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

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  • Slide 1
  • Let’s Talk About Beer Beer’s Law and Concentrations
  • Slide 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?
  • Slide 3
  • Why Does Color “Fade” as Concentration is Lessened? Conc is moles (number of ions/atoms/molecul e) per space More molecules means more light is “caught” Examples
  • Slide 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 towhy?)  Emissivity  Cell width  concentration
  • Slide 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
  • Slide 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
  • Slide 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.
  • Slide 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)
  • Slide 9
  • Spectroscopy: The kind you can see Spectroscopy involves the study of light that is absorbed or emitted by a substance Spectroscopy Visible spec. involves light we can see
  • Slide 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
  • Slide 11
  • Great! How do we do that? Calibrate the machine Calibrate Find the best wavelength ( max ) wavelength Test all solutions at that wavelength Test Graph or use “factor”
  • Slide 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
  • Slide 13
  • 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
  • Slide 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
  • Slide 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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