AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2022: Question 9

✅ Correct Answer

24.5 g

📐 Step-by-Step Working

1. Identify total initial mass before heating (time = 0 s): 26.5 g (Table 9).
2. Identify mass of hydrated cobalt chloride added: 2.0 g (Step 1).
3. Subtract substance mass from total mass:
   26.5 g - 2.0 g = 24.5 g

❌ Common Errors

• Using the final mass ( 25.6 g ) instead of the initial mass at 0 seconds ( 26.5 g ).
• Subtracting the lost mass rather than the mass of solid added.

✅ Correct Answer

Award 1 mark for each point:

• Water vapour (or steam / water as a gas) was produced. [1]
• The water vapour escaped from the test tube (so its mass was no longer measured). [1]

💡 Key Knowledge: Conservation of Mass in Open Systems

Total mass is always conserved in chemical reactions. However, if a reaction takes place in an open container (like an open test tube) and one product is a gas, the gas escapes into the surroundings, resulting in a measured mass decrease.

❌ Common Errors & Misconceptions

• Just saying "water was formed" without specifying it was a gas/vapour or steam.
• Stating that "mass was lost as heat/energy" (violates conservation of mass).
• Failing to mention that the gas actually escaped from the container.

🧠 Exam Technique: Two-Part Explanations

Whenever an exam asks why mass decreased in an open vessel, use the 2-step formula:

1. Identify the gas produced + 2. State that the gas escaped.

✅ Correct Answer

Any one of the following:

• To ensure that the reaction was complete. [1]
• To ensure that no more water (vapour) was produced / all water had been removed. [1]

🧠 Exam Technique: "Heating to Constant Mass"

This is a standard experimental technique across GCSE Chemistry. Heating, cooling, and re-weighing until the mass remains identical is done to guarantee that 100% of the volatile product (water/gas) has driven off and the reaction has gone to completion.

❌ Common Errors

• Vague answers like "to make it accurate" or "to be fair" (no marks awarded).
• Claiming it was done "to calculate an average" (this is not repeating the experiment, it is checking completion).

✅ Correct Final Answer

0.740 kJ (must be rounded to 3 significant figures)

📐 Step-by-Step Calculation

1. Identify given values:
   Mass 1 = 238 g requires 88.1 kJ
   Mass 2 = 2.00 g
2. Set up proportion (Mark 1):
   Energy = (2.00 / 238) × 88.1
3. Calculate unrounded value (Mark 2):
   = 0.740336134... kJ
4. Round to 3 significant figures (Mark 3):
   First 3 non-zero digits are 7, 4, 0. The next digit is 3 (rounds down).
   = 0.740 kJ

❌ Common Errors & Significant Figure Traps

• Writing 0.74 instead of 0.740. The trailing zero is necessary to show 3 significant figures!
• Inverting the ratio (e.g. calculating 238 / 2.00 × 88.1 ).
• Rounding too early during intermediate calculation steps.

🧠 Exam Technique: Significant Figures

Zeros at the start of a decimal (like 0. ) do not count as significant figures. The first significant figure is the first non-zero digit (here, 7). Therefore:

0 . 7 (1st) 4 (2nd) 0 (3rd)

✅ Correct Answer

Any one of the following terms:

• Endothermic (reaction)
• Reversible (reaction)
• Thermal decomposition (or decomposition)

💡 Why Multiple Answers are Accepted

• Endothermic: The question states: "Energy is taken in from the surroundings..." Taking in energy = endothermic.
• Reversible: The prompt uses the reversible arrow symbol ( ⇌ ).
• Thermal decomposition: Heating breaks down one substance into two simpler substances.

❌ Common Errors

• Confusing endothermic (energy taken in) with exothermic (energy given out / released).
• Writing descriptive phrases rather than naming the type of chemical reaction.