Chemistry Paper 3 Kenya High Post Mock Exams 2020 with Confidential - Questions and Answers

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INSTRUCTIONS TO CANDIDATES

  • Answer ALL the questions in the spaces provided.
  • You are not allowed to start working with the apparatus for the first 15 minutes of the 2¼ hours allowed time for the paper.
  • Use the 15 minutes to read through the question paper and note the chemicals you require
  • Mathematical tables and electronic calculators may be used.
  • All working MUST be clearly shown where necessary.
  1. Solution A is prepared by dissolving 6.3g of the organic acid H2C2O4.nH2O in water to make a litre of the solution.
    Solution B: 0.1M NaOH solution
    Phenolphthalein indicator
    Clamp and stand
    Burette and pipette.

    You are required to determine the value of n in the organic acid H2C2O4. nH2O
    Procedure.
    Fill the burette with solution A and adjust the volume to zero mark.
    Add 2 to 3 drops of phenolphthalein indicator and titrate solution A against solution B until thecolour just permanently changes. Record your results in the table below. Repeat the procedure two more times to obtain concordant results.

    1.  Titration   1   2   3 
       Final burette reading (cm3      
       Initial burette reading (cm3)      
       Volume of solution A used (cm3)      
      4marks
    2. Calculate the average volume of solution A used. 1mark
    3. Calculate the moles of sodium hydroxide in the volume of solution B used. 2marks
    4. Given that solution B - Sodium hydroxide and solution A organic acid react in the ration of 2:1, calculate the number of moles of the organic acid –solution A used? 2marks
    5. Calculate the moles of organic acid solution A used per litre of solution 2marks
    6. Calculate the relative formula masses of the organic acid solution A 3marks
    7. Calculate the value of n in H2C2O4.nH2O(H=1, C=12, O=16) 3marks
  2. You are provided with CBI. Carry out the test below. Write your observation and inferences in the spaces provided.
    1. Using a clean spatula, heat about one third of the solid CBI in a non- luminous Bunsen burner flame.
      Observation Inferences
      (1mk) (1mk)
    2. Put a half spatula endful of CBI in a test tube. Heat gently and then strongly. Test for any gas produced using litmus papers.
      Observation Inferences
      (1mk) (1mk)
    3. Put 2cm3 of dilute hydrochloric acid into a test tube. Add ¼ endful of CBI into the test tube.
      Test for any gas procedure.
      Observation Inferences
      (2mks) (2mks)
  3. You are provided with solid Q, carry out the test below. Record your observations and inferences in the table. Identify any gas (es) evolved.
    Place all the solid Q provided into boiling tube and add distilled water until the tube is ¼ full. Divide it into five portions.
    1. To the 1st portion add ammonia solution drop wise until excess.
      Observation Inferences
      1mk) (1mk)
    2.  
      1. To the 2nd portion add sodium hydroxide solution dropwise until in excess. Keep the resulting mixture for the next test.
        Observation Inferences
        (1mk) (1mk)
      2. Warm the preserved mixture from b (i) above
        Observation Inferences
        (1mk) (1mk)
    3.  
      1. To the 3rd portion add silver nitrate solution. Preserve the mixture for the next test.
        Observation Inferences
        (1mk) (1mk)
      2. To the preserved mixture in c (i) above add diluted nitric acid.
        Observation Inferences
        (1mk) (1mk)
    4. To the 4th portion add dilute Barium nitrate solution followed by dilute nitric acid.
      Observation Inferences
      (1mk) (1mk)
    5. To the 5th portion add 2-3 drops of conc. Nitric acid.
      Warm the mixture and allow to cool. Add sodium hydroxide solution dropwise until in excess.
      Observation Inferences
      (2mks) (1mk)

CONFIDENTIAL

Per Student

  1. Solution A (100ml)
  2. Solution B (100ml)
  3. Phenolphthalein indicator
  4. 3 conical flasks
  5. Funnel
  6. Burette
  7. Pipette
  8. Clamp
  9. Stand
  10. CBI(g) – NaHCO3(s)
  11. Clean spatula
  12. Test- tubes (5)
  13. Litmus papers ( 2 blue and 2 red)
  14. Distilled water
  15. Solid Q – 1g (NH4)2 SO4.FeSO4. 6H2O and NaCl (ration 1:1)
  16. 1 boiling tube

    Access to;

  17. 2M ammonia solution
  18. 2M Sodium hydroxide solution
  19. Source of heat
  20. Silver nitrate solution (0.05M)
  21. Dilute nitric acid (0.1M)
  22. Dilute hydrochloric acid (0.1M)
  23. Dilute Barium nitrate solution (0.1M)
  24. Conc. Nitric acid in dropper bottles
  25. White tile
  26. Test tube holder
  • Solution A is prepared by dissolving 6.3g of H2C2O4. 2H2O in 400cm3 of water and topped upto one litre of solution.
  • Solution B is prepared by dissolving 4g of Sodium hydroxide in 400cm3 of water and topped upto one litre of solution.

MARKING SCHEME

Question one

  1.  
    1. Complete table √1mk
      Complete table with 3 titres √1mk
      Incomplete table with 2 titres √½ mk
      Incomplete table with 1 titre −0 mk
      Conditions
      ✔ Penalize ½ mk for unrealistic values unless where explained
      ✔ Penalize ½ mk for any inversion of table
      ✔ Penalize ½ mk for any arithmetic error
      NB: penalize a maximum of ½ mk for any of the conditions above.
    2. Decimal √1mk
      Award 1mk for 1d.p. or 2 d.p used consistently
      If 2d.p used, 2ndd.p. can only be “0” or “5”
    3. Accuracy √1mk
      Award 1mk for any value + 0.1 of s.v.
      Award ½ mk for any value + 0.2 of s.v.
      Award 0mk (penalize fully) for any value beyond + 0.2 of s.v.
    4. Principles of averaging √1mk
      Values averaged must be consistent
      If 3 titres but only 2 are consistent and averaged award 1mk
      If 3 titres done and averaged award 1mk
      If 3 titres done and inconsistent and averaged award 0mk
      If 3 titres done and all are consistent but only 2 are averaged award 0mk
    5. Final answer √1mk
      Award 1mk for ans. + 0.1 of s.v.
      Award ½mk for ans. + 0.2 of s.v.
      Award 0mk if ans not within + 0.2 of s.v.
      Marks awarder as follows:
      • CT 1mk
      • D 1mk
      • A 1mk
      • PA 1mk
      • FA 1mk
        5mks
  2. Average titre = t1 + t2 + t3 = (√½ mk)Correct Ans ½ mk
                                  3
  3. Moles of NAOH = M x V
                              1000
    = 0.1 x 25  √1mk = 0.0025moles √1mk
         1000
  4. 2 NaOH(aq) + H2C2O4(aq) → Na2C2O4(aq) + 2H2O(l)
    Moles ratio = 2 :1
    Therefore moles of organic acid = ½ x 0.0025 moles √1mk
                                                  = 0.00125 moles √1mk
  5. Ans (b) cm3 has  → 0.00125moles
    1000cm3            →           ?
    = 0.00125 x 1000 √1mk
              Ans (b)
    = ______ Correct answer √1mk
  6. Ans (e) moles / L  →  has 6.3g/l
    1mole                   →      ?            √1mk
    = 6.3 x 1 √1mk
       Ans (e)
    = _______ Correct answer √1mk
  7. Value of n
    Ans (f) = H2C2O4. nH2O
    Ans (f) = 2 + 24 + 64 + 18n √1mk
            N = ans f – 90 √1mk
                        18
    = _______ correct ans √1mk

Question two

  Observation Inferences
 (a)   Yellow flame √1mk   Na+ ions √1mk
 (b)
  • Colourless, odourless gas produced
  • Gas turns moist blue litmus paper red
  • Red litmus paper remains red
  • Droplets of colourless liquid on cooler parts of test tube Any 2 x ½ = √1mk
Gas acidic
CO32- , HCO3- ions
Hydrated salt / water of crystallization
Any 2 correct x ½ = √1mk
 (c)
  • Effervescence / bubbles
  • Colourless , odourless gas produced
  • Gas turns moist blue litmus paper red
  • Red litmus paper remains red
    Any 4 x ½ = √2mks
CO32- , HCO3- ions
Gas acidic
Any 2 x 1 = √2mks

Question three

  Observation Inferences
 (a)  Pale green ppt√½ mk
insoluble in excess √½ mk
Fe2+√1mk
(b) (i) 

Pale green ppt√½ mk
Insoluble in excess √½ mk 

Fe2+√1mk
 (ii)
  • Gas with pungent . chocking smell 1mk
  • Moist red litmus paper turns to blue 1mk
  • Blue litmus paper remains blue any 2 x 1 = √2mks
Gas basic √½ mk
NH4+ ions present√½ mk
 (c) (i)   White ppt  CO32-, Cl-ions , SO32-
  (ii)  White ppt√½ mk
 Insoluble / persists √½ mk
 Cl- ions √1mk
 Confirmed
 (d)  White ppt√½ mk
 Insoluble √½ mk
 SO42- ions √1mk
 (e)  Pale green solution turns to yellow solution √1mk
 Brown ppt insoluble in excess √1mk
 Fe2+ oxidized to Fe3+ ions √½ mk
 Fe3+ ions confirmed √½ mk


 


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