Chemistry Paper 3 Questions and Answers with Confidentials - 2021 KCSE Eldoret Diocese Mock Exams

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Questions

INSTRUCTIONS TO CANDIDATES
- Write your name and admission number in the spaces provided.
- Sign and write the date of examination in the spaces provided above.
- Answer all questions in the spaces provided.
- KNEC Mathematical tables and silent non-programmable electronic calculators may be used.
- All working must be clearly shown where necessary.
- Candidates should answer all the questions in English.

  1. You are provided with:
    • Solution A, a mixture of two bases sodium hydroxide and sodium carbonate solids
      dissolved in a 1 litre solution.
    • Solution B, 0.2M hydrochloric acid.
    • Phenolphthalein and methyl orange indicators.
    • Solution C, barium chloride solution.

      You are required to determine the concentration of each of the reactants in the mixture.
    • Procedure 1
      • Pipette 25.0cm3 of solution A into a conical flask.
      • Add two drops of methyl orange indicator.
      • Titrate solution A with B until the yellow colour just changes to pink.
      • Record your results in the table below.
      • Repeat the procedure to obtain two more readings.
      • Table I (3 marks )
          1 2 3
        Final burette reading (cm3)                                    
        Initial burette reading (cm3)      
        Volume of solution B used (cm3)      

        1. Calculate the average volume (V1) of solution B used. (1 mark )
        2. Calculate the number of moles of hydrochloric acid that reacted. (1 ½ marks )

          Procedure II
          • Pipette 25.0cm3 of solution A into a conical flask. Measure 15.0cm3 of barium chloride solution (solution C) with clean measuring cylinder.
          • Add it to the solution A in the conical flask. Shake it gently and add three drops of phenolphthalein indicator.
          • Titrate solution B into the conical flask until the pink colour just changes to colourless.
            NB: The white precipitate should remain in the flask.
            Repeat the procedure to obtain two more readings.
            Table II ( 3 marks )
              1 2 3
            Final burette reading (cm3)                                    
            Initial burette reading (cm3)      
            Volume of solution B used (cm3)      
        3. Calculate the average volume (V2) of solution B used. (1 mark )
        4. The equation for the formation of white precipitate
          Na2CO3 (aq) + BaCl2 (aq) BaCO3 (s) + 2NaCl (aq)
          During titration II the white precipitate formed after adding barium chloride does not take part in the titration but all the hydroxide ions (OH-) in the solution are neutralized.
          1. Calculate the moles of the acid (solution B) reacting in titration II. ( 1 ½ marks )
          2. Calculate moles of sodium hydroxide (OH-) reacting during the titration. ( 1 ½ marks )
        5. Calculate number of moles of acid that reacted with sodium carbonate in the mixture. ( 1 ½ marks )
        6. Calculate the concentration of solution A in terms of sodium hydroxide in moles per litre. ( 1 ½ marks )
        7. Write an ionic equation for the reaction of the acid with sodium carbonate. ( 1 mark)
        8.        
          1. Calculate the number of moles of sodium carbonate in the mixture. (1 mark )
          2. Calculate concentration of solution A in terms of sodium carbonate in moles per litre. (1 ½ marks )
  2. You are provided with solid Z. Carry out the tests below and write your observations and inferences.
    1. Using a clean metallic spatula, heat a half of solid Z in a Bunsen burner flame.
      Observation Inferences
                                             
         
                                                     
    2. Dissolve the remaining portion of solid Z into 10cm3 of distilled water in a boiling tube.
      Divide the resulting solution into four portions.
      Observation Inferences
                                             
         
                                                     
    3. To 1st portion, add 3 drops of acidified potassium manganate (VII)
      Observation Inferences
                                             

         
                                                     
    4. To the 2nd portion, add 3 drops of acidified potassium dichromate (VI) and warm.
      Observation Inferences
                                             

         
                                                     
    5. To the 3rd portion, add all the NaHCO3 provided.
      Observation Inferences
                                             

         
                                                     
    6. To the 4th portion, add 3 drops of universal indicator and determine the pH value.
      Observation Inferences
                                             

         
                                                     
  3. You are provided with solid Y. Carry out the tests below and record your observations and inferences in the spaces provided.
    1. Place half of solid Y in a boiling tube and heat. Test any gas produced with litmus paper.
      Observation Inferences
                                             

         
                                                     
    2. Place the remaining solid Y into a boiling tube. Add about 10cm3 distilled water and shake. Divide the resulting solution into 5 portions.
      Observation Inferences
                                             

         
                                                     
    3. To the 1st portion, add NaOH (aq) dropwise till in excess.
      Observation Inferences
                                             

         
                                                     
    4. To the 2nd portion, add NH3 (aq) dropwise till in excess.
      Observation Inferences
                                             

         
                                                     
    5. To the 3rd portion, add 1cm3 of acidified hydrogen peroxide followed by NaOH (aq) dropwise till in excess.
      Observation Inferences
                                             

         
                                                     
    6. To the 4th portion, add 3 drops of lead (II) nitrate and then filter.
      Observation Inferences
                                             

         
                                                     
    7. To the 5th portion, add 3 drops of acidified barium nitrate solution.
      Observation Inferences
                                             

         
                                                     

Confidential

The information contained in this paper is to enable the head of Institution and the teacher in charge of Chemistry to make adequate preparations for the Form 4 entrance examination. NO ONE ELSE should have access to this paper or acquire knowledge of its contents. The teacher in charge of Chemistry should NOT perform any of the experiments in the same room as the candidates NOR make the results of the experiments available to the candidates or given any other information related to the experiments to the candidates. Doing so will constitute an examination irregularity which is punishable.

In addition to the apparatus and fittings found in a Chemistry Laboratory, each candidate will require the following:

  1. 1g solid Y
  2. 1g solid Z
  3. Metallic spatula
  4. 6 test tubes in a rack
  5. 2 boiling tubes
  6. Test tube holder
  7. Filter funnel
  8.  Filter paper (one)
  9. Distilled water
  10.  Acidified potassium manganate (VII) + dropper
  11. Acidified potassium dichromate (VI) + dropper.
  12. About 1g of NaHCO3 (s)
  13. pH chart
  14. Universal indicator.
  15. 2M NaOH (aq) + dropper
  16. 2M NH3(aq) + dropper.
  17. Blue and red litmus papers.
  18. Acidified hydrogen peroxide + dropper.
  19. Measuring cylinder (10 mls ).
  20.  Acidified 2M Barium Nitrate + dropper
  21. 2M Lead (II) nitrate + dropper.
  22. Bunsen Burner.
  23. 150cm3 of solution A.
  24. 150cm3 of solution B
  25. 50cm3 of solution C
  26. 25.0cm3 pipette.
  27. 50.0cm3 burette.
  28.  A clamp and stand.
  29. 100ml measuring cylinder.
  30. 3 conical flasks.
  31. White tile.
  32. Phenolphthalein indicator.
  33. Methyl orange indicator.

NOTE:

  • Solution A is prepared by dissolving a mixture of 8g sodium hydroxide and 10.6g sodium carbonate in distilled water to make 1 litre solution.
  • Solution B is prepared by measuring 17.2cm3 of concentrated hydrochloric acid ( specific gravity 1.18) to 500cm3 of distilled water and then making it to one litre.
  • Solution C 0.1M barium chloride solution.
  • Solid Y – Hydrated ammonium iron (II) sulphate
  • Solid Z – Maleic acid

Marking Scheme

  1. Procedure I
    Table 1
      1 2 3
    Final burette reading (cm3) 25.1 35.0 39.9
    Initial burette reading (cm3) 0.0 0.0 0.0
    Volume of solution B used (cm3) 25.1 25.0 24.9
    Award marks as follows:
    1. Complete table ( 1 mark )
      Conditions
      • Complete table with three titration 1 mark
      • Incomplete table with two titrations ½ ark
      • Incomplete table with one titration 0 mark
    2. Decimal place ( 1 mark)
      Conditions :
      • Accept only one or two decimal places used consistently.
      • If two decimal place the 2nd decimal place MUST be either o or 0.5
    3. C : Accuracy 1 mark
      Compare the student’s titre value with teachers titre values.
      Conditions
      • At least within ±  0.1 1 mark
      • At least within ± 0.2 ½ mark
      • Above ± 0.2 0 mark
    4. D: Principles of averaging 1 mark
      • Values averaged must be shown and within ± 0.1 of each other

        1. 25.1 + 25.0 + 24.9✓ ½
                  3
          = 25.0cm3✓ ½

        2. Moles of acid that reacted
          If 1000cm3 → 0.2 moles
          Then 25cm3 → 25.0 x 0.2✓1
                                      1000
          = 0.005 moles✓ ½

          Procedure II
          Table II
          Award according to procedure I table I
            1 2 3
          Final burette reading (cm3) 12.6 25.1 37.6
          Initial burette reading (cm3) 0.0 12.6 25.1
          Volume of solution B used (cm3) 12.6 12.5 12.5
          ( Compare / use the teacher’s value )

        3. 12.6 + 12.5 + 12.5✓ ½
                 3
          = 12.5333cm3✓ ½

        4.            
          1. Moles of the acid
            If 1000cm3 → 0.2 moles
            12.5333cm3 → 0.2 x 12.5333✓1
                                         1000
            = 0.002506 moles✓ ½

          2. Moles of sodium hydroxide
            Mole ratio
            H+ : OH-
            1 : 1✓1
            ∴ 0.002506 : 0.002506
            Ie 0.002506 moles✓ ½
        5. Moles of acid that reacted with sodium carbonate
          Ans (b) – ans (dii)
          0.005 moles – 0.002506 moles✓1
          = 0.002494 moles✓ ½

        6. Molarity of A in terms of NaOH
          If 25.0cm3 → 0.002506
          The 1000cm3 → 0.002506 x 1000✓1
                                              25cm3
          = 0.10024M✓ ½

        7. CO32- (aq) + 2H+(aq) H2O(l) + CO2(g)
          [Unbalanced 0 mark , Missing state symbol ½ mark]

        8.          
          1. Moles of sodium carbonate
            ½ x 0.002494✓ ½
            = 0.001247 moles✓ ½

          2. Molarity of A in terms of Na2CO3
            If 25cm3 → 0.001247
            Then 1000cm3 → 0.001247 x 1000✓1
                                                   25
            = 0.04988M✓ ½
  2.      
        Observations Inferences
    2




     
        
    a - Solid melts✓ ½
    - Burns in yellow✓ ½ sooty flame✓ ½
    Max 1 ½ marks
    - Presence of either

    Max 1 mark
    b Dissolves✓ ½ into a colourless✓ ½ solution
    Max 1 mark
    Solid is polar✓ ½
    Max ½ mark
    c Acidified KMnO4 changes from purple to colourless✓ ½
    Max ½ mark
    Presence of either
    d Acidified K2Cr2O7 changes from orange to green✓ ½ Presence of either
    e Effervences✓ ½ production of colourless gas✓ ½ H+✓ ½ 
    f pH 4 âœ“ ½ Weakly acidic✓ ½
    3 a Colourless liquid on cooler parts of test tube✓ ½
    Colourless gas with pungent smell✓ ½
    Gas turns moist red litmus blue✓ ½ /
    Blue litmus remains blue✓ ½
    Hydrated salt✓ ½
    Presence of NH4+✓ ½
    b Solid dissolves into pale green solution✓ ½ Polar solid✓ ½
    Presence of Fe2+✓ ½ 1
    c Green✓ ½ ppt insoluble✓ ½ in excess
    PPt turns brown on exposure to air✓ ½
    Presence of Fe2+✓ ½
    Fe2+ oxidized to Fe3+ by air✓ 1
    d Green ppt✓ ½ insoluble in excess
    Ppt turns brown on exposure to air✓ ½
    Presence of Fe2+✓ ½
    Fe2+ oxidized to Fe3+ by air✓ ½
    e Pale green solution turns yellow✓ ½
    Brown✓ ½ ppt insoluble✓ ½ in excess
    Presence of Fe3+✓ ½
    f Formation of white ppt✓ ½ as residue and green solution as filtrate✓ ½ Presence of Fe2+✓ ½
    Presence of either Ce-✓ ½ , SO42-✓ ½ or SO32-✓ ½ , CO32-✓ ½
    Max 2 marks
    g White ppt forms✓ ½ SO42- present✓ ½

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