Recrystallisation Of Benzoic Acid And Measuring Its Boiling Point

Question 1

Describe the method used to measure the melting point of a sample of benzoic acid

Answer 1

• fill melting point tube with crystals by dipping open end into sample of crystals to ensure its stuck in top of capillary tube
• drop capillary tube, closed end downwards through thick walled glass tube held vertically
• place in melting block and heat gently with hot plate
• observe the melting point range by finding the temperature at which liquid first appears and temperature at which no solid remains

Question 2

Name a lab technique that could be used to purify impure benzoic acid

Answer 2

Recrystallisation

Question 3

The melting points obtained by students were
Sample A - 117-120
Sample B - 120-121
Which was the purer sample and why?

Answer 3

B

Higher melting point with narrower range

Question 4

The students were required to recrystallise the impure benzoic acid, what solvent would be used for recrystallisation? Explain why solvent is suitable

Answer 4

Water - very soluble in hot but slightly less soluble in cold

Question 5

Name the instrumental technique that could have been used to confirm the identity of the product based on its absorbtion of a unique set of low energy electomagnetic radiations

Answer 5

Infra red spectroscopy

Question 6

Give one important use of benzoic acid or its salts

Answer 6

Food preservative, disinfectant

Question 7

In this experiment it is advisable to use the minimum amount of solvent possible. What is the reason for this and how is it achieved?

Answer 7

To maximise the yield - achieved by adding the water in small portions until the benzoic acid has just dissolved

Question 8

Is benzoic acid a covalent or ionic compound?

Answer 8

Covalent - only sparingly soluble in water and the melting point is reasonably low (molecular crystal)

Question 9

Describe how the speed of filtering is increased

Answer 9

Attaching the Buchner flask to a suction pump

Question 10

If little or no crystals appeared, what steps could be undertaken?

Answer 10

• the conical flask containing the filtrate could be cooled by surrounding it with ice
• scratching the inside of the flask with a glass rod helps to speed up the formation of crystals
• if both fail, then too much solvent has been used. Allow water to evaporate either by boiling or by placing it in an evaporating basin overnight

Question 11

Describe the apperance of pure benzoic acid

Answer 11

White crystalline solid

Question 12

Outline how you would know the sample is pure? (give to differences that would be observed when measuring the melting points of the pure and impure samples)

Answer 12

• melt at sharp temperature

- melts at higher temperature than impure

Question 13

How does Recrystallisation work?

Answer 13

It works on the principle that more solute is soluble in a hot solvent than in a cold solvent. The
substance to be purified is dissolved in the minimum amount of hot solvent possible and then this is
filtered to remove any insoluble impurities.

Question 14

Why must recrystallisation occur quickly?

Answer 14

To prevent it from cooling

Question 15

Method for recrystallisation

Answer 15

1. About 5 g of benzoic acid was placed in a beaker.
2. This was dissolved in the minimum amount of hot water. Minimum so that the benzoic acid does
   not remain in solution on cooling to maximise the yield.
3. First filtration - This hot solution was filtered through a Buchner funnel (under suction, so, fast).
   Insoluble impurities remain on the filter paper.
4. The filtrate was allowed to cool slowly.
5. Second filtration - The excess solvent was filtered off and the crystals remain on the filter paper and
   were allowed to dry.