Purifying And Filtering Methods

Question 1

How do you remove water-soluble impurities from a product?

Answer 1

•Separation - to remove any water-soluble impurities.

Question 2

Describe the method of separation for the removal of water-soluble impurities from a product. (3)

Answer 2

•1). Once reaction to form product is completed, pour the mixture into a separating funnel and add water.
•2). Shake the funnel and then allow it to settle - organic layer and aqueous layer (which contains any water-soluble impurities are immiscible (don’t mix), so separate out into two distinct layers.
•3). Open the tap and run each layer off into a separate container.
(See revision guide page 170 for diagrams)

Question 3

How do you separate a product from its impurities when they are both soluble in water? (4)

Answer 3

• Do solvent extraction - take an organic solvent in which the product is more soluble than it is in water.
• Add solvent to impure product solution and shake well.
• Product will dissolve into the organic solvent, leaving impurities dissolved in the water.
• Solvent containing product can be run off using a separating funnel.

Question 4

How do you remove water from a purified product? (4)

Answer 4

• 1). Separation = drying - organic layer will end up containing trace amounts of water (needs to be dried).
• 2). Add anhydrous salt e.g. magnesium sulfate (MgSO4) or calcium chloride (CaCl2), salt used as a drying agent which will bind to any water present to become hydrated.
• 3). When you first add the salt to the organic layer it will clump together, you keep adding the drying agent until it disperses evenly when you swirl the flask.
• 4). Filter the mixture to remove the solid drying agent - add filter paper into a funnel that feeds into a flask and then pour the mixture into the filter paper.

Question 5

How can filtering be improved?

Answer 5

•Fold it (it is fluted) to increase the surface area.

Question 6

How can impurities be removed by washing? (2)

Answer 6

• By washing the product to remove any contamination (leftover reagents or unwanted side products).
• E.g. Adding sodium hydrogencarbonate to remove acid from it, acid reacts with sodium hydrogencarbonate to give CO2 gas and the organic product is removed using a separating funnel.

Question 7

How do you purify volatile liquids? (2)

Answer 7

• By distillation - due to different boiling points.
• Works by gently heating a mixture in distillation apparatus, substances will evaporate out of the mixture in order of increasing boiling point.

Question 8

Describe the method for distillation. (4)

Answer 8

• 1). Connect a condenser to a round-bottomed flask containing the impure product in solution.
• 2). Place a thermometer in the neck of the flask so that the bulb sits next to the entrance to the condenser - temperature will show the boiling point of the substance that’s evaporating at any given time.
• 3). Heat the impure product - many organic chemicals are flammable, so an electric heater may be used.
• 4). When the product you want boils (see thermometer reading), place a flask at the open end of the condenser to collect the pure product.

Question 9

Draw and label a diagram of distillation. (8)

Answer 9

•Thermometer. 
•Water out. 
•Condenser. 
•Impure product. 
•Heat. 
•Water in. 
•Pure product in flask. 
•No lid. 
(See revision guide page 170)

Question 10

How do you purify an organic solid? (3)

Answer 10

• By recrystallisation.
• Dissolve the solid in a hot solvent to make a standard solution (the maximum possible amount of solid is dissolved in the solvent), then let it cool.
• As the solution cools, the solubility of the product falls and when it reaches the point where it can’t stay in solution it forms pure crystals.

Question 11

Describe the method for recrystallisation. (5)

Answer 11

• 1). Add very hot solvent/water to impure solid until it just dissolves (not too much added = minimum volume), should give a saturated solution of the impure product.
• 2). Filter the hot solution through a heated funnel to remove any insoluble impurities.
• 3). Leave the solution to cool down slowly - crystals of the product will form as it cools.
• 4). Remove the liquid containing the soluble impurities from the crystals by filtering the mixture under reduced pressure (in Büchner funnel).
• 5). Wash the crystals with ice-cold solvent to remove any soluble impurities from their surface and leave the purified crystals to dry.

Question 12

When crude aspirin sample is dissolved in a minimum volume of hot water during recrystallisation, what does this ensure? (3)

Answer 12

• A saturated sample is obtained.
• It increases the yield/reduces the amount left in solution.
• It enables easier crystallisation; crystals form on cooling.

Question 13

Why is hot water used in the recrystallisation of crude aspirin? (2)

Answer 13

• To remove insoluble impurities.

* To prevent crystals from forming during filtration.

Question 14

Why is the sample cooled in ice?

Answer 14

•To increase the amount of crystals formed.

Question 15

Why is the sample washed with cold water?

Answer 15

•To remove soluble impurities.

Question 16

Describe the method for filtration using a Büchner funnel for the separation of crude aspirin. (2)

Answer 16

• Cold water is added to bring the dissolved aspirin out of solution as a solid.
• Under reduced pressure in Büchner funnel (flat-bottomed funnel with holes in the base, sitting in a side-arm flask attached to a vacuum line) - pour the mixture into a filter paper lined.

Question 17

What are the advantages of using a Büchner funnel for filtration? (2)

Answer 17

• It is much quicker than allowing the liquid to pass through the filter on its own.
• Low-pressure suction allows the wet compound to dry out a little, resulting in dried aspirin on the filter paper (sometimes further drying in a warm oven may be needed).

Question 18

Draw and label a diagram of a Büchner funnel. (9)

Answer 18

• Large enough particles of solid cannot fit through tiny holes in filter paper, so remain in the top.
• Büchner funnel.
• Rubber bung.
• Büchner flask.
• Filtrate (liquid that passes through filter paper) collected in flask.
• Suction from aspirator creates a partial vacuum.
• Rubber tubing.
• Porous plate.
• Moistened filter paper.

Question 19

How are melting and boiling points good indicators of purity? (3)

Answer 19

• If sample is impure, the melting point is lowered and the boiling point is raised.
• If very impure, melting and boiling will occur across a wide range of temperatures.
• Use a melting point apparatus to accurately determine the melting point of an organic solid.

Question 20

Describe the method of using a melting point apparatus. (5) (to show that a sample is pure)

Answer 20

• 1). Pack a small sample of the solid into a glass capillary tube and place it inside the heating element.
• 2). Increase the temperature until the sample turns from solid to liquid.
• 3). Usually measure melting range - range of temperatures from where the solid begins to melt to where it has melted completely; melting point measured using a melting point apparatus/oil bath.
• 4). Look up the melting point of a substance in data books and compare it to measurements.
• 5). Impurities in sample will lower the melting point and broaden the melting range.

Question 21

Recrystallisation mark scheme. (4)

Answer 21

•Dissolve the product in the minimum volume of water/solvent (in a
boiling tube/beaker).
•Hot water/solvent.
•Allow the solution to cool and allow crystals to form.
•Filter off the pure product under reduced pressure/using a Buchner.

Question 22

Outline a simple laboratory process to show that the recrystallised product is a pure sample. (3)

Answer 22

• Measure the melting point
• Use of melting point apparatus or oil bath.
• Sharp melting point/melting point matches data source value.

Question 23

Suggest some practical reasons why the percentage yield for the recrystallisation of the reaction may not be 100%. (3)

Answer 23

• Product left in the beaker or glassware.
• Sample was still wet.
• Sample lost during recrystallisation.

Question 24

Suggest why the flask was cooled to room temperature before the crystals were filtered off. (2)

Answer 24

• The yield is lower if warm.

* The solubility is higher if warm.

Question 25

Why were the crystals compressed in the funnel (Buchner funnel)?

Answer 25

•Air passes through the sample, not just round it.

Question 26

Suggest the most likely impurity to have causes a low percentage yield value and an improvement.

Answer 26

•Water, press the sample of crystals between filter papers.