WJEC A-Level Chemistry AS Unit 1, June 2025: Question 10

13 marks · Medium difficulty · Practical Methods and Analysis

Determine the water of crystallisation in hydrated strontium chloride from thermal decomposition data, calculate percentage error, predict experimental effects, and suggest improvements.

Practise this question

Question

Question 10 displays a laboratory experiment to determine the value of x in SrCl2·xH2O. A data table lists masses: empty crucible (12.12 g), crucible + hydrated salt before heating (13.71 g), crucible + anhydrous salt after heating (13.09 g), mass of hydrated salt before heating (1.59 g), with blank spaces for mass of anhydrous salt and mass of water removed. Subsequent parts ask to complete the table, calculate the whole number value of x, determine the percentage error in mass assuming a maximum error of ±0.005 g per reading, predict and explain the effect of using a yellow flame instead of blue, predict and explain the effect of crystals spitting out, and suggest two improvements to the method with explanations.

Mark scheme

Show the mark scheme Mark scheme for Question 10 provides answers: (a)(i) Mass of SrCl2 = 0.97 g, mass of H2O = 0.62 g (or 0.65 g depending on subtraction) (1 mark). (a)(ii) n(SrCl2) = 0.0061 mol, n(H2O) = 0.0344 mol, giving x = 6 (3 marks). (a)(iii) Percentage error = (0.01 / 1.59) * 100 = 0.6% (1 mark). (b)(i) Value of x decreases because not all water is lost or soot deposits on crucible (2 marks). (b)(ii) Value of x increases because apparent mass of water lost increases (2 marks). (c) Improvements such as heating to constant mass, using a lid on the crucible, or using a larger mass of solid, each with full explanations (4 marks). Total marks: 13.

How to answer it

Determining the Formula of Hydrated Strontium Chloride

📋 What This Question Tests

Core Practical & Stoichiometry Skills Assessed:

  • Mass data interpretation from crucible decomposition experiments.
  • Mole calculations and determining water of crystallisation ( x in SrCl₂·xH₂O).
  • Calculating percentage measurement uncertainty from balance readings.
  • Evaluating procedural errors (incomplete heating, thermal convection/soot, spitting of solids) and their mathematical effect on calculated ratios.
  • Proposing and explaining specific practical improvements (heating to constant mass, using a lid, scaling sample size).

Part (a) Data Processing, Molar Ratio & Uncertainty

Part (a)(i) Completing Mass Table [1 Mark]

✅ Correct Values

  • SrCl₂ after heating: 0.97 g
    (13.09 g − 12.12 g = 0.97 g)
  • H₂O removed: 0.62 g
    (13.71 g − 13.09 g = 0.62 g)
[1 mark] awarded for both correct values.

🧠 Exam Technique

Always double check rows by cross-subtraction:

Mass of hydrate (1.59 g) − Mass of anhydrous SrCl₂ (0.97 g) = 0.62 g of H₂O

Note: The official mark scheme print contains a small typo printing "0.65" in the header line, but accurately calculates with 0.62 g throughout.

Part (a)(ii) Finding the Value of x [3 Marks]

📐 Step-by-Step Calculation

  1. Calculate moles of anhydrous SrCl₂:
    Molar mass, Mr(SrCl₂) = 87.62 + (2 × 35.45) = 158.52 g mol⁻¹ (or 158.6 g mol⁻¹)
    n(SrCl₂) = 0.97 ÷ 158.6 = 0.00612 mol [1 mark]
  2. Calculate moles of water, H₂O:
    Molar mass, Mr(H₂O) = (2 × 1.01) + 16.00 = 18.02 g mol⁻¹
    n(H₂O) = 0.62 ÷ 18.02 = 0.0344 mol [1 mark]
  3. Find molar ratio (x):
    x = n(H₂O) ÷ n(SrCl₂) = 0.0344 ÷ 0.00612 = 5.62
    Nearest whole number: x = 6 [1 mark]

Final Formula: SrCl₂·6H₂O

❌ Common Errors in (a)(ii)

  • Dividing water moles by total hydrate mass instead of anhydrous salt moles.
  • Inverting the ratio ( n(SrCl₂) ÷ n(H₂O) ), giving a fractional value.
  • Forgetting to round to the nearest whole integer when explicitly asked.

🧠 Error Carried Forward (ECF)

If your subtractions in (a)(i) were incorrect, you can still gain all 3 marks in (a)(ii) via ECF provided you execute the mole steps and ratio calculation correctly.

Part (a)(iii) Percentage Uncertainty [1 Mark]

📐 Percentage Error Calculation

Mass of hydrated SrCl₂ = 1.59 g, obtained by taking two weighings (crucible, and crucible + sample).

Total uncertainty = 2 × (±0.005 g) = ±0.010 g

% Error = (0.010 ÷ 1.59) × 100 = 0.629% ≈ 0.63% (or 0.6%)

[1 mark] for 0.6% or 0.63%

❌ Single-Weighing Trap

Many students used only 0.005 ÷ 1.59 = 0.31% . The mass of the hydrated salt is a difference between two balance readings, so the maximum uncertainty is doubled: 2 × 0.005 = 0.010 g .

Part (b) Evaluating Experimental Errors

Part (b)(i) Heating with a Yellow Bunsen Flame [2 Marks]

✅ Mark Scheme Requirements

  • Prediction: The value of x decreases / is lower. [1 mark]
  • Reasoning: Any sensible reason [1 mark] :
    • A yellow flame is cooler / not hot enough, so not all water of crystallisation is lost / driven off.
    • Apparent mass of water removed decreases.
    • Alternative: Soot (carbon) deposits on the crucible base, increasing the final mass and reducing the apparent mass lost.

💡 Chemical Logic

Calculated x is proportional to recorded mass loss: x ∝ Δm(water) .

If heating is incomplete, apparent water mass is lower, yielding fewer calculated moles of water and a smaller x .

Part (b)(ii) Crystals Spat Out of Crucible [2 Marks]

✅ Mark Scheme Requirements

  • Prediction: The value of x increases / is higher. [1 mark]
  • Reasoning: Loss of solid is mistaken for loss of water / apparent mass of water removed increases. [1 mark]

❌ Common Misconception

Students often think losing crystals decreases x because "there is less solid left". The balance cannot distinguish between lost steam and lost solid! It records the lost solid as extra "water loss", artificially inflating x .

Part (c) Method Improvements [4 Marks]

Suggest two distinct improvements and explain each. Awarded as [1 mark] for suggestion + [1 mark] for corresponding explanation.

Option 1: Reheating

  • Suggestion: Heat to constant mass / reheat, cool, and reweigh until mass does not change. [1 mark]
  • Explanation: To ensure all water of crystallisation has been driven off / removed completely. [1 mark]

Option 2: Physical Containment

  • Suggestion: Place a lid on the crucible (loosely fitted). [1 mark]
  • Explanation: Prevents crystals/solid from spitting out during heating while letting steam escape. [1 mark]

Option 3: Sample Size

  • Suggestion: Use a larger / greater mass of hydrated strontium chloride. [1 mark]
  • Explanation: Reduces the percentage uncertainty / percentage error in weighing. [1 mark]

🧠 Examiner Tip for 4/4 Marks

Always pair each suggestion directly with its chemical rationale. Vague statements like "to make it more accurate" or "to get better results" will not secure the explanation mark.

Topics

Physical Chemistry · Practical · 1.3 Chemical calculations · AS Unit 1 practical work

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