Required Practical 10 - Preparation Of A Pure Organic Solid And Test Of Its Purity

Front 1

Dissolve the impure compound in a minimum volume of hot solvent

Why?

Back 1

• hot - an appropriate solvent will dissolve both the compound and impurities when hot, and which the compound itself does not dissolve well when cold

• min vol - obtain a saturated solution, enables crystallisation on cooling

Front 2

Hot filter solution through fluted filter paper quickly

Why?

Back 2

Removes any insoluble impurities

Heat - prevents crystals reforming during filtration

Front 3

Cool the filtered solution by inserting beaker in ice

Why?

Back 3

Crystals will reform but soluble impurities will remain in solution form because they are present in small quantities so solution is not saturated

Ice - increases the yield of crystals by allowing cooling to a lower temp

Front 4

Suction filtrate with a Buchner flask

Why?

Back 4

Separate out the crystals

Buchner flask speeds up filtration

Front 5

Wash the crystals with distilled water

Why?

Back 5

To remove soluble impurities

Front 6

Dry the crystals between absorbent paper

Why?

Back 6

Dry the crystals

Front 7

What are the reasons for loss of yield in purifying organic solid?

Back 7

• Crystals lost when filtering or washing

• Some product stays in solution after recrystallisation

• Other side reactions occur

Front 8

What can cause the percentage yield to be greater than 100% in purifying organic solid?

Back 8

If the crystals are not dried properly, their mass will be larger than expected

Front 9

Why are ethanoic anhydrides used in the preparation of aspirin instead of acyl chlorides?

Back 9

• cheaper

• less corrosive

• less vulnerable to hydrolysis

• less dangerous to use

Front 10

What is the process of purifying an organic solid called?

Back 10

Recrystallisation

Front 11

How do you get the recrystallisation solution hot?

Back 11

Water bath

Front 12

How do we test for the degree of purity of an organic solid?

Back 12

Determine the melting point (or range)

Front 13

If the sample is very pure, what will you observe when you go to determine the melting point of the organic solid?

Back 13

The melting point will be a sharp one, at the same value as quoted in data books

Front 14

If impurities are present, what will you observe when you go to determine the melting point of an organic solid?

Back 14

• The melting point will be lowered

• The sample will melt over a range of several °C

Front 15

What could cause an error when measuring the melting point of an organic solid to determine the degree of purity?

Back 15

If the temperature on the thermometer is not the same as the temperature in the actual sample tube

Front 16

Describe the general method to purify an organic liquid?

Back 16

• put the distillate of impure product into a separating funnel

• Add NaCl - helps the layers separate

• Discard the aqueous lower layer

• Run the organic layer into a clean dry round bottom flask

• Add CaCl2 drying agent - to dry the organic liquid - when dry the organic liquid should appear clear

• Carefully decant the liquid into the distillation flask

• Distil to collect pure product

Front 17

The drying agent used in purifying an organic liquid should be:

Back 17

• Insoluble in the organic liquid

• Not react with the organic liquid

Front 18

Describe how to determine the purity of an organic liquid?

Back 18

Measuring a boiling point - distillation set up

• If the liquid is pure it will have the boiling point referred to in data books

• If impure the boiling point tends to be higher than the pure liquid

Front 19

When determining the boiling point of a organic liquid, why is it important to note the pressure?

Back 19

Changing pressure can change the boiling point of a liquid

Front 20

One might measuring the boiling point of an organic liquid be the most accurate method of identifying a substance?

Back 20

Several substances may have the same boiling point