Edexcel A-Level Chemistry AS Paper 1, June 2025: Question 5

15 marks · Medium difficulty · Calculations

Deduce electron configurations and explain ionisation energy trends for zinc and magnesium, determine isotope properties including calculating the mass of an unknown isotope and the Avogadro constant, and find the value of x in hydrated zinc sulfate.

Practise this question

Question

Question 5 consists of multiple parts spanning atomic structure, isotopes, and formula determination: (a)(i) complete electronic structures of Zn and Mg2+; (a)(ii) explain why Zn and Mg form stable 2+ ions in terms of ionisation energies; (b)(i) complete a table with proton and neutron counts for 64_30Zn; (b)(ii) state why relative isotopic mass is not exactly 64; (b)(iii) a table showing abundances and relative isotopic masses of four isotopes of zinc, requiring calculation of the isotopic mass and symbol of the fifth isotope given average Ar = 65.396; (b)(iv) calculate the Avogadro constant given the mass of a single 64_30Zn atom; (c) calculate x in ZnSO4·xH2O using data from heating 450 mg of hydrated salt to leave 253 mg of anhydrous salt.

Mark scheme

Show the mark scheme Mark scheme for Question 5 showing: 5(a)(i) 2s2 2p6 3s2 3p6 3d10 4s2 for Zn and 2s2 2p6 for Mg2+; 5(a)(ii) large jump in energy between 2nd and 3rd ionisation energies as the 3rd electron is removed from an inner shell; 5(b)(i) 30 protons and 34 neutrons; 5(b)(ii) reference to standard of carbon-12 or proton/neutron masses not being exact integers; 5(b)(iii) calculation yielding abundance 18.75%, relative isotopic mass 67.928, and formula 68_30Zn; 5(b)(iv) calculation of NA = 63.929 / (1.06157 x 10^-22) = 6.0221 x 10^23; 5(c) mass of water = 197 mg, mol of water = 10.94 mmol, mol of ZnSO4 = 1.567 mmol, giving x = 7.

How to answer it

Zinc & Magnesium: Structure, Isotopes & Formulae

📋 WHAT THIS QUESTION TESTS

A comprehensive 15-mark structured question spanning fundamental physical and inorganic chemistry concepts:

  • Writing full sub-shell electronic configurations for d-block atoms ( Zn ) and s-block ions ( Mg²⁺ ).
  • Explaining stability of oxidation states using successive ionisation energy trends and inner shell removal.
  • Deducing subatomic particles and understanding the definition of relative isotopic mass based on the standard carbon-12 scale.
  • Calculating unknown isotopic mass and identifying isotope symbols from relative atomic mass ( Aᵣ ) data.
  • Determining the Avogadro constant from single atom isotopic mass data using appropriate significant figures.
  • Experimental moles calculation to determine the water of crystallisation ( x ) in hydrated zinc sulfate ( ZnSO₄·xH₂O ).

Part (a)(i): Electronic Configurations

Full sub-shell configurations of Zn atom and Mg²⁺ ion (2 Marks)

✅ Correct Answer

Zn (atomic number 30):
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s²
(or 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ ) [1 mark]

Mg²⁺ (12 - 2 = 10 electrons):
1s² 2s² 2p⁶ [1 mark]

❌ Common Errors

  • Writing the configuration for neutral Mg ( 1s² 2s² 2p⁶ 3s² ) instead of the Mg²⁺ ion.
  • Zinc electron omissions or forgetting that the 3d subshell is completely filled ( 3d¹⁰ ).
  • Writing shorthand noble gas cores (e.g. [Ar] 3d¹⁰ 4s² ) when the prompt explicitly starts with 1s² .
Examiner Insight: Read the prompt carefully. The line starts with " 1s² ... ", requiring the full electronic structure, not condensed notation.

Part (a)(ii): Ionisation Energy & 2+ Ion Stability

Explaining why both Zn and Mg form stable 2+ ions (2 Marks)

✅ Correct Answer

  • There is a large jump / increase in energy between the second and third ionisation energies (or both have relatively low 1st and 2nd IEs). [1 mark]
  • The third electron is removed from a new inner shell / sub-shell (closer to the nucleus, experiencing significantly less shielding and greater attraction). [1 mark]

🧠 Exam Technique

Always state which specific ionisation energies are involved. Saying "there is a jump in ionisation energy" is too vague; specify "between the 2nd and 3rd ionisation energies". For the second mark, link the jump directly to the removal of an electron from an inner quantum shell.

Part (b)(i) & (b)(ii): Nuclear Structure & Isotopic Mass

Subatomic particles and isotopic mass standard (2 Marks)

✅ Correct Answers

(b)(i) Particles in ⁶⁴₃₀Zn:
Number of protons = 30
Number of neutrons = 64 − 30 = 34 [1 mark for both]

(b)(ii) Why isotopic mass is not exactly 64:
The mass is measured relative to the standard carbon-12 isotope ( ¹²C = 12.000 ).
(OR: Protons and neutrons do not have a mass of exactly 1 amu / mass defect due to binding energy). [1 mark]

💡 Key Knowledge

The standard definition of relative isotopic mass is the mass of an atom of an isotope compared to 1/12th the mass of an atom of carbon-12. Because individual nucleons have masses slightly different from 1, and nuclear binding energy causes a mass defect, isotopic masses are not integers.

Part (b)(iii): Calculation of Fifth Isotope

Finding the relative isotopic mass and identity of the isotope (4 Marks)

📐 Step-by-Step Calculation

  1. Find the percentage abundance of the fifth isotope:
    % abundance = 100 − (48.63 + 27.90 + 4.10 + 0.62)
    % abundance = 100 − 81.25 = 18.75% [1 mark]
  2. Set up the relative atomic mass equation:
    Let m = relative isotopic mass of the fifth isotope.
    65.396 = [(63.929 × 48.63) + (65.926 × 27.90) + (66.927 × 4.10) + (69.925 × 0.62) + (m × 18.75)] ÷ 100 [1 mark]
  3. Calculate the intermediate sum and solve for m:
    6539.6 = [3108.867 + 1839.335 + 274.401 + 43.354] + (18.75 × m)
    6539.6 = 5265.957 + (18.75 × m)
    18.75 × m = 6539.6 − 5265.957 = 1273.643
    m = 1273.643 ÷ 18.75 = 67.928 (to 3 d.p.) [1 mark]
  4. Deduce the symbol of the isotope:
    Mass number = 68 (nearest integer). Atomic number of Zn = 30.
    Symbol = ⁶⁸₃₀Zn (or ⁶⁸Zn) [1 mark]

🧠 Rounding Advice

Keep unrounded intermediate values in your calculator! Rounding too early when summing the four isotopes can introduce rounding errors that alter the 3rd decimal place.

❌ Common Errors

  • Forgetting to round the mass to exactly 3 decimal places as explicitly requested.
  • Omitting the element symbol or atomic/mass numbers in the final isotope notation.

Part (b)(iv): Determining the Avogadro Constant

Calculating Avogadro's constant using isotopic mass (1 Mark)

📐 Calculation

1 mole of ⁶⁴Zn has a mass of 63.929 g.
Mass of 1 atom = 1.06157 × 10⁻²² g.

L = Mass of 1 mole ÷ Mass of 1 atom
L = 63.929 ÷ (1.06157 × 10⁻²²)
L = 6.0221 × 10²³ mol⁻¹ (or 6.022 × 10²³) [1 mark]

🧠 Significant Figures Trap

The question specifies an appropriate number of significant figures. The input values have 5 SF (63.929) and 6 SF (1.06157 × 10⁻²²). Therefore, your answer must be given to 4 or 5 significant figures ( 6.022 × 10²³ or 6.0221 × 10²³ ). Writing just 6.02 × 10²³ loses the mark!

Part (c): Water of Crystallisation in Hydrated Zinc Sulfate

Finding x in ZnSO₄·xH₂O from thermal decomposition data (4 Marks)

📐 Step-by-Step Calculation

  1. Find mass of water lost:
    Mass of water lost = Mass of hydrated salt − Mass of anhydrous salt
    Mass of H₂O = 450 mg − 253 mg = 197 mg (or 0.197 g) [1 mark]
  2. Calculate amount of water (H₂O):
    Molar mass (Mᵣ) of H₂O = (2 × 1.0) + 16.0 = 18.0 g mol⁻¹
    Moles of H₂O = 197 ÷ 18.0 = 10.944 mmol (or 0.010944 mol) [1 mark]
  3. Calculate amount of anhydrous zinc sulfate (ZnSO₄):
    Molar mass (Mᵣ) of ZnSO₄ = 65.4 + 32.1 + (4 × 16.0) = 161.5 g mol⁻¹
    Moles of ZnSO₄ = 253 ÷ 161.5 = 1.5666 mmol (or 0.0015666 mol) [1 mark]
  4. Determine mole ratio x:
    x = moles of H₂O ÷ moles of ZnSO₄ = 10.944 ÷ 1.5666 = 6.986 ≈ 7
    x = 7 (Formula is ZnSO₄·7H₂O ) [1 mark]

🧠 Unit Tip

You do not need to convert milligrams to grams, provided both masses remain in milligrams (yielding millimoles). If you prefer grams: 0.197 g and 0.253 g work identically.

❌ Common Errors

  • Leaving x as a decimal (e.g. 6.99). The prompt requires the value of x , which must be an integer.
  • Inverting the mole ratio ( ZnSO₄ ÷ H₂O ).
  • Incorrect Mᵣ for ZnSO₄ (forgetting to multiply oxygen by 4).

Topics

Physical Chemistry · Topic 1: Atomic Structure and the Periodic Table · Topic 5: Formulae, Equations and Amounts of Substance

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.