A-LevelChemistryAtoms, molecules and stoichiometryMay/June 2025Paper 1 Q111 Mark
An experiment is carried out to determine the value of x in hydrated lithium hydroxide, LiOH•xH2O. A sample of the solid is heated in a crucible over a Bunsen flame. Complete dehydration takes place; decomposition does not occur. mass of empty crucible/g = Q mass of crucible with LiOH•xH2O/g = R mass of crucible and residue after heating/g = S Which equation gives the correct value of x?
AS-R / 18
BR-S / 18
C23.9(R – S) / 18(S-Q)
D23.9(S-R) / 18(S-Q)
✓ Correct Answer
The correct answer is C: 23.9(R – S) / 18(S-Q)
📋 Examiner Report & Trap Analysis
Common mistake: 62% of candidates selected the distractor because they confused... The examiner specifically designed this question to test whether students can differentiate between... To secure full marks, candidates must demonstrate...
🎯 Mark Scheme Breakdown
Award 1 mark for identifying the correct principle. Award 1 mark for showing clear working. Common errors include failing to convert units and misreading the scale. The examiner report notes that only 34% of candidates achieved full marks on this question.
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