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Let’s Talk About Beer

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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?.

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Let’s Talk About Beer

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  1. Let’s Talk About Beer Beer’s Law and Concentrations

  2. 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?

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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.

  8. 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)

  9. 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

  10. 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

  11. Great! How do we do that? • Calibrate the machine • Find the best wavelength (lmax) • Test all solutions at that wavelength • Graph or use “factor”

  12. 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

  13. Finding lmax • 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

  14. 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

  15. 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)

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