Questions on apparatus, techniques and safety: the twelve required practicals in outline

Question 1

Describe the hazards, and a control measure for each, when a student prepares ethanoic acid by heating ethanol under reflux with potassium dichromate(VI) acidified with sulfuric acid, which she first dilutes from the concentrated acid.

Accepted Answer

Indicative content

concentrated sulfuric acid is corrosive to skin and eyes, so it is dispensed in small volumes wearing goggles and gloves
diluting the concentrated acid releases a great deal of heat, so the acid is added slowly to the water and never water to the acid
ethanol is flammable and its vapour spreads along a bench, so the flask is heated with a water bath or electric heater and no naked flame
potassium dichromate(VI) is a carcinogen and an oxidising agent, so it is handled in small amounts with gloves and any spill is cleared at once
a sealed apparatus that is heated builds up pressure and can burst, so the top of the reflux condenser is left open to the air
anti-bumping granules make the mixture boil smoothly, preventing sudden surges that could throw hot acidic liquid out of the flask

Levels

Level 3 (5–6 marks) all the hazards are identified with a control measure for each: the corrosive concentrated acid and the order of dilution, the flammable ethanol and the heat source, the toxic dichromate(VI), the open top of the reflux condenser and the anti-bumping granules; each control is matched to its hazard rather than listed separately
Level 2 (3–4 marks) three or four hazards each have a sensible control; the order of dilution, the open top of the condenser, or the flammability of ethanol is left out or matched to the wrong control
Level 1 (1–2 marks) one or two hazards such as the corrosive acid are named, with controls no more specific than 'wear goggles', or with water poured into the concentrated acid to dilute it

Question 2

Describe how exactly 25.0 cm³ of a solution is transferred into a conical flask for a titration, naming the apparatus used.

Accepted Answer

B1 a 25.0 cm³ volumetric pipette, used with a pipette filler; reject a measuring cylinder or a burette B1 rinse the pipette with distilled water and then with the solution it will hold B1 draw solution above the line, then release it until the bottom of the meniscus is on the line at eye level B1 run the solution into the flask and touch the tip on the inside of the flask, leaving the last drop in the tip

Question 3

Suggest which CPAC criterion each of the four lab-book entries given in the table falls short of, and what the student should do instead.

Accepted Answer

B1 entry 1 falls short of recording observations (CPAC 4): every burette reading is recorded to two decimal places, as 23.40 and 23.50 B1 entry 2 falls short of using equipment safely (CPAC 3): flammable cyclohexane is heated with a water bath or electric heater B1 entry 3 falls short of controlling variables (CPAC 2): the volume of acid is kept the same in every run B1 entry 4 falls short of referencing (CPAC 5): the source is cited fully enough for a reader to find it

Question 4

Explain why the cooling water enters a reflux condenser at the bottom and leaves at the top, and why the top of the condenser must not be stoppered.

Accepted Answer

B1 water entering at the bottom fills the outer jacket completely B1 water entering at the top would drain straight through, leaving part of the condenser uncooled so vapour could escape B1 heating a closed apparatus builds up pressure from the vapour B1 the pressure could make the apparatus burst, so the top is left open to the air

Question 5

Describe how a liquid organic product contaminated with an acid and with water is purified, using a separating funnel.

Accepted Answer

B1 shake the product with sodium hydrogencarbonate solution in a separating funnel, opening the tap often to release carbon dioxide B1 let the layers settle and run off the lower layer through the tap, keeping the organic layer B1 dry the organic layer with an anhydrous salt such as magnesium sulfate until the liquid is clear, then filter or decant B1 distil the liquid and collect the fraction that boils within a degree or two of the product's boiling point

Question 6

Describe how an impure organic solid is purified by recrystallisation.

Accepted Answer

B1 dissolve the solid in the minimum volume of hot solvent B1 filter the hot solution through a warmed funnel to remove insoluble impurities B1 let the filtrate cool so the product crystallises while soluble impurities stay in solution B1 collect the crystals by filtration under reduced pressure, wash them with a little cold solvent and leave them to dry

Question 7

Explain the low yield, and what the melting point shows, when a student recrystallises 5.0 g of impure benzoic acid from a large volume of hot water and obtains 1.2 g of crystals that melt between 118 °C and 121 °C. Pure benzoic acid melts at 122 °C.

Accepted Answer

B1 too much hot solvent was used, so the solution was not saturated when it cooled B1 much of the benzoic acid stayed dissolved in the cold solution and was lost in the filtrate, lowering the yield; allow washing with warm solvent B1 the crystals melt below 122 °C, so impurities are still present, since impurities lower a melting point B1 they melt over a range of 3 °C instead of sharply, which also shows the solid is impure

Question 8

Explain the difference between a hazard and a risk, using the dilution of concentrated sulfuric acid as the example, and how the risk of the dilution is kept low.

Accepted Answer

B1 a hazard is a substance, object or activity with the potential to cause harm: the concentrated acid is corrosive wherever it is B1 a risk is the likelihood that the harm will occur in the way the activity is done, together with its severity B1 the acid is added slowly to water, never water to acid, because the heat released would boil water dropped onto the acid and spatter it B1 small volumes are used, wearing goggles and gloves, which lowers the chance and the severity of harm

Question 9

Evaluate a student's plan to prepare butanal by heating butan-1-ol with acidified potassium dichromate(VI) under reflux for thirty minutes and then distilling the mixture. Butanal boils at 75 °C, butan-1-ol at 118 °C and butanoic acid at 164 °C.

Accepted Answer

B1 under reflux the butanal formed stays in the flask, in contact with the oxidising agent B1 it is oxidised further to butanoic acid, so the plan gives the acid and little or no aldehyde B1 the butanal should be distilled off as it forms, adding the oxidising agent slowly to the warm alcohol in a distillation apparatus B1 butanal boils well below the alcohol and the acid, so the fraction collected near 75 °C is the aldehyde; the plan as written is unsuitable

Question 10

Explain why ethanol is heated under reflux with acidified potassium dichromate(VI) to make ethanoic acid.

Accepted Answer

B1 ethanol and the intermediate ethanal are volatile, and the reaction is slow, so the mixture must be heated for a long time B1 the vertical condenser condenses the vapours and returns them to the flask, so no reactant or product is lost B1 ethanal stays in contact with the oxidising agent until it is oxidised completely to ethanoic acid

Question 11

Suggest the effect of the fault in the distillation apparatus drawn, in which the thermometer bulb dips into the boiling liquid, and how the set-up should be corrected.

Accepted Answer

B1 the thermometer then reads the temperature of the liquid mixture, not of the vapour passing into the condenser B1 the reading is not the boiling point of what is distilling, and the right fraction cannot be identified or collected B1 the bulb should be level with the side arm of the still head; accept 'at the entrance to the condenser'

Question 12

Compare filtration under gravity with filtration under reduced pressure.

Accepted Answer

B1 filtration under gravity uses a fluted filter paper in a funnel, whereas reduced pressure uses a flat paper in a Büchner funnel on a side-arm flask connected to a pump B1 filtration under reduced pressure is much faster and leaves the solid almost dry B1 gravity filtration is chosen when the filtrate is wanted, reduced pressure when the solid is wanted

Question 13

Calculate the mass of solid transferred, and its percentage uncertainty, when a weighing bottle and solid weigh 17.62 g before the transfer and 15.48 g after, on a balance whose display reads to 0.01 g.

Accepted Answer

M1 mass transferred = 17.62 − 15.48 = 2.14 g M1 the mass is the difference of two readings, each ±0.01 g, so its uncertainty is ±0.02 g A1 percentage uncertainty = 0.02 ÷ 2.14 × 100 = 0.93%

Question 14

Explain why liquid bromine is dispensed only in a fume cupboard, and why bromine water is used for the test for an alkene.

Accepted Answer

B1 liquid bromine is very volatile and its vapour is toxic, damaging the lungs B1 the fume cupboard draws the vapour away from the user and out of the room B1 bromine water is a dilute solution that gives the same colour change with an alkene at a much lower risk

Question 15

Justify the choice of water as the solvent for recrystallising impure benzoic acid, given that benzoic acid dissolves at 2.9 g dm⁻³ in water at 20 °C and at 68 g dm⁻³ at 95 °C.

Accepted Answer

B1 a recrystallisation solvent must dissolve the compound well when hot and poorly when cold, and the twenty-fold change between 20 °C and 95 °C gives exactly that B1 the impure solid is dissolved in the minimum volume of near-boiling water, so the solution is saturated and crystals form as it cools, while soluble impurities stay in the small volume of cold solution B1 only about 2.9 g dm⁻³ stays dissolved at 20 °C, so most of the benzoic acid is recovered; insoluble impurities are removed by filtering the hot solution before it cools

Question 16

Suggest why a mixture containing cyclohexane is heated with a water bath or an electric heating mantle and not with a Bunsen burner.

Accepted Answer

B1 cyclohexane is flammable, and its vapour can spread to a flame some distance away B1 a water bath or heating mantle has no naked flame, so the vapour cannot be ignited by the heat source

Question 17

Identify the technique used to separate a liquid product that boils at 56 °C from an unreacted starting material that boils at 118 °C, and give the reason it works.

Accepted Answer

B1 simple distillation, collecting the fraction that comes over close to 56 °C B1 the product boils at a much lower temperature, so it vaporises and condenses in the receiver while the starting material stays in the flask

Entry	What the lab book says
1	Titres: 23.4, 23.45, 23.5
2	Heated the cyclohexane mixture over a Bunsen burner
3	Used 25 cm³ of acid in run 1 and 30 cm³ in run 2 to speed things up
4	Melting point of aspirin 135 °C, from a website