Beer's Law Virtual Lab: Absorbance and Concentration

This Beer's law virtual lab matches the AP Chemistry spectrophotometry lab on how the concentration of a colored solution affects the light it transmits. Students shine light through a cuvette of copper(II) sulfate, record absorbance and percent transmittance at five concentrations, and plot a calibration line. They use the line to find the concentration of a sample from its absorbance, then test path length and wavelength. One period gives a straight-line graph through the origin, a gradient equal to εb, and a clear reason why chemists measure at the wavelength of maximum absorbance.

Curriculum links

• AP Chemistry Unit 3, Topic 3.13 Beer–Lambert law (SAP-8.C): the absorbance of a solution depends on its concentration, the path length and the molar absorptivity. It also matches the AP Chemistry guided-inquiry lab on concentration and transmitted light, and prepares students for the colorimetry lab that finds the copper content of brass.
• Other courses: the same calibration-curve method is used in IB Chemistry and A-level colorimetry practicals.

Simulic is not affiliated with or endorsed by the College Board or AQA.

Before the lab (5 min)

Ask students to commit to a prediction, on paper or as question 1 of the class link:

"You double the concentration of copper(II) sulfate in the cuvette. The cuvette and the wavelength stay the same. What happens to the absorbance?"

Expect a split between "doubles", "halves" (confusing absorbance with transmittance) and "rises a little, then levels off".

Method in the simulation

1. Open the Beer's Law tab. Choose Copper(II) sulfate.
2. Set Path length b to 1.0 cm and press Best λ (635 nm). Press Light on.
3. Set the Concentration c to 0.10 mol/L. Record A, then press Show %T and record %T. Press Record data to add the point to the graph.
4. Repeat for 0.20, 0.30, 0.40 and 0.50 mol/L.
5. Plot A against c, draw the best-fit line and calculate its gradient.
6. Path length test: at 0.20 mol/L, set b to 0.5 cm and then 2.0 cm. Moving the path-length or wavelength slider clears the graph, so record readings in the table.
7. Wavelength test: back at b = 1.0 cm and 0.20 mol/L, set λ to 450, 500, 550, 600 and 700 nm.

c (mol/L)	0.10	0.20	0.30	0.40	0.50
A (b = 1.0 cm, 635 nm)
%T

Expected results

All readings come from the simulation. Its absorptivity values are teaching values, so don't compare them with data tables.

c (mol/L)	0.10	0.20	0.30	0.40	0.50
A	0.300	0.600	0.900	1.200	1.500
%T	50.12	25.12	12.59	6.31	3.16

• A against c is a straight line through the origin with a gradient of 3.00 L/mol. Since A = εbc and b = 1.0 cm, ε = 3.00 L mol⁻¹ cm⁻¹.
• %T against c is a curve, not a line: each extra 0.10 mol/L halves the light that gets through.
• Path length at 0.20 mol/L: 0.5 cm gives A = 0.300, 1.0 cm gives 0.600, 2.0 cm gives 1.200. A is proportional to b.
• Wavelength at 0.20 mol/L: 450 nm gives 0.073, 500 nm 0.195, 550 nm 0.384, 600 nm 0.556, 635 nm 0.600, 700 nm 0.462. The peak is at 635 nm, in the orange-red: the solution absorbs orange-red light and looks blue.
• Reading a sample: an absorbance of 0.840 on the calibration line gives c = 0.840 ÷ 3.00 = 0.280 mol/L. Students can check by setting c to 0.280 mol/L.

Questions for students

1. (Prediction, asked again after the lab) If you double the concentration, what happens to the absorbance?
2. Which settings must stay the same while you build the calibration line?
3. What is the gradient of your A against c line at b = 1.0 cm and 635 nm?
4. A sample gives A = 0.840 at the same settings. What is its concentration?
5. Why do chemists measure at the wavelength of maximum absorbance?

Answers for teachers: (1) It doubles: 0.300 at 0.10 mol/L, 0.600 at 0.20 mol/L. (2) The path length and the wavelength (and the solution). (3) 3.00 L/mol (accept 2.95–3.05). (4) 0.280 mol/L (accept 0.275–0.285). (5) The line is steepest there, so each change in concentration gives the largest change in A. The method is most sensitive, and a small error in reading A causes the smallest error in c.

Common misconceptions

• "Absorbance and transmittance are opposites, so both are straight lines." Only A is proportional to c. %T falls on a curve, which is why spectrophotometers report A for calibration.
• "Any wavelength works." At 450 nm the line is about eight times flatter than at 635 nm, so readings carry much larger relative errors.
• "A darker color means a different substance." A deeper blue here just means a higher concentration or a longer path.

Extension

• Unknown-sample challenge: the simulation has no hidden-sample mode, so play it in pairs. One student sets a concentration and reads out only A, while the partner keeps their eyes off the slider and the readout line. The partner uses the calibration line to find c, then checks.
• Compare solutions: switch to potassium permanganate (best λ 525 nm). Its concentration slider stops at 0.00080 mol/L, hundreds of times lower than for copper sulfate, yet the absorbances are similar. Ask what that says about ε.

FAQ

Do students need to zero the instrument with a blank?

Not in the simulation: c = 0 always gives A = 0.000. In the wet lab, a cuvette of water sets the zero. Mention it so students know why the line passes through the origin.

Does the line ever curve at high concentration?

No. The simulation follows the ideal Beer–Lambert law over its whole range. Real solutions can deviate at high absorbance, so labs keep standards below about A = 1.

Can students find copper in brass with this?

They can practice the calibration-curve step. Dissolving the brass sample and the mass-percent calculation are not in the simulation.

Related simulations and guides

Preparing solutions – percentage concentration, molar concentration and dilution Molarity – moles, volume and concentration

For more chemistry activities, see interactive chemistry lesson ideas. For help with question wording, see writing good questions for virtual labs.