Spectrophotometry and Beer's Law: Quiz

Spectrophotometry and Beer's Law: Quiz

7 questions

1. Question 1EasyA solution transmits 50.0 % of the light. What is its absorbance?

   A0.301B0.500C1.70D−1.70
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   Answer: 0.301

   T = 0.500, so A = −log(0.500) = 0.301. Using 50.0 instead of 0.500 gives −1.70, which is impossible for a real sample.

2. Question 2EasyIf the concentration of a solution is doubled (same cell), the absorbance:

   AhalvesBstays the sameCdoublesDis squared
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   Answer: doubles

   Beer's law, A = εbc: A is directly proportional to c.

3. Question 3MediumWhat are the units of molar absorptivity, ε?

   Amol L⁻¹ cm⁻¹BL mol⁻¹ cm⁻¹Ccm mol L⁻¹Dno units
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   Answer: L mol⁻¹ cm⁻¹

   A has no unit, so ε = A ÷ (b × c) has units 1 ÷ (cm × mol/L) = L mol⁻¹ cm⁻¹.

4. Question 4MediumA = 0.450, ε = 1.50 × 10⁴ L mol⁻¹ cm⁻¹, b = 1.00 cm. What is c?

   A3.00 × 10⁻⁵ mol/LB6.75 × 10³ mol/LC3.33 × 10⁴ mol/LD3.00 × 10⁻³ mol/L
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   Answer: 3.00 × 10⁻⁵ mol/L

   c = A ÷ (εb) = 0.450 ÷ (1.50 × 10⁴ L mol⁻¹ cm⁻¹ × 1.00 cm) = 3.00 × 10⁻⁵ mol/L.

5. Question 5EasyWhat is the purpose of the blank?

   Ato set the wavelengthBto zero the instrument so that only the analyte absorbance is measuredCto give the most concentrated standardDto check the path length
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   Answer: to zero the instrument so that only the analyte absorbance is measured

   The blank contains everything except the analyte, so absorption by the solvent, reagents and cell is subtracted.

6. Question 6MediumThe same solution is moved from a 1.00 cm cell to a 2.00 cm cell. Its absorbance changes from 0.250 to:

   A0.125B0.250C0.500D1.00
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   Answer: 0.500

   A is proportional to the path length b: doubling b doubles A.

7. Question 7HardAn unknown gives A = 1.85, but the standards only go up to A = 0.95. What should you do?

   Aextend the line and read the concentrationBdilute the unknown by a known factor, measure again and multiply by the factorCuse a longer cellDreport the result as the highest standard
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   Answer: dilute the unknown by a known factor, measure again and multiply by the factor

   The reading must fall inside the calibration range; Beer's law may not hold at high absorbance. A longer cell would raise A even further.