Planning experiments

Sample questions

Original practice questions written in the style of the exam.

1. Question 13 marks

   A student plans to oxidise ethanol by heating it with potassium dichromate(VI) and concentrated sulfuric acid.

   Identify three hazards in this procedure and give a precaution to control each one.

   Mark scheme

   • Ethanol is flammable — heat with a water bath or electric heating mantle, with no naked flames (1)
   • Potassium dichromate(VI) is toxic / carcinogenic — wear gloves (and avoid skin contact) (1)
   • Concentrated sulfuric acid is corrosive — wear eye protection and gloves; add it slowly with cooling (1)
2. Question 24 marks

   A brand of vinegar contains ethanoic acid at a concentration of about $1.0\,\text{mol dm}^{-3}$. A student plans to find its exact concentration by titrating $25.0\,\text{cm}^3$ portions against $0.100\,\text{mol dm}^{-3}$ sodium hydroxide, aiming for a titre of about $25\,\text{cm}^3$.

   (a) Show that the vinegar must be diluted, and by what factor.

   (b) Describe how to carry out the dilution accurately.

   (c) Choose a suitable indicator, giving a reason.

   Mark scheme

   • (a) A $25\,\text{cm}^3$ titre contains $2.50 \times 10^{-3}\,\text{mol}$ NaOH, so the diluted acid must be about $$ — a dilution factor of 10 (1)