Edexcel A-Level Chemistry AS Paper 2, June 2024: Question 6

✅ Correct Answer

Sulfur is formed, which is insoluble in water / precipitates as a solid.

💡 Key Knowledge

Of all four reaction products, only elemental sulfur ( S ) is insoluble in water. It forms a pale yellow precipitate that suspended in solution causes light scattering (cloudiness).

❌ Common Errors

Stating that "sulfur dioxide gas causes bubbles/cloudiness". SO₂ is a soluble toxic gas; only the solid sulfur precipitate causes turbidity.

📐 Step-by-Step Calculation

1. Identify total volume in any experiment:
   For Exp 5: V(thiosulfate) = 50 cm³, V(water) = 0 cm³, V(acid) = 20 cm³.
   Total volume = 50 + 0 + 20 = 70 cm³.
2. Use the dilution equation:
   C₁ × V₁ = C₂ × V₂
   C(original) × 50 cm³ = 0.15 mol dm⁻³ × 70 cm³
3. Solve for initial concentration:
   C(original) = (0.15 × 70) / 50 = 10.5 / 50 = 0.21 mol dm⁻³

✅ Correct Answer

C (0.21 mol dm⁻³)

❌ Common Distractor Traps

• A (0.03): Concentration of Exp 1 after dilution by both water and acid.
• B (0.15): Concentration of Exp 5 after adding the 20 cm³ acid. Students forget the acid also dilutes the stock solution!
• D (0.71): Incorrect volume ratio inversion ( 50 / 70 = 0.71 ).

✅ Correct Answer

To keep the total volume of the reaction mixture constant (at 70 cm³) across all experiments.

🧠 Exam Technique

Always state what is being kept constant and why. If the volume varied, the concentration of the acid and the depth of the liquid would also change, introducing multiple uncontrolled variables.

❌ Common Errors

Vague answers such as "to dilute the solution" or "to make it a fair test" without specifying that volume is the variable being kept constant.

📐 Step-by-Step Calculation

1. Locate 0.10 mol dm⁻³ on the x-axis:
   Read vertically up to the line of best fit.
2. Read the corresponding y-axis value:
   1/time = 0.030 s⁻¹ (allow ±½ square: 0.0300 to 0.0305 s⁻¹).
3. Calculate the time taken (t):
   time = 1 / (1/time) = 1 / 0.030 = 33.3 s
   (Range: 32.8 s to 33.3 s).

✅ Correct Answer

33.3 s (accept 33 s, 33.33 s, or range 32.8 to 33.3 s)

❌ Common Calculation Traps

• Forgetting to invert: Quoting the y-axis reading 0.03 directly as the time!
• Rounding incorrectly: Truncating to 1 significant figure ( 30 s ) loses the mark. Standard guidance permits 2 or 3 sig figs.

✅ Correct Answer

Rate is directly proportional to concentration because the graph is a straight line passing through the origin (0,0).

🧠 Exam Technique: "State and Justify"

Two essential elements are required for a complete answer:

• State: "Directly proportional" (first order).
• Justify: Must mention both that it is a straight line AND it passes through the origin.

❌ Insufficient Responses

Writing "as concentration increases, rate increases" only describes a positive correlation, NOT direct proportionality. This scores 0 marks.

✅ Correct Answer

A (rinsing the flask with deionised water before each new experiment)

💡 Why Option A Decreases Accuracy

Rinsing leaves residual water droplets inside the flask. This unmeasured extra water dilutes the reaction mixture in subsequent runs, altering the intended concentrations and decreasing accuracy.

❌ Why Other Options Are Incorrect

• B (stirring continuously): Improves mixing consistency and ensures homogeneity across all runs.
• C (different cylinders): Prevents cross-contamination between reagents.
• D (same paper): Ensures the darkness/thickness of the cross remains a strictly controlled visual standard.

✅ Correct Answer

B (the time taken will decrease using the 100 cm³ flask)

💡 The Physics of the "Disappearing Cross"

A smaller conical flask (100 cm³ vs 200 cm³) has a narrower base.

• The same total volume of liquid (70 cm³) will reach a greater depth (height).
• Looking down from above, the light must pass through a longer path length of solution.
• Fewer sulfur particles per unit volume are required to obscure the cross, so the cross disappears sooner (shorter time).

❌ Common Misconception

Thinking that because the quantities/concentrations are identical, the time taken must be the same (Option D). Flask geometry directly dictates liquid column height!