let’s talk about beer beer’s law and concentrations
TRANSCRIPT
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Let’s Talk About Beer
Beer’s Law and Concentrations
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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?
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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
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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
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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
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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
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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.
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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)
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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
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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
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Great! How do we do that?
Calibrate the machine
Find the best wavelength (max)
Test all solutions at that wavelength
Graph or use “factor”
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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
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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
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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
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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)