Chemistry WCH16 May 2020 Questions & Answers

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Questions & Worked Solutions For Chemistry WCH16 May/June 2020

IAL Chemistry WCH16 May/June 2020 Questions - Complete Paper (pdf)

  1. Compound A is a green solid containing one cation and one anion.
    A sample of compound A was dissolved in distilled water, forming a green solution.
    Aqueous sodium hydroxide was added to the solution of A until there was no further change.
    A pale blue precipitate B and a yellow solution C were formed.
    (a) The pale blue precipitate B was separated and tested.
    (i) B dissolved in excess ammonia to form a deep blue solution containing a complex ion D.
    Identify, by name or formula, B and D
  2. This question is about three organic compounds P, Q and R.
    These compounds are isomers with the molecular formula C4H8O2.
    (a) Compound P is a colourless liquid with a sweet fruity smell.
    When a sample of P was heated with sodium hydroxide, a volatile product was formed which had a molecular ion peak in its mass spectrum at m / z = 46.
    The mass spectrum of P has a strong peak at m / z = 43.
    Deduce the structure of P. Justify your answer using all this information
    (b) (i) When sodium hydrogencarbonate solution is added to separate samples of Q and R, effervescence occurs and a gas is evolved which turns limewater milky.
    Deduce the two possible structures of Q and R.
    Justify your answer using all this information.
    (ii) A simplified high resolution proton NMR spectrum of Q is shown.
    The relative peak areas are given above each set of peaks.
    Deduce the structure of Q. Fully justify your answer by referring to the number of peaks, the relative peak areas and the splitting patterns in the proton NMR spectrum

  1. A group of students carried out an experiment to determine the rate equation for the reaction between bromide and bromate(V) ions in acid conditions.
    The equation for this reaction is
    Procedure (to determine the order of reaction with respect to bromate(V) ions)
    Step 1 Measure 10.0cm3 of aqueous phenol solution into a boiling tube and add five drops of methyl red indicator. The mixture turns yellow (the alkaline colour of methyl red).
    Step 2 Add 5.0cm3 of aqueous potassium bromide and 10.0cm3 of dilute sulfuric acid to the boiling tube. The mixture turns red (the acid colour of methyl red).
    Step 3 Measure 15.0cm3 of aqueous potassium bromate(V) into a second boiling tube.
    Step 4 Mix the contents of the two boiling tubes and start a timer.
    Step 5 Record the time (t) when the colour of the methyl red is bleached from red to colourless by excess bromine.
    Step 6 Repeat the experiment using different volumes of aqueous potassium bromate(V), adding distilled water so that the total volume of the reacting solution is always 40.0cm3.
    (a) Two of the hazard warning signs for phenol are
    State the most important hazard associated with phenol in this experiment and the precaution you would take to reduce the risk, apart from wearing safety goggles and a laboratory coat.
  2. A group of students prepared aspirin from 2-hydroxybenzoic acid using ethanoic anhydride
    (a) Give a reason for placing the flask in cold water in Step 1.
    (b) Suggest the purpose of the concentrated sulfuric acid added in Step 2.
    (c) Show, by calculation, that the ethanoic anhydride is in excess in this preparation.
    (d) One student drew a diagram of the apparatus used for reflux in Step 4
    (e) Suggest the purpose of adding crushed ice and distilled water in Step 5.
    (f) The filtration in Step 7 is carried out under reduced pressure.
    State two advantages of this method compared with ordinary (gravity) filtration.
    (g) Describe how the purity of the recrystallised aspirin could be tested. Experimental details are not required.

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