Paper 3 Exam Questions

Question 1

Explain why a pH meter should be calibrated before use.

Answer 1

over time meter does not give accurate readings

Question 2

Describe a method for determining the melting point of an organic sample. State how the result obtained could be used to evaluate its purity.

Answer 2

→ ensure sample solid is dry & finely powdered
→ place solid into a capillary tube attached to a thermometer
→ place this in oil bath and gently heat
→ record the temperatures at which sample begins to melt
→ pure substances have sharp well defined melting points
→ impure substances have a broad melting point range

Question 3

Describe the apparatus used to remove the drying agent by filtration under reduced
pressure.

Answer 3

→ Buchner funnel (flat bottom) containing filter paper
→ Buchner flask connected to vacuum pump

Question 4

Outline a method for recrystallisation.

Answer 4

→ dissolve sample in minimum volume of hot water
→ filter (hot to remove insoluble impurities)
→ allow solution to cool + form crystals
→ filter off the pure product under reduced pressure // using a Buchner funnel

Question 5

Suggest two practical reasons why the percentage yield of a recrystallisation reaction may not be 100%.

Answer 5

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

Question 6

Explain why during the sample is filtered hot during recrystallisation.

Answer 6

to avoid crystals forming during filtration