Week 2: Spectrophotometric Analysis Beer-Lambert Law (Beer’s Law)

What does spectrophotometry measure?

It measures how much light a substance absorbs to determine the concentration of a solution.

What is the equation for Beer’s Law?

A = ε L C, where:

• A = Absorbance

• ε = Molar absorptivity constant

• L = Path length of light

• C = Concentration of the solution

What is the relationship between absorbance (A) and transmittance (T)?

Transmittance (T) measures how much light passes through a solution.

Absorbance (A) measures how much light is absorbed.

A=−log(T)

Absorbance and transmittance are inversely related—higher absorbance means lower transmittance.

How do you choose the best wavelength (λmax) for Beer’s Law?

Use the wavelength where absorbance is highest (complementary to the solution’s color).

What happens if a solution is too concentrated in Beer’s Law?

Absorbance readings become inaccurate. Dilute the solution to keep absorbance between 0.1 - 1.0.

What is a blank solution in spectrophotometry?

A solution that contains all chemicals except the analyte to correct for background interference.

What is the formula to find an unknown concentration in Beer’s Law?

A2 / A1 = C2 / C1

Example: If solution A has:

• Absorbance = 0.15

• Concentration = 0.24 M
  And solution B has:

• Absorbance = 0.45

• Unknown concentration (C)

Using A1 / A2 = C1 / C2

0.45 / 0.15 = C / 0.24

C = (0.24) × 0.45 / 0.15 = 0.72 M