Chem 101L: Beers Laws

What causes color in a substance?

The wavelengths of light not absorbed by the substance (the transmitted light).

What wavelengths does a substance absorb relative to its observed color?

It absorbs light near the complementary color on the color wheel.

What determines whether a molecule can absorb visible light?

If the energy of a photon matches the energy gap between electron levels.

What does a spectrometer measure?

How much light at each wavelength is transmitted vs absorbed.

What is transmittance?

T=I/I​₀, where I is transmitted intensity and I₀ is incident intensity.

What is % transmittance?

%T=100×I/I​₀

Definition of absorbance (A)?

A=−log(I/I​₀)

What is Beer’s Law?

A=εlc

What does ε represent?

Molar absorptivity (M⁻¹ cm⁻¹), a constant describing how strongly a substance absorbs light at a specific wavelength.

What does l represent?

Path length (cm), usually 1.00 cm in standard cuvettes.

What does c represent?

Concentration (M)

Why is absorbance unitless?

It is a log of a ratio of intensities.

What is λmax?

The wavelength at which a substance’s absorbance is highest.

Why use λmax for Beer’s Law measurements?

Maximizes sensitivity and makes Beer’s Law most linear.

What is a calibration curve?

A plot of absorbance (y) vs. concentration (x) for standard solutions.

What is the linear regression form of a calibration curve?

A=mc+b

How does the slope (m) relate to Beer’s Law?

m=εl

What does the y-intercept (b) represent?

Background absorbance (instrument noise, scratches, cuvette imperfections).

Why should b ideally be near zero?

high b indicates poor calibration or background errors.

How to calculate an unknown concentration from the calibration line?

c=A-b/m

What does R² measure?

How well the data fits a straight line (linearity). Over 0.98 is considered good.

What is the dilution equation?

M1​V1​=M2​V2​

What does M₁ represent?

Initial (more concentrated) molarity.

What does V₁ represent?

Volume taken from the stock.

What does M₂ represent?

Final (diluted) molarity.

What does V₂ represent?

Final total volume.

What important idea underlies dilutions?

Moles of solute stay constant before and after dilution.

What is a “stock solution”?

A concentrated solution used to make dilutions.

What are “standard solutions”?

Solutions of known concentration used to create a calibration curve.

What does a cuvette do?

Holds the solution so light can pass through a fixed path length.

Why must the cuvette be clean and unscratched?

Scratches or fingerprints increase absorbance artificially.

Why rotate the cuvette during measurements?

To average out imperfections in the plastic path.

What does the spectrometer compare to compute absorbance?

Sample cuvette vs. blank cuvette.

How does concentration affect absorbance?

Directly proportional. Higher concentration = higher absorbance.

Why does a very concentrated solution give inaccurate absorbance?

It may exceed the linear range (usually A > 1.2), causing deviations from Beer’s Law.

Why must standards span the range of expected unknowns?

Extrapolating outside the calibration range is unreliable.

General process for determining concentration of an unknown solution?

• Measure absorbance at λmax

• Use c=(A−b)/mc=(A−b)/m

• Correct for any dilution performed before measurement

Why might real samples deviate from ideal Beer’s Law behavior?

Interference from sugars, acids, or other absorbing species.

Units of ε (molar absorptivity)?

M⁻¹ cm⁻¹

Units of absorbance?

None (unitless)

Typical cuvette path length?

1.00 cm

How do you convert moles to mass?

mass=moles×molar mass

What is the acceptable daily intake (ADI)?

Maximum safe daily amount per kilogram of body weight.