Let’s Talk About Beer
Beer’s Law and
Concentrations
Solution Concentration Reminders
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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?
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Conc is moles
(number of
ions/atoms/molecul
e) per space
More molecules
means more light is
“caught”
Examples
Beer’s Law
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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

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
Emissivity
Cell width
concentration
So, What Does This Means
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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?
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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?
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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
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Spectroscopy
involves the study
of light that is
absorbed or
emitted by a
substance
Visible spec.
involves light we
can see
The plan:
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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?
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Calibrate the
machine
Find the best
wavelength (lmax)
Test all solutions at
that wavelength
Graph or use
“factor”
Calibration of Spec 20
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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 lmax
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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
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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
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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)