Chemistry WCH13 January 2021 Questions & Answers

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Questions And Worked Solutions For Chemistry WCH13 January 2021

IAL Chemistry WCH13 January 2021 Questions - Complete Paper (pdf)

  1. (a) A student was provided with five test tubes labelled A, B, C, D and E, each containing a colourless aqueous solution.
  2. Sodium hydroxide solution reacts with carbon dioxide in the air and should be standardised before use. Ethanedioic acid may be used for this standardisation. (a) A standard solution of ethanedioic acid, (COOH)2, is prepared.
  • 2.40 g of solid ethanedioic acid is dissolved in approximately 100 cm3 of deionised water in a beaker.
  • The solution is transferred into a 250.0 cm3 volumetric flask and made up to the mark with deionised water.
    (i) Give a possible reason why any solution remaining in the beaker is washed into the volumetric flask before making up to the mark.
    (ii) Calculate the concentration of this standard solution of ethanedioic acid in mol dm-3.
    Give your answer to an appropriate number of significant figures.
    (b) A different standard solution of ethanedioic acid is used to determine the concentration of a sodium hydroxide solution J.
    Step 1 A burette is rinsed with deionised water.
    Step 2 The burette is then rinsed with 0.0900 mol dm-3 ethanedioic acid and filled with this acid solution.
    Step 3 A pipette is used to transfer 25.0 cm3 portions of solution J to conical flasks.
    Step 4 The portions are titrated with the ethanedioic acid solution using phenolphthalein indicator.
    (i) Explain why the burette is rinsed with ethanedioic acid solution in Step 2.
    (ii) The diagram shows how the student read the filled pipette in Step 3.
    Identify the two mistakes the student made.
    (iii) The student completely emptied the pipette for each transfer in Step 3.
    Explain the effect on the titre of completely emptying the pipette rather than leaving a small amount of solution in the tip.
    (iv) State the colour change in the conical flask at the end-point.
    (c) The titration results are shown
    (ii) Calculate the concentration of the sodium hydroxide solution in mol dm-3.

  1. This question is about the thermal decomposition of Group 2 carbonates.
    A student heated a sample of a Group 2 carbonate until no more gas was produced.
    The equation for the decomposition is
    (a) Give a reason why the delivery tube must be removed from the water bath before removing the test tube from the heat source.
    (b) The results of the experiment are shown
    (i) Using the results of the experiment identify the Group 2 metal.
    (ii) The student suggested that the experiment could be made more accurate by increasing the mass of carbonate from 0.33 g to 1.00 g.
    No changes to the size of the apparatus or the method of measurement of the gas produced would be made.
    Comment on this suggestion.
    (c) The enthalpy change for the thermal decomposition of a carbonate, is difficult to measure directly.
    An example of a Hess’s Law cycle to determine it indirectly is
  2. The halogenoalkane 2-chloro-2-methylpropane may be prepared from 2-methylpropan-2-ol.
    Step 1 Add 35 cm3
    of concentrated hydrochloric acid to 8.00 g of 2-methylpropan-2-ol in a conical flask.
    Swirl the mixture gently for 20 minutes.
    Step 2 Two distinct layers form. The upper (organic) layer contains the required product. The lower aqueous layer is removed using a separating funnel.
    Step 3 Add a solution of sodium hydrogencarbonate to the organic layer.
    Swirl gently. Stopper the separating funnel and shake it
    Invert the separating funnel and open the tap.
    Step 4 Return the separating funnel to its upright position, remove the stopper and run off the aqueous layer. Transfer the organic layer into a clean conical flask.
    Step 5 Add some anhydrous sodium sulfate.
    Leave the flask to stand and decant off the liquid.
    Step 6 Distil the liquid, collecting the product between 50°C and 52°C.
    (a) (i) The concentrated hydrochloric acid used in Step 1 was labelled
    Suggest two safety precautions, other than wearing safety spectacles and a laboratory coat, to minimise the risk when using this reagent in Step 1.
    (ii) Explain why the product in the organic layer in Step 2 does not mix with the aqueous layer.
    (iii) State why the tap of the separating funnel must be opened in Step 3.
    (iv) State why anhydrous sodium sulfate is added to the organic layer in Step 5.
    (v) Draw the apparatus required to distil the product and collect the distillate between 50°C and 52°C in Step 6.
    (b) The equation for the reaction is
    (c) The choroalkane produced is used in an experiment to compare its rate of hydrolysis with two other halogenoalkanes.
    A student dissolves separate 1.0 cm3
    samples of each halogenoalkane in ethanol and adds 2 cm3 of silver nitrate solution.
    The time taken for a precipitate to form is recorded. The results are shown

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