Practical skills and techniques

Question 1

Colorimetry

Answer 1

Uses relationship between colour intesnity of solution and the concentration of the coloured species present.

Question 2

Technique of colorimetry

Answer 2

• series of standard solutions prepared
• suitable wavelength/filter selected
• Using a blank
• absorbance of unknown measured and concentration determined using calibration graph

Question 3

Hows a calibratiuon graph prepared

Answer 3

Serial dilution using a standard solution, absorbance of each is measured by colorimeter, plot on graph.

Question 4

Distillation

Answer 4

Used for purification and determination of organic compounds,
* Purifies by separating compound from volatile substances in the mixture.
* Determines boiling points, a physical property used to confirm its identities.

Question 5

Heating under reflux

Answer 5

Allows heat energy to be applied to a chemical reaction over extended period of time without volatile substances escaping

Question 6

Vacuum Filtration

Answer 6

Involves carrying out a filtration with reduced pressure, separates precipitate from filtrate faster. Use a Buchner funnel.

Question 7

Gravity filtration

Answer 7

Increased surface area allows for fast filtration.

Question 8

Recrystallisation

Answer 8

Purifies solids, the solids have to be very soluble at hot temps, not verry at cold!!
1. Impure solid gently dissolved in minimum volume of hot solvent
2. Hot filtration of resulting mixture to remove insoluble impurities
3. Filtrate is cooled slowly, allowing crystals of pure compound to form, leaving dissolved impurities in solvent.
4. Filter wash and dry pure crystals.

Question 9

During solvent extraction

Answer 9

Two immiscible solvents form two layers in a separating funnel. Solute dissolves in both and establishes an equilibirum. Ratio is determined by K. Lower layer run off, and repeated to maximise solute extracted.

Question 10

How can identity of pure compound be determined

Answer 10

Melting point analysis and comparison. Presence of impurities lowers melting point and widens its range.

Question 11

Rf

Answer 11

Rf = distance travelled by sample
distance travelled by solvent