3.13+Beer-Lambert+Law

3.13 Beer-Lambert Law

Enduring Understanding and Learning Objective

  • Enduring Understanding SAP-8: Spectroscopy can determine the structure and concentration in a mixture of chemical species.

  • Learning Objective SAP-8C: Understand the relationship between light absorption, concentration, path length, and molar absorptivity of a solution.

Absorbance

  • Definition: Absorbance is the measure of how much light is stopped by or absorbed by a solution.

  • Measurement Tools: Absorbance can be measured using a spectrophotometer or a colorimeter.

  • Concept: The greater the concentration of colored substances in a solution, the less light passes through it.

Molarity

  • Definition: Concentration refers to the amount of particles in a solution, describing how densely packed the particles are.

  • Common Measurement: Molarity (M) is often used for measuring concentration in chemistry.

Molarity Example

  • If you have 2 liters of a 0.25 M solution, the number of moles of solute can be calculated as follows:

    • Formula: M = moles of solute / Liters of solution

    • Calculation: M = 0.25 mol/L * 2 L = 0.5 mol

Using Absorbance to Measure Concentration

  1. Wavelength Determination:

    • Place the sample in a spectrophotometer to generate an absorbance vs. wavelength graph.

    • Optimal Wavelength: Choose a wavelength close to an absorbance of 1.

  2. Complementary Colors: Different colors of solutions absorb different colors of light.

    • For a red solution, use green light (495-570 nm) for measurement.

Calibration Graph

  • Creation: Measure standard solution concentrations and their corresponding absorbance to create a linear calibration graph.

  • Equation: Beer-Lambert Law is expressed as[ A = \epsilon b c ]

    • Where:

      • A = Absorbance

      • \epsilon = Molar absorptivity constant (slope of the graph, in 1/M*cm)

      • b = Path length (usually 1 cm)

      • c = Concentration (Molarity, M)

Calibration Procedure

  • Starting Measurement: Before measurement, calibrate the spectrophotometer using a blank (solvent only, usually water) to eliminate solvent data from the reading.

Data Analysis

  • After calibration, use the absorbance of an unknown sample to determine concentration based on the established calibration curve.

Example Problem

  • Given a calibration graph, if the absorbance of an unknown solution is 0.34, determine its molarity.

  • Data from the Beer-Lambert Law graph will be used for this calculation.

Practice Problems

  • Engage in practice problems for further understanding and review of the Beer-Lambert Law.