The evidence is narrow but concrete: know when effervescence has stopped, then transfer, rinse, make up to the mark and mix in a volumetric flask.
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)
Ethanoic acid solution is added, drop by drop, to the neutralised mixture until the precipitate redissolves.
Describe how the solution in the beaker should then be made into 250.0 cm3 of a homogeneous solution before Stage 2.
A description that makes reference to three of the following points:
- (transfer contents of beaker to) and washings to a
(1) volumetric flask
- make up to the mark with (distilled/deionised)
(1) water
- shake / mix
If beaker/measuring cylinder used penalise in M1 only If incorrect solvent e.g. ethanol/ethanoic acid used penalise in M2 only
Allow standard / graduated flask
Allow any indication of mixing e.g. inverting/swirling