3.13 Beer-Lambert Law

One way that we can measure the concentration of a colored substance is by:

Seeing how much light can be passed through it or how much light is stopped by the solution

The measure of the light that is stopped by or absorbed by a solution is referred to as the:

Absorbance

A ______ can be used to measure the absorbance.

Spectrophotometer or a colorimeter

Simplified schematic of spectrophotometer or colorimeter:

The more concentrated the solution is, the more/less light can pass through the solution

Less

A measure of the amount of particles that are distributed in a solution

Concentration

You can think of concentration as:

How densely packed the particles are

Concentration is measured in a lot / a few of ways

A lot

Chemists use ______ to measure concentration

Molarity

What is molarity in terms of solute and solution?

Moles of solute divided by the volume of solution

The term used for the substance that is dissolved

Solute

Most commonly we use ______ as the solvent

Water

The solute and solvent together are called the:

Solution

A solution with water as the solvent is:

An aqueous solution

Units for molarity:

M, mol/L

Can molarity be used as a conversion factor?

Yes

Why can molarity be used as a conversion factor?

It has two units, moles and liters

Different colors of solutions absorb different:

Colors (or wavelengths) of light

How to measure a red solution?

By using green light

Wavelength of green light:

560-520 nm

Red and green are:

Complementary colors

For colored solutions, ______ can be used to measure the concentration of solutions

Absorbance

To measure absorbance, you first have to:

Determine the best wavelength to use to measure your sample

How to determine the best wavelength to use to measure your sample?

Place your sample into a spectrophotometer and generate a graph of absorbance vs. wavelength

On a graph showing absorbance vs wavelength, the best choice for a wavelength is one where the absorbance is:

Close to 1

What should you do after determining the wavelength used for measuring absorbance?

Create a calibration graph with standard solution concentrations and their corresponding absorbance value

Calibration graph with concentrations and corresponding absorbance values should be:

Linear

The slop of the calibration graph with concentrations and corresponding absorbance values gives the:

Molar absorptivity constant

The more concentrated a solution is the more/less light is absorbed by the solution

More

Beer-Lambert Law:

A = ԑ b c. (A = Absorbance, ԑ = molar absorptivity constant (1/M*cm), b = path length (usually 1 cm), c = concentration (Molarity, M))

How to determine concentration of unknown sample using absorbance?

Using the calibration graph for the solution

In Beer-Lambert Law, the path length is:

Width of the cuvette that the sample is placed in

Is path length usually held constant?

Yes

When you calibrate the spectrophotometer you should start with:

A “blank”

The “blank” on the spectrophotometer is:

Your solvent only, usually water

By calibrating the spectrophotometer with the blank it will:

Remove the data from the solvent

By removing data from the solvent, the spectrophotometer:

Only outputs the absorbance of the solute

Calibrating the spectrophotometer with the blank is analogous to:

The “tare” button on a digital scale