Modern Analytical Techniques I (Edexcel AS Chemistry)

Exam Questions

17 mins2 questions
1a1 mark

This question is about organic compounds containing fluorine and chlorine.

The use of chlorofluorocarbons as refrigerants has ceased due to concerns about their effects on the ozone layer. One such compound is dichlorodifluoromethane.

Give the molecular formula of dichlorodifluoromethane.

1b
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5 marks
i)
A different refrigerant contains 34.0% chlorine and 54.5% fluorine by mass, with the remainder carbon.

Calculate the empirical formula of this compound.

(3)

ii)
Use the mass spectrum to show that the empirical and the molecular formulae of this compound are the same.

(1)

q1bii-8cho-as-2-nov-2020-qp-edexcel-a-level-chem

iii)
Suggest the species responsible for the peak at m/z = 69.

(1)

1c4 marks

Compounds containing carbon and fluorine but no chlorine can be used as refrigerants as they are not harmful to the ozone layer.
These can be made by the reaction of fluorine with alkanes or fluoroalkanes.
A refrigerant currently in use contains the compound trifluoromethane, CHF3.

i)
Write the equation for the formation of trifluoromethane by the reaction of difluoromethane with fluorine. State symbols are not required.
(1)

ii)
The mechanism for this reaction is similar to that of the reaction between chlorine and methane.

Give the equations for the following steps in the mechanism for the reaction between fluorine and difluoromethane. Curly arrows are not required.

(3)

Initiation step


First propagation step


Second propagation step


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2a
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1 mark

This question is about compounds with the molecular formula C4H8O.

What is the percentage by mass of each element in C4H8O?

      Percentage carbon Percentage hydrogen Percentage oxygen
  A 66.67 11.11 22.22
  B 60.00 20.00 20.00
  C 41.38 3.45 55.17
  D 30.77 61.54 7.69

2b6 marks

Three different compounds, A, B and C, have the molecular formula C4H8O.

Information about these three compounds includes:

  • all three compounds have infrared absorptions at about 3500 cm–1
  • the infrared spectra of A and B each contain a peak at about 1650 cm–1, while that of C does not
  • only A has a branched carbon chain
  • B is the E-isomer of a pair of stereoisomers.

Deduce a possible displayed formula for each of the compounds A, B and C.

You must use all the information and the Data Booklet to fully justify each of your structures.

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