Let’s Talk About Beer

Beer’s Law and Concentrations

Solution Concentration Reminders

Concentration expresses how the amount of solute and the amount of solution compare

Our unit of choice: Molarity = mols/L

What do you notice about the solutions?

Why Does Color “Fade” as Concentration is Lessened?

Conc is moles (number of ions/atoms/molecule) per space

More molecules means more light is “caught”

Examples

Beer’s Law

Beer’s Law quantifies the relationship between color and concentration

Beer’s Law states that the absorbance (why?) of light by a solution is directly proportional to Emissivity Cell width concentration

So, What Does This Means

It means that a graph of absorbance of light by a “kind” of solution TO the concentration is linear.

It means that predictions are accurate and reliable

We can find concentrations by comparison

How Do We Do This?

We make several solutions with known concentrations

We determine which wavelength or color of light the solution responds to best

We measure that transmittance and/or absorbance for the known solutions and the unknown solution

We graph the knowns and interpolate for the unknown

A Successive Dilution Example

Design a process of successive dilution to make 250.0 mls of the following solutions from a 0.85 M stock solution: 0.50 M, 0.30 M, 0.10 M.

Why Not Transmittance?

Imagine an area enclosed by a curtain containing a mystery number of people

Each person can catch and hold 2 tennis balls

We throw 1000 balls into the area We measure what comes out (Trans) But we find the number of people by what

doesn’t come out (Abs) # People is related to number of balls

caught (Abs)

Spectroscopy: The kind you can see

Spectroscopy involves the study of light that is absorbed or emitted by a substance

Visible spec. involves light we can see

The plan:

We’ll shine light that is absorbed well into solutions of known concentration

We’ll make a plot of known concentrations versus absorbance

We’ll test our unknown solution We’ll interpolate to find the

concentration that matches its absorbance

Great! How do we do that?

Calibrate the machine

Find the best wavelength (max)

Test all solutions at that wavelength

Graph or use “factor”

Calibration of Spec 20

Allow the machine to warm up Set wavelength to desired value (400 nm) With sample chamber empty set %T to zero

using left knob With water (or some other solvent) in

sample chamber set %T to 100% using right knob

Repeat for each wavelength

Finding max

Calibrate machine at 400 nm Place one solution (usually a “middle”

concentration) into sample chamber Record data Reset machine to 425 (or 450) nm Calibrate and test same solution Repeat until you reach 750 nm

Testing the solutions & unknown

Once you’ve found the best wavelength (light is absorbed best) test all solutions and unknown at this wavelength

Use data to find concentration of unknown

Finding the unknown

Suppose this graph is generated

The unknown absorbance is 0.500

Find 0.500 absorbance Across and down The concentration is

0.775 (or so)