EXPERIMENTAL TECHNIQUES

Question 1

Using a colorimeter

Answer 1

1. Select a filter with the complimentary color to the solution being tested
2. Zero the colorimeter using cuvette of distiller water
3. Make a series of solutions of known concentration, above and below unknown sample. Measure absorbance of each and plot calibration curve (absorbance against concentration)
4. Measure unknown sample, read this of calibration curve.

Question 2

Thin Layer Chromatography

Answer 2

1. Spot any mixtures and reference sample on a pencil line 1cm from bottom of T.L.C paper
2. Place t.l.c plate in a beaker containing solvent, ensuring solvent below pencil line. Cover with watch glass or cling film
3. Remove the t.l.c when solvent is near top. Mark how far solvent has reached.
4. Allow tlc to dry. Locate any spots with locating agent - iodine or ninhydrin (a.a) or UV lamp.
5. Find rf for each value (rf= distance by spot/distance by solvent)
6. compare rf values with known sample (reference samples)

Question 3

Heating under reflux

Answer 3

1 put reactants in a pear-shaped or round-bottomed flask. Add a few anti bumping granules. Attach a condenser vertically. Do NOT stopper.

2. Connect condenser to water supply
3. Heat so liquid boils gently, using Bunsen burner.

Question 4

Recrystallization

Answer 4

1. Dissolve solid in minimum volume of hot solvent.
2. Filter, and retain filtrate
3. Allow filtrate to cook until crystals form
4. Collect crystals by vacuum filtration. Wash with minimum volume of cold solvent
5. Dry in air, in an oven set at below the melting point of the crystals or in a desiccator.

Question 5

Good solvent for Recrystallization???

Answer 5

One in which the solid to be Recrystallised is insoluble at low temperatures but very soluble at high temperatures. Solution is saturated when hot, and on cooling the substance crystallizes out leaving impurities in solution

Question 6

Standard electron potential

Answer 6

1 construct the half - cell whose electrode potential you wish to measure . Ensure standard conditions.

2. Connect the half-cell to a standard hydrogen half-cell using high-resistance voltmeter and salt bridge.
3. Check voltmeter reading is positive. If it is then the half cell connected to the positive terminal is the positive electrode.
4. Record voltmeter reading - this is the cell e.m.f (Ecell)
5. Standard electrode potentials, E0, can be measured by connecting any half cell to a standard hydrogen half-cell.

Question 7

Carrying out an organic preparation 3stages etc

Answer 7

Usually 3 stages:
Reaction
Extraction of product
Purification of product

After purification product is often analysed for purity. (Melting/boiling points or by titration)