rounded mass numbers (like 39, 40, 41) instead of the precise relative isotopic masses provided in the table loses the first mark.
  • Failing to round to the requested 4 significant figures will lose the final accuracy mark.
🎯 2 Marks available: 1 for correct working/calculation, 1 for correct evaluation to exactly 4 SF.
Question 4 (b) (i) - Mass Spectrometry

Identifying the Deflection Region

✅ Correct Answer

Deflection by an electromagnetic field (or magnet / electromagnet).

❌ Common Errors

Do not write "electric field" for region X. The curved electromagnet bends the ions based on their mass-to-charge ratio.

🎯 1 Mark available.
Question 4 (b) (ii) - Ion Pathways

Explaining Ion Pathways A, B, and C

💡 Key Knowledge

In the magnetic field, ions are deflected according to their mass-to-charge ratio (m/z). Lighter ions or those with higher charges deflect more.

🧠 Exam Technique: Breakdown

  • Pathway B: Ions hit the detector normally as they have the targeted/standard mass-to-charge ratio.
  • Pathway A: Ions with a greater mass (or higher m/z ratio) are deflected less and curve wider.
  • Pathway C: Ions with a smaller mass (lower m/z) or multiple charges are deflected more and curve tighter.
🎯 3 Marks available: 1 mark for each properly explained pathway (A, B, and C).
Question 4 (b) (iii) - Vacuum Conditions

Why Operate Under Vacuum?

✅ Correct Answer

To prevent ions from colliding with gas / air molecules or particles, which would deflect or neutralize them before reaching the detector.

❌ Examiner Warning

Generic answers like "to keep it clean" or "to remove impurities" do not score. You must link it specifically to collisions with air particles interfering with ion flight paths.

🎯 1 Mark available.

Topics

Physical Chemistry · Organic Chemistry · Topic 1: Atomic Structure and the Periodic Table · Topic 7: Modern Analytical Techniques I

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.