Chemistry Paper 3 Questions and Answers - KCSE 2022 Kapsabet Highschool Trial 1 Pre-Mock Examination

INSTRUCTIONS TO CANDIDATES

• Write your name and index number in the spaces provided above.
• 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.

FOR EXAMINER’S USE ONLY.

QUESTIONS

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
   1 mark	1 mark

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.
3. Put 2cm³ of dilute hydrochloric acid into a test tube. Add ¼ endful of CBI into the test tube.

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.
2.                        
   1. To the 2nd portion add sodium hydroxide solution dropwise until in excess. Keep the resulting mixture for the next test.
   2. Warm the preserved mixture from b (i) above
3.                  
   1. To the 3rd portion add silver nitrate solution. Preserve the mixture for the next test.
   2. To the preserved mixture in c (i) above add diluted nitric acid.
4. To the 4th portion add dilute Barium nitrate solution followed by dilute nitric acid.
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.

MARKING SCHEME

Question one

1. Complete table 1mk
   1. 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 ifansnot 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 251mk    =          0.0025moles    1mk
                               1000
4. 2 NaOH _(aq) + H₂C₂O_{4 (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