Questions on preparing, purifying and testing an organic solid and liquid

Question 1

Describe how a sample of crude aspirin is purified by recrystallisation and how its purity is then tested.

Accepted Answer

Indicative content

dissolve the crude solid in the minimum volume of hot solvent, such as an ethanol and water mixture, so the solution is saturated
filter the hot solution through a warmed funnel if any insoluble impurities remain, so the product does not crystallise in the funnel
cool slowly to room temperature and then in ice, so the aspirin crystallises while soluble impurities stay in solution
collect the crystals by filtration under reduced pressure in a Büchner funnel and wash with a little ice-cold solvent
dry the crystals in a desiccator or a low oven, then pack a little of the powder into a capillary tube sealed at one end
heat slowly near the expected value in a melting point apparatus; a sharp melting point at 135 °C shows the sample is pure

Levels

Level 3 (5–6 marks) every stage of the recrystallisation is present with its reason: minimum hot solvent, hot filtration, slow cooling then ice, suction filtration, washing with ice-cold solvent and drying, followed by the melting point test with a sharp melting point at the expected value as the sign of purity
Level 2 (3–4 marks) dissolving in minimum hot solvent, cooling and filtering are described and the melting point named; why the solvent is minimal, why the wash is ice-cold, or what a sharp melting point shows is left unsaid
Level 1 (1–2 marks) the solid is dissolved and recrystallised, often in a large volume of solvent or washed with warm solvent, both of which lose product

Question 2

Explain why a mixture of ethanol, ethanoic acid and concentrated sulfuric acid is heated under reflux rather than in an open flask, why anti-bumping granules are added, and why the top of the condenser is left open.

Accepted Answer

B1 the reaction is slow, so the mixture must be heated for a long time, and the reactants and ester are volatile B1 the vertical condenser condenses the vapour and returns it to the flask, so nothing is lost and the mixture cannot boil dry B1 anti-bumping granules let bubbles form smoothly, so the liquid boils evenly rather than in sudden violent bursts B1 a closed system would build up pressure as it is heated and could burst or blow apart; reject 'to let air in'

Question 3

Explain why, when recrystallising crude aspirin, a student warms the funnel before the hot filtration, and why the final crystals are washed with a small volume of ice-cold solvent.

Accepted Answer

B1 the solution is saturated while hot and the product is much less soluble in cooler solvent B1 a cold funnel would cool the solution, so the product would crystallise in the funnel and paper and be lost with the insoluble impurities B1 the wash removes the solution of soluble impurities left on the surface of the crystals B1 the solvent is ice-cold and the volume small so that as little of the product as possible dissolves; accept 'to keep the yield high'

Question 4

Describe how crude ethyl ethanoate is washed with sodium carbonate solution in a separating funnel.

Accepted Answer

B1 the crude ester and sodium carbonate solution are put in the funnel and the stopper is inserted B1 the funnel is inverted and the tap opened to release the carbon dioxide, before and between gentle shakes B1 this is repeated until no more gas forms, and the funnel is left until two distinct layers separate B1 the stopper is removed and the lower aqueous layer is run off through the tap and discarded, leaving the ester

Question 5

Calculate the percentage yield of ethyl ethanoate (Mr 88.0) from the data in the table, given that 5.85 g of pure ester is collected.

Accepted Answer

M1 ethanoic acid: 10.0 × 1.05 = 10.5 g, so 10.5 ÷ 60.0 = 0.175 mol M1 ethanol: 12.0 × 0.789 = 9.47 g, so 9.47 ÷ 46.0 = 0.206 mol; the reaction is 1 : 1, so ethanoic acid is limiting M1 theoretical mass of ester = 0.175 × 88.0 = 15.4 g; error carried forward if ethanol is taken as limiting A1 percentage yield = 5.85 ÷ 15.4 × 100 = 38.0%, to three significant figures

Question 6

Explain why cold water is added to the aspirin reaction mixture after heating under reflux, and why the flask is then stood in ice.

Accepted Answer

B1 water hydrolyses the excess ethanoic anhydride to ethanoic acid, which stays dissolved in the water B1 aspirin is only slightly soluble in cold water, so it crystallises out of the solution B1 cooling in ice lowers its solubility further, so more of the aspirin crystallises and the yield of crude solid is higher

Question 7

Explain why the minimum volume of hot solvent is used to dissolve the crude solid during recrystallisation.

Accepted Answer

B1 the product is much more soluble in the hot solvent than in the cold B1 with the minimum volume the hot solution is saturated, so as much product as possible crystallises on cooling B1 extra solvent keeps more product dissolved in the cold filtrate, which lowers the yield; ignore references to purity

Question 8

Explain how a melting point determination shows whether a sample of aspirin is pure.

Accepted Answer

B1 a pure sample melts sharply, over a range of about 1 °C B1 at a temperature that matches the data-book value, 135 °C for aspirin B1 impurities disrupt the lattice, so an impure sample melts lower and over a wider range

Question 9

Calculate the percentage uncertainty in the mass of a product weighed by difference on a balance reading to 0.01 g, when the empty watch glass weighs 24.36 g and the watch glass with the product 26.51 g.

Accepted Answer

M1 mass of product = 26.51 − 24.36 = 2.15 g M1 each reading is ±0.01 g, so the mass by difference carries ±0.02 g A1 percentage uncertainty = 0.02 ÷ 2.15 × 100 = 0.93%, to two significant figures; allow 0.47% from ±0.005 g a reading

Question 10

Explain why crude ethyl ethanoate is washed with sodium carbonate solution, and why the separating funnel must be vented during the wash.

Accepted Answer

B1 sodium carbonate neutralises the unreacted ethanoic acid and the sulfuric acid catalyst B1 the acids become ionic salts that dissolve in the aqueous layer and are removed with it B1 the neutralisation releases carbon dioxide, which builds up pressure that could force out the stopper unless it is released through the tap

Question 11

Describe how anhydrous calcium chloride is used to dry the washed ester, and how the end point of drying is recognised.

Accepted Answer

B1 the ester is run into a dry conical flask and anhydrous calcium chloride is added and swirled B1 more is added until fresh solid no longer clumps and the cloudy liquid turns clear, showing the water has been absorbed B1 the dry liquid is decanted or filtered away from the solid before it is redistilled

Question 12

Calculate the percentage yield of aspirin from the data in the table. The reaction is 1 : 1 and ethanoic anhydride is in excess.

Accepted Answer

M1 moles of 2-hydroxybenzoic acid = 3.00 ÷ 138.0 = 0.02174 mol M1 theoretical mass of aspirin = 0.02174 × 180.0 = 3.913 g; error carried forward from the moles A1 percentage yield = 2.71 ÷ 3.913 × 100 = 69.3%, to three significant figures

Question 13

Suggest, for crude 1-bromobutane (density 1.27 g cm⁻³) washed with water in a separating funnel, which layer holds the product, how a student could confirm which layer is aqueous, and which layer is kept.

Accepted Answer

B1 1-bromobutane is denser than water, so it forms the lower layer B1 adding a few cm³ of water to the funnel: the layer that increases in volume is the aqueous layer B1 the lower organic layer is run off through the tap into a clean flask and kept; the upper aqueous layer is discarded

Question 14

Suggest what a melting range of 126 to 131 °C shows about a student's aspirin, two possible causes, and what the student should do next.

Accepted Answer

B1 the range is wide and below 135 °C, so the sample is impure B1 it may still contain unreacted 2-hydroxybenzoic acid or ethanoic acid, or it may not be fully dry B1 the student should dry it to constant mass and recrystallise again, then repeat the melting point

Question 15

Suggest three reasons why the yield of purified ethyl ethanoate is well below 100%.

Accepted Answer

B1 esterification is reversible, so the mixture reaches equilibrium with acid and alcohol still unreacted B1 ester is lost on each transfer, dissolved in the aqueous wash and absorbed by the drying agent B1 the ester is volatile and evaporates whenever the liquid is exposed, and fractions outside the collection range are discarded

Question 16

Compare arrangement A with arrangement B, naming each, in the arrangement of the condenser and in what each is used for.

Accepted Answer

B1 A is heating under reflux: the condenser is vertical and the vapour returns to the flask; B is distillation: it slopes away and the condensed liquid is collected separately B1 reflux is used to heat a reaction for a long time without losing volatile substances B1 distillation is used to separate a liquid by its boiling point or to remove a product as it forms, with a thermometer at the side arm

Question 17

Evaluate a student's claim that a sample of ethyl ethanoate must be pure because it distilled between 76 and 78 °C.

Accepted Answer

B1 the range matches the boiling point of ethyl ethanoate, 77 °C, and shows that high-boiling impurities have been left behind B1 ethanol boils at 78 °C, inside the same range, so the fraction could still contain ethanol B1 the claim is not justified: a boiling range is a weak test here, and an infrared spectrum with no broad O–H, or a gas chromatogram with one peak, would be needed

Question 18

Explain why phenylamine, which boils at 184 °C, distils at about 98 °C when steam is passed through the reaction mixture.

Accepted Answer

B1 phenylamine and water are immiscible, so each exerts its own vapour pressure independently of the other B1 the mixture boils when the sum of the two vapour pressures equals atmospheric pressure B1 the two together reach atmospheric pressure below 100 °C, so phenylamine distils with the steam without being heated to 184 °C, where it could decompose

Question 19

Describe how the boiling temperature of a purified liquid is measured during its redistillation, and the result that shows it is pure.

Accepted Answer

B1 the thermometer bulb is placed level with the side arm, so it reads the temperature of the vapour leaving B1 the temperature is read once it is steady and distillate is dripping from the condenser B1 a pure liquid distils at a steady temperature matching the data-book value; a mixture distils over a rising range

Question 20

Give two hazards in the preparation of aspirin from ethanoic anhydride, with a precaution for each.

Accepted Answer

B1 ethanoic anhydride is corrosive and its vapour irritates the eyes and lungs, so it is handled in a fume cupboard with gloves and goggles B1 concentrated phosphoric or sulfuric acid is corrosive, so it is added a drop at a time wearing goggles and gloves; ignore 'wear a lab coat' on its own

Question 21

Suggest two reasons why a student's purified ethyl ethanoate distils steadily at 76 °C, when the data-book boiling point is 77 °C.

Accepted Answer

B1 atmospheric pressure in the laboratory was below standard pressure, which lowers the boiling temperature B1 the thermometer reads low, because it is inaccurately calibrated or its bulb sits above the side arm rather than level with it

Quantity	Value
Mass of 2-hydroxybenzoic acid used	3.00 g
Mr of 2-hydroxybenzoic acid	138.0
Mass of dry aspirin obtained	2.71 g
Mr of aspirin	180.0