The questions reward specific hazard-control links: identify toxic, oxidising or corrosive risks, then choose gloves or a fume cupboard only when the context supports it.
Brass is an alloy of copper. The percentage by mass of copper in a sample of brass can be determined by a three-stage process.
Stage 1 Oxidation of the copper to copper(II) ions with excess concentrated nitric acid.
Stage 2 Reduction of the copper(II) ions to copper(I) ions with excess iodide ions.
Stage 3 Titration of the iodine produced in Stage 2 against a standard solution of sodium thiosulfate.
In Stage 1, 2.53 g of brass is carefully warmed with 25 cm³ concentrated nitric acid in a 250 cm³ beaker.
The equation for the oxidation of copper metal by nitric acid is shown.
Cu(s) + 4H⁺(aq) + 2NO₃⁻(aq) → Cu²⁺(aq) + 2NO₂(g) + 2H₂O(l)
State two hazards in this experiment, giving the precautions you would take to minimise the risk associated with each hazard.
Assume safety spectacles and a laboratory coat are worn.
An answer that makes reference to the following points:
- (concentrated) nitric acid is corrosive and wear/use gloves
- nitrogen (di)oxide/ NO_2 (gas) is toxic/corrosive and carry out experiment in a fume cupboard
Allow well ventilated room Ignore reference to flames/bunsens/gas mask/face mask
Excess nitric acid must be neutralised before the iodide ions are added in Stage 2.
Sodium carbonate solution is slowly added to the cooled solution. The solution is neutralised when a faint precipitate appears.
Suggest why the solution is cooled before the addition of the sodium carbonate solution.
An answer that makes reference to the following point:
- to prevent too much effervescence/fizzing / the reaction mixture spilling over / CO_2 being formed too quickly
Allow the reaction is vigorous/ to slow down the reaction