Use this Unit 6 bank to revise practical reasoning, graph plotting, concentration calibration, uncertainty calculations and method improvements across unfamiliar experiments.
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.
State what would be seen during the reaction.
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.
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 of a homogeneous solution before Stage 2.
This question is about the nitration of methyl benzoate.
The equation for the reaction is shown.
Procedure
Step 1 Weigh between 1.9 g and 2.1 g of methyl benzoate in a conical flask.
Step 2 Slowly add of concentrated sulfuric acid to the methyl benzoate with swirling and place the flask in an ice-water bath to cool.
Step 3 Place of concentrated nitric acid into a test tube.
Cool the nitric acid by immersing the test tube in an ice-water bath before slowly adding of concentrated sulfuric acid.
Allow this nitrating mixture to cool.
Step 4 Using a teat pipette, add the nitrating mixture very slowly to the conical flask, ensuring the temperature does not exceed .
Step 5 Allow the flask to stand at room temperature for about 15 minutes and then pour the contents into a beaker containing some crushed ice. Impure methyl 3-nitrobenzoate will form.
Step 6 Recrystallise the methyl 3-nitrobenzoate using methanol as the solvent.
Step 7 Weigh the dry crystals and determine their melting temperature.
A bottle of concentrated nitric acid has two hazard warning signs.
State the two hazards.
Give a precaution to reduce the risk when using concentrated nitric acid. Assume that safety goggles and a laboratory coat are used.
Explain why the nitrating mixture is added slowly in Step 4.
1.95 g of methyl benzoate reacted with an excess of nitric acid to form
1.51 g of methyl 3-nitrobenzoate.
[Molar mass values: methyl benzoate,
methyl 3-nitrobenzoate, ]
Give one possible reason why the yield in (d)(i) is less than 100\%.
Sodium thiosulfate can be used to determine the concentration of an iodine solution by titration using starch indicator.
5.00 g of sodium thiosulfate was dissolved in deionised water and the solution made up to in a volumetric flask.
The volumetric flask has an uncertainty of .
What is the minimum uncertainty of the balance required to match the uncertainty of the volumetric flask?
Assume two weighings are needed.
±0.0025 g
±0.005 g
±0.01 g
±0.05 g
Ethyl butanoate reacts with water.
A student investigated the kinetics of this reaction.
Procedure
Step 150 cm^3 of ethyl butanoate was heated under reflux with a very large excess of water.
Step 2 After 30 minutes a sample of known volume was removed from the reaction mixture and placed in a conical flask.
Step 3 This sample was titrated using aqueous sodium hydroxide solution.
Step 4 Further samples were removed from the refluxing mixture at 30-minute intervals. Each sample was titrated until the titres of three consecutive samples were the same.
Name the apparatus which is most suitable to remove the samples from the reaction mixture.
The student suggested placing ice in the conical flask before carrying out each titration.
Explain whether or not this suggestion would improve the validity of the data collected.