Quick answer: IUPAC nomenclature names an organic compound from its longest carbon chain, its functional group and its substituents, with numbers giving their positions. This Chemistry 11-12 lab on organic nomenclature gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Can we measure how a chiral molecule bends light? In this lab you build a simple polarimeter from two polarising filters and measure how much a sugar solution rotates plane-polarised light at different concentrations, then use a calibration line to find an unknown concentration.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB S3.2
Aim
To show that a sugar solution rotates plane-polarised light, to find how the rotation depends on concentration, and to use the result to determine the concentration of an unknown solution.
Equipment
- Two polarising filters (for example from sunglasses or a lens kit) mounted in cardboard holders, one with a protractor scale
- White lamp or bright phone screen, clear tube or cuvette 10.0 cm long, caps
- Sucrose, balance, 100 cm³ beaker, measuring cylinder, stirring rod
- Standard solutions of 0.10, 0.20, 0.30 and 0.40 g cm⁻³ and an unknown
Safety
Do not look directly at a bright lamp. Wipe up sugar spills at once because they make floors slippery. Do not taste any laboratory solution.
Method
- Fill the tube with water, place it between the two filters and rotate the analyser until the light is darkest. Record this as 0°.
- Replace the water with each sugar solution in turn. Rotate the analyser until it is darkest again and record the angle turned.
- Repeat each measurement three times and take the mean.
- Measure the unknown solution in the same way.
Results
| Concentration (g cm⁻³) | 0 | 0.10 | 0.20 | 0.30 | 0.40 | unknown |
|---|---|---|---|---|---|---|
| Rotation (°) | 0 | 7 | 13 | 19 | 27 | 16 |
Analysis questions
Question 1 Graph
Plot rotation against concentration and describe the graph.Question 2 Specific rotation
The tube is 1.0 dm long. Calculate the slope of the best-fit line through the origin from these data and state its units; this is the specific rotation.Question 3 Unknown concentration
Use the slope to find the concentration of the unknown solution.Question 4 Longer tube
Predict the rotation for the 0.30 g cm⁻³ solution in a 2.0 dm tube.Question 5 Enantiomers
Explain why a solution containing equal amounts of two enantiomers would give no rotation.Question 6 Uncertainty
Estimate the percentage uncertainty in the 7° reading if the angle can be read to ±1°.Question 7 Improvement
Suggest two improvements that would make the measurements more reliable.
