Beer-Lambert law lab answers, AP Chemistry (PDF)

The method and the results are on the Beer-Lambert law lab. Do the analysis yourself first, then check the answers below.

Quick answer: Molarity is moles of solute per litre of solution, and dilution follows M₁V₁ = M₂V₂. These answers go with the AP Chemistry lab on Beer-Lambert law: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.

Beer-Lambert law lab answers for AP Chemistry (free PDF download)

Here are the Beer-Lambert law lab answers for AP Chemistry: graph, calculations, uncertainties and conclusion. First do the lab yourself, then compare, and go back to the lesson if needed.

Model answers use the sample data. If your values differ slightly, the method and the proportionality should still hold. Report concentrations in mol/L with scientific notation and give results to two or three significant figures.

Level: AP Chemistry · Curriculum: AP Chem Unit 3

Contents

Beer-Lambert law lab answers: the analysis explained

Question 1 Check the concentrations

M₂ = M₁V₁/V₂ = 1.00 × 10⁻⁵ × 10.0 / 25.00 = 4.00 × 10⁻⁶ mol/L.

Question 2 Why a blank

The blank removes the absorbance of the cuvette and solvent, so the reading is due only to the dye.

Question 3 Plot the graph

The points lie on a straight line through (almost) the origin: A is proportional to c, so the law is obeyed over this range.

Question 4 Gradient

Gradient = (1.036 − 0.131) / ((8.00 − 1.00) × 10⁻⁶) = 0.905 / 7.00 × 10⁻⁶ = 1.29 × 10⁵ L/mol. With b = 1.00 cm, ε = 1.29 × 10⁵ L/(mol·cm).

Question 5 Concentration of the drink

c = 0.405 / (1.30 × 10⁵) = 3.1 × 10⁻⁶ mol/L.

Question 6 Mass concentration

m = 3.1 × 10⁻⁶ × 792.9 = 2.46 × 10⁻³ g = 2.5 mg/L.

Question 7 Choice of wavelength

A blue solution absorbs red-orange light most strongly. Red light gives the highest absorbance and so the greatest sensitivity.

Question 8 Improvements

Pipetting small volumes (2.50 mL) has a large percentage uncertainty. Use a larger stock volume and a larger flask, or prepare standards by a serial dilution. This lowers the percentage uncertainty in c.

← Back to the Beer-Lambert law lab