Lesson 13: 3.13 Beer-Lambert Law
Duration of Days: 1
Lesson Objective
Explain the quantitative relationship between the light absorbance of a solution and the concentration of the absorbing chemical species.
How can we use light to non-destructively determine the exact concentration of a colored solution?
Why must a spectrophotometer be calibrated at the analytical wavelength of maximum absorbance (lambda max)?
Beer-Lambert Law
Absorbance
Transmittance
Molar Absorptivity (epsilon)
Path Length (b)
Cuvette
Calibration Curve
Analytical Wavelength (lambda max)
Learning Objective 3.13.A; Suggested Skill 2.E (Question and Method - Identify source of error).
Covers the Beer-Lambert Law (A = epsilon b c). Absorbance is directly proportional to concentration. Students must understand how to construct a standard calibration curve and analyze how standard laboratory errors (e.g., leaving fingerprints on a cuvette, adding extra water droplets) skew the final calculated concentration.
Support: Walk through a virtual simulator of a colorimeter, showing visually that a dark solution blocks more light (higher absorbance) than a pale, dilute solution.
Extension: Give students an experimental scenario where a student fails to wipe the outside of a cuvette before measuring. Have them trace how this extra light scattering impacts the recorded Absorbance reading and subsequent concentration calculation.
A student uses a spectrophotometer to find the concentration of an unknown sample of CuSO4(aq). They build a calibration curve by plotting Absorbance versus Concentration, obtaining a straight line equation: A = 42.5c.
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If the unknown sample yields an absorbance value of 0.340, calculate its concentration.
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If the student accidentally left water droplets inside the cuvette before filling it with the unknown solution, would the calculated concentration be falsely high, falsely low, or unaffected? Justify your answer.