A-LevelChemistryAtoms, molecules and stoichiometryOct/Nov 2020Paper 5 Q110 Marks
Calcium hydroxide, Ca(OH)₂, is slightly soluble in water. Aqueous calcium hydroxide is also known as limewater. A student measures the concentration of saturated aqueous calcium hydroxide using the following method. step 1 Add a spatula measure of solid calcium hydroxide to 250 cm³ of distilled water in a conical flask. Stopper the flask and shake the flask thoroughly. step 2 Leave the flask stoppered for 24 hours. step 3 Filter the aqueous calcium hydroxide into a clean, dry beaker. step 4 Transfer 25.00 cm³ of the aqueous calcium hydroxide to a clean conical flask. step 5 Carry out a titration with 0.100 mol dm⁻³ hydrochloric acid. step 6 Record the volume of hydrochloric acid required to react with the aqueous calcium hydroxide.
📋 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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