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4 marksUltra Violet-Visible (UV/VIS) Spectroscopy

CAPE Chemistry Unit 2 · May/June 2019 · Paper 2 · Question 2(c)(iii)

When a covalent molecule absorbs UV-visible radiation, electrons are promoted. This phenomenon forms the basis for UV-VIS spectroscopy.

The absorbance of an iron thiocyanate solution containing 0.005 mol Fe/mL was reported as 0.4900 at 540 nm. Using Beer–Lambert's law, A = εcl, calculate the specific absorptivity, including units, of iron thiocyanate, assuming that a 1.00 cm cuvette was used.

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  1. 2(a)State the meaning of EACH of the following terms: Electromagnetic radiation, Wavelength, Frequency.[3 marks]
  2. 2(b)(i)Write the formula which relates the speed of light (c), the wavelength of light (λ) and its frequency (v).[1 mark]
  3. 2(b)(ii)Using the formula from (b) (i), calculate the missing frequencies and wavelengths in Table 2 AND identify the corresponding type of electromagnetic radiation.…[4 marks]
  4. 2(c)(i)Complete Figure 2 by inserting arrows to show ALL possible electron transitions in the molecular orbitals indicated.[3 marks]
  5. 2(c)(ii)List the transitions from (c) (i) which will normally produce absorption in the ultraviolet and visible regions of the electromagnetic spectrum.[3 marks]
  6. 2(d)(i)Identify the compound and provide justification for Figure 3.[3 marks]
  7. 2(d)(ii)Identify the compound and provide justification for Figure 4.[2 marks]
  8. 2(d)(iii)Identify the compound and provide justification for Figure 5.[2 marks]
  9. 2(e)(i)Describe FOUR steps to be taken by the student in preparing the sample for analysis.[4 marks]
  10. 2(e)(ii)Give ONE reason why the plates used for IR spectroscopy are made from NaCl.[1 mark]

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