WJEC A-Level Chemistry AS Unit 1, June 2025: Question 9

✅ Correct Answers

I. Labels:

• A: Electron gun (or heated filament / ionisation chamber) [1 Mark]
• B: (Electro)magnet / magnetic field [1 Mark]

II. Need for a vacuum:

• Ensures ions are not slowed down or deflected by air particles, OR prevents ions from colliding with air/other particles. [1 Mark]

💡 Key Knowledge

• Ionisation: High-energy electrons from an electron gun bombard gaseous sample atoms, knocking out electrons to form positive ions: X(g) + e⁻ → X⁺(g) + 2e⁻ .
• Deflection: The electromagnet deflects positive ions based on their mass-to-charge ratio ( m/z ). Lighter ions and ions with higher charges are deflected more.
• Vacuum: The mean free path must be large enough so ions travel freely straight to the detector.

❌ Common Errors

• Vaguely identifying A as just "heater" or "wire" instead of electron gun or ionisation source.
• Calling B an "electric field" (which accelerates ions) rather than the electromagnet (which bends/deflects ion paths).
• Stating the vacuum is to "prevent oxidation" or "keep it clean" rather than specifically focusing on ion collisions with gas molecules.

Mark Breakdown:
• Part I: 1 mark for A, 1 mark for B.
• Part II: 1 mark for clear reference to avoiding collisions with air molecules or preventing ions from slowing down.

📐 Step 1: Find Missing Isotope Abundance

The three isotopes are ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb. The sum of all abundances must equal 100%:

Peak Summary:

• m/z = 206 : peak height = 24.0%
• m/z = 207 : peak height = 22.0%
• m/z = 208 : peak height = 54.0%

✅ Correct Answer & Graph Setup

• Y-axis: Labeled as percentage abundance (or % abundance / relative abundance) with a uniform scale from 0 to at least 60% (e.g. 1 major division = 10%). [1 Mark]
• X-axis: Labeled as m/z (or mass / charge). Range focused around 205–210. Do not start with 0 on the x-axis. [1 Mark]
• Peaks: Three vertical lines drawn precisely at m/z values 206, 207, and 208 to heights corresponding to 24%, 22%, and 54%. [1 Mark]

🧠 Exam Technique: Drawing the Grid

• Use a sharp pencil and a ruler to draw clean, single vertical lines.
• Never draw broad Gaussian/bell curves for a standard mass spectrometer bar spectrum.
• Make sure your x-axis has a broken-axis symbol or simply starts near 205. Starting from 0 squashes all peaks into an unreadable vertical sliver.

❌ Common Errors

• Omitting the units / label on the axes (especially missing m/z ).
• Starting the x-axis scale at 0, which directly contradicts the mark scheme instructions.
• Plotting relative abundance ratios instead of percentages without updating axis scale.

✅ Correct Answers

(b)(i) I. Health Hazard [2 Marks]:

• Radon is a gas, so it is easily inhaled/breathed into the lungs. [1 Mark]
• Alpha (α) particles are highly ionising and cause cell/DNA damage or mutation leading to cancer. [1 Mark]

(b)(i) II. Half-life Calculation [1 Mark]:

0.8 g → 0.4 g → 0.2 g → 0.1 g (3 half-lives)

Time = 3 × 3.8 days = 11.4 days

(b)(ii) Decay Series Completion [2 Marks]:

²¹⁸Po ──( α )──> Pb ──( β )──> Bi ──( β )──> ²¹⁴Po

The three missing arrow labels in order are: α, β, β

💡 Key Knowledge: Nuclear Changes

• Alpha (α) decay (⁴₂He): Mass number decreases by 4, atomic number decreases by 2.
  ²¹⁸₈₄Po → ²¹⁴₈₂Pb + ⁴₂α
• Beta (β) decay (⁰₋₁e): A neutron turns into a proton; mass number remains constant, atomic number increases by 1.
  ²¹⁴₈₂Pb → ²¹⁴₈₃Bi + ⁰₋₁β
  ²¹⁴₈₃Bi → ²¹⁴₈₄Po + ⁰₋₁β

🧠 Exam Technique

• For (b)(i) I, you must link the state of matter to ingestion route: it is a gas, therefore it is inhaled. Outside the body, alpha particles cannot penetrate skin; inside lungs, they cause direct tissue ionization.
• For (b)(ii), award 2 marks for all three correct (α, β, β). Award 1 mark if two are correct, or if two β and one α are listed in the wrong order.

❌ Common Errors

• Writing that alpha particles "penetrate the body from outside" (alpha radiation has very low penetration power and is stopped by paper/skin).
• Miscounting half-lives (e.g. calculating 0.8 / 0.1 = 8 , then doing 8 × 3.8 instead of powers of 2).
• Confusing the proton numbers for Po (84), Pb (82), and Bi (83) when tracking alpha and beta steps.

Step 1: Calculate moles of AgCl precipitate formed
Molar mass of AgCl = 107.9 + 35.45 = 143.4 g mol⁻¹ (or 143.5 using standard data sheet)
n(AgCl) = mass / M_r = 0.8490 g / 143.5 g mol⁻¹ = 5.9164 × 10⁻³ mol

Step 2: Use stoichiometric ratio to find moles of RaCl₂
Chemical reaction: RaCl₂ + 2AgNO₃ → 2AgCl(s) + Ra(NO₃)₂
Since each unit of RaCl₂ contains 2 chloride ions:
n(RaCl₂) = n(AgCl) / 2 = (5.9164 × 10⁻³) / 2 = 2.9582 × 10⁻³ mol

Step 3: Calculate molar mass (M_r) of RaCl₂
M_r(RaCl₂) = mass / moles = 0.8779 g / (2.9582 × 10⁻³ mol) = 296.77 g mol⁻¹

Step 4: Calculate relative atomic mass of Ra to 4 significant figures
A_r(Ra) = M_r(RaCl₂) - 2(A_r(Cl)) = 296.77 - (2 × 35.45) = 296.77 - 70.90 = 225.87 → 225.8 (or 225.9 depending on exact A_r values used)
Final Answer: A_r = 225.8

❌ Common Calculation Traps

• Forgetting the 1:2 ratio: Assuming n(RaCl₂) = n(AgCl) loses 2 marks immediately because Ra is divalent ( Ra²⁺ , Group 2 like Barium).
• Subtracting only one Cl: Forgetting that RaCl₂ contains two chlorine atoms when finding A_r(Ra) .
• Significant figures penalty: Giving 226 or 225.84 instead of the requested four significant figures (225.8) loses the final mark.
• Premature rounding: Rounding intermediate values too early (e.g. using 0.003 instead of 2.958 × 10⁻³ ) leads to an incorrect final value.

🧠 Top Exam Tips for Gravimetric Problems

• Error Carried Forward (ecf): If you miscalculate the M_r of AgCl, you can still gain marks 2, 3, and 4 if your method is chemically sound and carried through consistently.
• Keep the unrounded number stored in your calculator memory ( ANS or STO A ) at each step to prevent rounding drift.
• Check feasibility: Radium is in Period 7 below Barium ( A_r ≈ 137 ). A calculated value around 226 confirms your answer makes chemical sense.