Prac 10 Flashcards
Required Practical 10a
Purification and Analysis of an organic solid
Dissolve in a minimum volume of hot (near boiling) solvent.
Hot filter solution quickly through fluted filter paper by inserting beaker in ice.
Reduce pressure with a Büchner flask to speed filtration.
Cool the filtered solution under reduced pressure to separate out crystals.
Wash the crystals with distilled water.
Dry the crystals between absorbent paper or in a drying cabinet.
Weigh the amount of dry product. Calculate the % yield.
Test the purity by conducting a melting point determination.
Comment on the purity.
Dissolve in a hot solvent: Why?
Creates a ‘saturated solution’ of the organic compound. It will also contain soluble impurities but not insoluble impurities.
Filter through filter paper: Why?
Filter through filter paper: This removes the insoluble impurities.
Cool in an ice bath: Why?
Crystallizes the compound because the saturated solution can no longer be maintained in the beaker.
Büchner apparatus: Why, apparatus
A faster, more efficient method of filtration. A thick-walled flask is connected to a vacuum which pushes the filtrate through the filter paper.
Buchner flask
vacuum
clamp
Buchner funnel
Filter paper
Stand
Melting point determination (Mpt): What are signs of purity?
The pure product should look whiter and have finer, needle-like crystals. Pure substances have a sharp melting point. Impure substances melt over a range of temperatures.
Wash with deionised water: Why
This will remove soluble impurities.
Dry: Why?
This will attempt to get an accurate mass of product for the % yield calculation.
Required Practical 10b
Purification and Analysis of an organic liquid.
Put the impure product into a separating funnel.
Add NaCO₃ solution which neutralises any excess acid and produces CO₂ gas, which needs to be vented regularly by inverting the separating funnel and opening the tap.
Separate the aqueous and non-aqueous layers by running the bottom aqueous layer into a beaker and discard it.
Run the organic layer into a clean, dry flask.
Add anhydrous sodium sulphate.
Decant the organic layer into a distillation flask.
Distil the product a final time, making a note of the temperature of the major fraction.
Record the mass of the product and calculate % yield.
Add NaHCO₃ solution. : Why
The NaHCO₃ neutralises excess acid.
This is also the reaction of a metal hydrogen carbonate and an acid, so it produces CO₂ gas. The CO₂ gas is released regularly by inverting the separating funnel and opening the tap.
Separate the aqueous and non-aqueous layers. Why?
This is tricky because the layers look similar and it is sometimes difficult to identify where one layer stops and the other starts.
Distil the product. Why?
The temperature of the major fraction should give an indication of purity.
Add anhydrous sodium sulphate. Why?
This is a drying agent and will absorb any water impurities in the product. The drying agent must be inert and insoluble in the organic layer.
Why are there layers in this practical?
Liquids are immiscible
Density difference
