18C.3 Practical preparation and purification Quiz: Pearson Edexcel Chemistry, Unit 18

The 20 questions

1. What is recrystallisation used for?

   • Purifying a solid by dissolving it in minimum hot solvent, then cooling to crystallise the pure product
   • Separating a dissolved solid from a solution by evaporating the solvent to dryness in an evaporating dish
   • Removing water from a liquid by adding a drying agent and then filtering off the solid salt formed
   • Separating two liquids with different boiling temperatures by heating them in a fractionating column

2. Why is the minimum volume of hot solvent used in recrystallisation?

   • So that the solvent evaporates faster during the cooling stage and the crystals form more quickly
   • So that the impurities dissolve completely in the solvent and are then removed in the filtration step
   • So the solution is saturated when hot and crystallises well on cooling, giving a good yield
   • So that the crystals are smaller and easier to dry on the filter paper after the solution is cooled

3. What is the purpose of washing an organic liquid with sodium hydrogencarbonate solution?

   • To remove acidic impurities by converting them into soluble salts that dissolve in the aqueous layer
   • To heat the product and remove the last traces of solvent by evaporating the water phase to dryness
   • To dissolve the organic product in the water layer so that it can be extracted and collected separately
   • To lower the boiling temperature of the impurities so that they distil off before the product does

4. What is the purpose of anhydrous magnesium sulfate in drying an organic liquid?

   • To react with the organic liquid and remove its double bonds
   • To dissolve the water into a salt layer that separates
   • To catalyse the reaction between the liquid and the drying agent
   • To absorb traces of water from the liquid, after which the solid is filtered off

5. What is solvent extraction used for?

   • Measuring the boiling point of a liquid mixture under reduced pressure, using a vacuum pump and a thermometer
   • Transferring a compound from an aqueous solution into an immiscible organic solvent in which it is more soluble
   • Separating two solids by their different melting temperatures, using a heated block and a melting point tube
   • Converting a solid into a liquid by heating it with a solvent until the solid dissolves completely in the mixture

6. What is reflux used for?

   • Cooling the reaction to stop it at a fixed time, so that the amount of product formed can be measured
   • Separating a mixture by boiling off the lower-boiling liquid, which is then collected in a receiving flask
   • Removing solvent by evaporation until a solid is left behind, which is then weighed to find the yield
   • Heating a reaction for a long time without losing volatile vapours, since they condense and return to the flask

7. What does a sharp melting temperature close to the literature value suggest?

   • The melting temperature gives the molar mass directly
   • The solid is likely to be pure
   • The solid contains oxygen
   • The solid is an ionic compound

8. What does a sharp boiling temperature close to the literature value suggest for a liquid?

   • The boiling temperature gives the molecular formula of the liquid
   • The liquid is a gas at room temperature
   • The liquid is likely to be pure
   • The liquid contains a chiral centre

9. Which technique separates a volatile liquid product from a non-volatile residue?

   • Refluxing with a catalyst
   • Distillation
   • Recrystallisation
   • Drying with an anhydrous salt

10. Why is steam distillation used for a water-insoluble organic compound with a high boiling temperature?

    • It dissolves the compound into the steam for collection, which keeps the product from decomposing
    • It removes the water from the product as a gas, so the product can be collected as a dry, pure liquid
    • It converts the compound into a vapour that does not condense, so the product is collected as a gas
    • It lets the compound distil at a temperature below its boiling point, reducing decomposition

11. When a solution containing an acid is washed with sodium hydrogencarbonate, what is observed?

    • The solution turns green with no gas, as the acid reacts with the hydrogencarbonate to form a salt
    • Effervescence, as CO2 is released when the acid is neutralised
    • A white precipitate forms immediately as the acid and the hydrogencarbonate solution are mixed together
    • The organic layer turns black from the reaction, as the acid is oxidised by the hydrogencarbonate ion

12. After washing, two layers form in a separating funnel. What is the next step to separate them?

    • Run off the lower layer through the tap, then collect the upper layer separately
    • Filter the mixture through paper to remove the top layer, which is then discarded before the lower layer is used
    • Heat the mixture until one layer disappears, so that only the product layer remains for collection
    • Evaporate the top layer to dryness directly, so the product is recovered without any further separation

13. A recrystallisation starts with 5.00 g of crude solid and gives 4.20 g of pure product. What is the percentage yield?

    • 119%
    • 84.0%
    • 42.0%
    • 16.0%

14. A pure compound melts at 122 C, but a sample melts over the range 110 to 115 C. What is the best conclusion?

    • The sample is pure, since a range of melting temperatures shows high purity
    • The sample has decomposed to a gas, so the melting range is meaningless
    • The sample is a different compound with a higher melting temperature
    • The sample is impure, since impurities lower and broaden the melting range

15. Why is a single recrystallisation solvent not always suitable?

    • Recrystallisation removes all water from the product, which is not wanted in the final crystal form
    • Recrystallisation works only for liquids, not solids, so it cannot be used for crystalline products
    • Some compounds are too soluble or too insoluble in every solvent, so no suitable solvent may exist
    • Recrystallisation always gives a 100% yield, so it is rarely used because it wastes no product at all

16. Which purification sequence is best for an organic liquid product made in a reaction that produces an acid by-product?

    • Dry with an anhydrous salt, then wash with water, then filter, so that the product is separated from the acid
    • Wash with sodium hydrogencarbonate, separate, dry with anhydrous magnesium sulfate, then distil
    • Distil straight away without washing, since distillation removes all acids and then the product is dried
    • Recrystallise from hot ethanol, then dry over sodium hydroxide, which removes the acid and then the water

17. Why is the yield of a purification step always less than 100%?

    • Some product is lost during transfers, filtrations and washing, so the mass is never fully recovered
    • Purification removes only impurities, so the product is never affected by any of the separation steps
    • Purification always creates new product, so the yield after purification is greater than 100 percent overall
    • Impurities always react to form product during purification, which is why yields are often above 100 percent

18. A 5.00 g crude sample gives 3.00 g of pure product. What is the percentage recovery?

    • 3.00%
    • 40.0%
    • 166.7%
    • 60.0%

19. A student adds anhydrous drying agent to a liquid, but the liquid is still cloudy. What should they do?

    • Heat the liquid until the cloudiness disappears completely, so that the water is driven off as a vapour
    • Add more acid to convert the cloudy particles into a salt, which then settles out of the liquid
    • Add water to dilute the cloud and then distil, so that the cloudy particles are carried over
    • Add more drying agent until some swirls freely without clumping, then filter

20. Why is a separating funnel used after washing an organic liquid?

    • To filter a solid from a liquid, using paper and a funnel after the wash
    • To distil a liquid from a solid residue, using a condenser and flask
    • To heat a mixture under reflux, keeping the vapour in the flask
    • To separate two immiscible liquids of different densities

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