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

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 2^nd 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 3^rd 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 4^th portion add dilute Barium nitrate solution followed by dilute nitric acid.
      

      Observation	Inferences
      (1mk)	(1mk)

   5. To the 5^th 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) – NaHCO_3(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, 2^ndd.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) → Na₂C₂O_4(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

Question three