Quick answer: Molarity is moles of solute per litre of solution, and dilution follows M₁V₁ = M₂V₂. This AP Chemistry lab on Beer-Lambert law gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Many drinks and medicines are coloured by dyes whose amount can be measured by light absorption. In this lab you make standards by dilution, measure their absorbance with a colorimeter and test whether absorbance is proportional to concentration. You then use the calibration line to find the dye concentration in an unknown sample.
Level: AP Chemistry · Curriculum: AP Chem Unit 3
Aim
To verify the Beer-Lambert law for a blue food dye and to find the concentration of the dye in a sample of drink.
Equipment
- stock dye solution of concentration 1.00 × 10⁻⁵ mol/L
- five 25.00 mL volumetric flasks
- 10 mL graduated pipette with filler
- colorimeter with red filter (about 630 nm) and matched cuvettes
- distilled water, drink sample, tissue
Safety
Wear goggles and a lab coat. Food dyes stain skin and clothes. Never pipette by mouth. Clean spills on the floor at once, because they are slippery.
Method
- Pipette 2.50, 5.00, 10.0, 15.0 and 20.0 mL of stock dye into five 25.00 mL flasks and fill each to the mark with distilled water.
- Fill a cuvette with distilled water and zero the colorimeter (blank).
- Rinse a cuvette with a little of standard 1, fill it, wipe the sides and measure the absorbance.
- Repeat for standards 2 to 5, working from the least to the most concentrated.
- Measure the absorbance of the drink sample, diluted if it is outside the range.
Results
Sample data (1.00 cm cuvette, 630 nm). The drink sample gave A = 0.405.
| Standard | Stock volume (mL) | c (10⁻⁶ mol/L) | Absorbance |
|---|---|---|---|
| 1 | 2.50 | 1.00 | 0.131 |
| 2 | 5.00 | 2.00 | 0.258 |
| 3 | 10.0 | 4.00 | 0.521 |
| 4 | 15.0 | 6.00 | 0.783 |
| 5 | 20.0 | 8.00 | 1.036 |
Analysis questions
Question 1 Check the concentrations
Show that the concentration of standard 3 is 4.00 × 10⁻⁶ mol/L.Question 2 Why a blank
Explain why the colorimeter is zeroed with distilled water.Question 3 Plot the graph
Plot A against c and describe the shape. State whether Beer’s law is obeyed.Question 4 Gradient
Calculate the gradient using standards 1 and 5 and state the molar absorptivity.Question 5 Concentration of the drink
Use the line A = 1.30 × 10⁵ c to find c for the drink sample, A = 0.405.Question 6 Mass concentration
The dye has a molar mass of 792.9 g/mol. Calculate the mass of dye per litre of drink in mg.Question 7 Choice of wavelength
Why was a red filter chosen for a blue dye?Question 8 Improvements
State one source of uncertainty and one way to reduce it.
