KCSE 2010 Chemistry Paper 3 Questions and Answers

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  1. You are provided with:
    Acid A labeled solution A;
    2.0 M sodium hydroxide solution labeled solution B.
    Solution C containing 25.0 g per litre of alkanoic acid.

    You are required to:
    1. Prepare a dilute solution of sodium hydroxide, solution B
    2. Determine the:
      1. Molar mass of the alkanoic acid.
      2. Reaction ratio between sodium hydroxide and acid A.

        Procedure 1
        Using a pipette and a pipette filler, place 25.0cm3 of solution B in a 250.0 ml volumetric flask. Add about 200 cm3 of distilled water. Shake well. Add more distilled water to make up to the mark. Label this solution D. Retain the remaining solution B for use in procedure II.

        Fill a burette with solution C. Using a clean pipette and a pipette filler, place 25.0 cm3 of solution D into a 250 ml conical flask. Add two drops of phenolphthalein indicator and titrate with solution C. Record your results in table 1. Repeat the titration two more times and complete the table.

        chemistry practicals kcse 2010

        Determine the:
        (i) Average volume of solution C used; (1 mark)
        (ii) Concentration of solution D in moles per litre; (1 mark)
        (iii) Concentration of the alkanoic acid in solution C in moles per litre (1 mole of the acid reacts with 3 moles of the base); (1 mark)
        (iv) Molar mass of the alkanoic acid. (1 mark)

        Procedure II
        Fill a clean burette with solution A. Place 5 cm3 of solution A into a 100 ml beaker. Measure the initial temperature of solution A in the beaker and then record it in table II. Using 10 ml or 100 ml measuring cylinder, measuring 25 cm3 of solution B. Add it to solution A in the beaker and immediately stir the mixture with a thermometer. Record the maximum temperature reached in table II. Repeat the experiment with other sets of volumes of solution A and B and complete the table.

        chemistry practicals kcse 2010

        (a) On the grid provided, plot graph of ΔT (Vertical axis) against the volume of solution A. (3 marks)


        (b) From the graph, determine the volume of solution A which gave the maximum change in temperature. (1 mark)
        (c) Determine the volume of solution B that reacted with the volume of solution A in (b) above. (1 mark)
        (d) Calculate the:
        (i) Ratio between the volumes of solutions A and B that neutralised one another. (1 mark)
        (ii) Concentration in moles per litre of the acid in solution A.
        (Assume that the volume ratio is the same as the mole ratio). (1 mark)

  2. You are provided with solids E, F and G.
    Carry out the tests below and write your observations and inferences in the spaces provided.
    1. Place all of solid E in a boiling tube. Add 20 cm3 of distilled water and shake until all the solid dissolves. Label this as solution E.
      1. To about 2 cm3 of solution E in a test tube, add four drops of 2 M sulphuric (VI) acid.


      2. To about 2 cm3 of solution E in a at test-tube, add 2 M sodium hydroxide dropwise until in excess.


      3. Place one half of solid F in a test tube. Add 2 cm3 of distilled water and shake well. Add 4 drops of this solution to about 2 cm3 of solution E in a test-tube.


      4. To about 2 cm3 of solution E in a test tube, add two drops of aqueous potassium iodide.


      1. Using a metallic spatula, ignite about one half of solid G in a Bunsen burner flame.


      2. Place the other half of solid G into a boiling tube.
        Add 15 cm3 of distilled water and shake well. Label this solution G.
        Use this solution for the following tests.

        1. Place 2 cm3 of solution G in a test tube and determine its pH.

        2. To about 2 cm3 of the solution obtained in (ii) above, add 3 drops of acidified potassium manganate (VII).


        3. To about 2 cm3 of the solution obtained in (ii) above, add 2 drops of bromine water.


      3. To the remaining solution G in the boiling tube, add the other half of solution F.


Marking Scheme

       I II  III 
     Final burette reading 13.80  27.80  40.70 
     Initial burette reading 0.00  13.80  27.30 
     Volume of solution used (cm3) 13.80  13.50  13.40 

    Average volume used  
    13.50 + 13.40 = 13.45 cm3
    Mava = Mbvb
    2 x 25 = 250 x Vb = 0.20M
    Moles of NaOH used = 0.2 x 25/1000 = 0.005 moles
    Moles of acid used = 1/3 x 0.0005
    Concentration of acid =     0.005 x 100     = 0.12M
                                                  13.45 x 3
    Molar mass of acid = 25/0.12 = 208.3

    Table 2
     Volume of solutin A   (cm3) 13  17  21  25 
     Volume of soluton B (cm3) 25 21  17  13 
     Maximum temperature (°C) 30.5 34.0  36.5  36.5  34.0  30.5 
     Initial temperature (°C) 26.5 26.5  26.5  26.5  26.5  26.5 
     ΔT change in temperature  4.0 7.5  10.0  10.0  7.5  4.0 
    2. 15cm3
    3. 30 - 15 = 15cm3
      1. 15:15 = 1:1
      2. MaVa = MbVb
          Ma x 15    1  
            2 x 15         1
        Ma = 2 x 15 = 2
        Ma = 2M
     (i)  Observations Inferences 
       White ppt formed
    No effervesence
    CO2-3 and SO2-3 ion absent
    Probably Pb2+, Ba2+ or Ca2+ may be present 
     (ii)   Observations  Inferences
      White ppt which
    Dissolves in excess
    Pb2+ present 
     (iii)   Observations  Inferences
       White ppt formed Insoluble cpd of Pb2+ is formed 
     (iv)   Observations  Inferences
      Yellow ppt Pb2+ ions confrimed or Pbl2 formed
    (i) Observations Inferences
      Burns with a smoky flame Unsaturated organic cpd or long chain hydrocarbon
    (ii) Observations Inferences
      Colourless solution, turns red pH  1- 2  Carboxylic acid present
    (iii) Observations Inferences
      Effrevesence colourless gas evolved
    Odourless gas
    Confirm G was acid and F was carbonate
    I Observations Inferences
      Decolourised KMnO4(l) Unsaturated alkene or alcohol present
    II Observations Inferences
      Bromine water decolourised Unsaturated alkene present or alkyne
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