Sample AP Model Drawing Question

Today: How is the color of a solution related to concentration and how can that be used to our advantage?

I0 I1 Concentration and Color: Solution 1: Low concentration Incident Light I0 I1 (The wavelength must be specific to this solute)

I0 I2 Concentration and Color: Solution 2: Higher concentration, same solute Incident Light I0 I2

I1 > I2 Concentration and Color: Solution 1 = lower M, Solution 2 = higher M How does the observed light compare between the two solutions? Solution 1 Solution 2 I1 > I2

Concentration and Color: Why do we care? The amount of light that is able to pass through a solution can be used as a way to measure the concentration!

Concentration and Color: Some terms … Transmittance: A measure of the light that was able to pass through the sol’n Absorbance: A measure of the amount of light that was absorbed by the solution A sol’n with high transmittance, will have low absorbance.

Concentration and Color: Beer-Lambert Law A = a x b x c where A is the absorbance a = constant, called molar absorptivity b = path length, in cm c = concentration, in mol/L This law shows that there is a linear relationship between absorbance and concentration

Concentration and Color: Beer-Lambert Law

Example Problem: What color light is a red dye likely to absorb? The following absorbances were measured for different concentrations of red dye at 505 nm (greenish-blue) Solution Concentration Absorbance blank 0.00 M 0.00 standard #1 0.15 M 0.24 standard #2 0.30 M 0.50 standard #3 0.45 M 0.72 standard #4 0.60 M 0.99 Unknown ???? M 0.39 Determine the M of the unknown sample.

Example Problem: Graph the known concentrations … Solution Concentration Absorbance blank 0.00 M 0.00 standard #1 0.15 M 0.24 standard #2 0.30 M 0.50 standard #3 0.45 M 0.72 standard #4 0.60 M 0.99 Unknown ???? M 0.39 Example Problem: Graph the known concentrations … Determine the M of the unknown sample.

Try #1 and #2 on your own: