Chemistry Paper 3 Questions and Answers with Confidentials - KCSE 2022 Mock Exams Set 2

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Questions

Instruction to Candidates:

  • Answer ALL the questions in the spaces provided after EACH question in the question-paper.
  • You are NOT allowed to start working with the apparatus for the first 15minutes of the 2¼ hours allowed for this paper. This time is to enable you read the question-paper and make sure you have ALL the chemicals and apparatus that you may need.
  • Mathematical tables and silent electronic calculator may be used.
  • ALL working MUST be clearly shown where necessary.

 

QUESTION 1.
You are provided with:

  • Sulphuric acid solution A
  • 0.5M sodium hydroxide solution B
  • Magnessium ribbon labelled C

You are required to:-

  • Investigate the rate of reaction between solution A and metal C
  • Determine the concentration of sulphuric acid in moles per litre

Procedure I

  1. Using a ruler, make 6 marks at 2cm length interval on the Magnesium ribbon provided. Cut the magnesium ribbon into 2 cm long pieces.
  2. Transfer 50cm3 of acid solution using a measuring cylinder into a clean dry 100ml beaker.
    Place 2cm length piece of magnesium ribbon into the beaker with the acid and immediately
    start the stop watch/clock. Shake gently and note the time taken for the piece of
    magnesium ribbon to react completely.
  3. Record in table I below. Place another piece of magnesium ribbon (2cm) to the same solution and again note the time taken.
  4. Repeat the procedure until all six pieces of magnesium ribbon have reacted with
    the same solution initially placed in the beaker
  5. Complete the table I below:
    Note: Keep the solution obtained in this experiment for use in procedure II
    1. Table I
       Piece of magnesium added
       Length of magnesium added (cm) 10  12 
       Time taken t(second)            
       Reciprocal of time 1/t(s-1)            
      (4 marks)
    2.        
      1. On the grid provided, plot a graph of total length of magnesium ribbon added against reciprocal of time (1/t) for the reaction to go to completion. (3 marks)
      2. From your graph, determine the time taken when 4.5cm length of magnesium ribbon to react completely. (1 mark)
      3. Write a chemical equation for the reaction between magnesium and sulphuric acid. (1 mark)
      4. Given that the mass of solid V, which reacted was 0.12g and that atomic mass of magnesium is 24.0g, determine the number of moles of sulphuric (VI) acid that were used up during the reaction. (1 mark)
      5. From your graph, state and explain the relationship between the length of magnesium ribbon and the reciprocal of time (1/t) (1 mark)

Procedure II
Place all the solution obtained in procedure I in a clean 100ml measuring cylinder.
Add distilled water to make 100cm3 of solution. Transfer all the solution into a beaker
and shake well. Label it solution D. Fill the burette with solution B. Pipette 25.0cm3
of solution D into a conical flask. Add 2-3drops of phenolphthalein indicator and titrate
with solution. Record your results in the table II below. Repeat the titration two more times

Table II

 Titration II  III 
 Final burette reading (cm3)      
 Initial burette reading (cm3)      
 Volume of solution B (cm3) used      


(4 marks)

  1.      
    1. Determine the average volume of solution B used . (1 mark)
    2. Calculate the number of moles of sodium hydroxide solution B used. (1 mark)
  2. Calculate:
    1. The number of moles of sulphuric acid in 25.0cm3 of solution D. (1 mark)
    2. The number of moles of sulphuric acid in 100cm3 of solution D. (1 mark)
  3. Determine the total number of moles of sulphuric acid in 50cm3 of solution A. (1 mark)
  4. Calculate the concentration of the original sulphuric acid solution A in moles per litre. (1 mark)

QUESTION 2.
You are provided with solid E. Carry out the following tests and write your observations and
inferences in the table below:

  1. Place all the solid E in a boiling tube. Add about 15cm3 of distilled water and shake vigorously for about 2 minutes.
     Observation Inferences 
     



     
  2. Divide the solution into five equal portions in five different clean test tubes.
    1. To the first portion, add 2M ammonia solution drop wise until in excess.
       Observation Inferences 
       



       
    2. To the second portion add 2M Sodium hydroxide solution drop wise until in excess.
       Observation Inferences 
       



       
    3. To the third portion add 4 drops of 2M Lead (II) nitrate solution.
       Observation Inferences 
       



       
    4. To the fourth portion, add 4 drops of 0.2M silver nitrate solution.
       Observation Inferences 
       



       
    5. Clean one end of the glass rod provided. Dip the clean end of the glass rod in the fifth portion. Remove the end and heat it in the non-luminous part of a Bunsen burner flame. Note the colour of the flame and record below.
       Observation Inferences 
       



       

QUESTION 3.
You are provided with solid F. Carry out the tests below. Write your observations and inferences
in the spaces provided

  1. Place about a half of solid F on a metallic spatula and burn it using a Bunsen burner flame.
     Observation Inferences 
     



     
  2. Place the remaining of solid F in a boiling tube. Add about 10cmof distilled water and shake the mixture well.
     Observation Inferences 
     



     
  3. Divide the mixture obtained into three portions.
    1. To the first portion, add a small amount of solid sodium hydrogen carbonate.
       Observation Inferences 
       



       
    2. To the second portion, add about 1cm3 of acidified potassium dichromate (VI) and warm.
       Observation Inferences 
       



       
    3. To the third portion, add two drops of acidified potassium magnate (VII)
       Observation Inferences 
       



       

Confidential

INSTRUCTIONS.
Apart from the normal fittings in the laboratory, each candidate will need the following chemicals and apparatus.

  1. 500ml of distilled water supplied in a wash bottle
  2. 50ml burette
  3. 25ml
  4. a pipette filler
  5. 2 conical flasks (250ml)
  6. Source of heat (means of heating)
  7. Stop watch/clock
  8. A ruler
  9. 100ml measuring cylinder
  10. 50ml measuring cylinder
  11. Complete retort stand
  12. 12cm long magnesium ribbon labelled C
  13. 100ml of solution A (sulphuric acid)
  14. 80ml of solution B (Sodium hydroxide soltn.)
  15. 100ml empty beaker
  16. Funnel
  17. Sand paper
  18. 3g of solid E
  19. 1g of solid F
  20. Means of labeling
  21. Six clean test tubes in a test tube rack
  22. 3 boiling tubes in a rack
  23. Metallic spatula
  24. About 0.2g of sodium hydrogen carbonate
  25. Glass rod.

Access

  1. 2M Ammonia solution supplied with a dropper
  2. 2M Sodium hydroxide solution supplied with a dropper
  3. 2M Lead (II) Nitrate supplied with a dropper
  4. 0.2M Silver Nitrate solution supplied with a dropper
  5. Acidified potassium dichromate (VI) supplied with a dropper
  6. Acidified Potassium Manganate (VII) supplied with dropper

N/B

  1. Solution A is prepared by accurately measuring 27.5cm3 of concentrated
  2. Sulphuric acid, then adding it to 700ml of distilled water then topping it to one litre. Density of acid 1.84g/cm3
  3. Solution B is prepared by accurately measuring 20g of NaOH pellets and dissolving it in 800cm3 of distilled water then topping to one litre with distilled water.
  4. Solid E – sodium chloride
  5. Solid F – maleic acid

Marking Scheme

  1.            
    1. Table 1
       Piece of magnesium added
       Length of magnesium added (cm) 10  12 
       Time taken t(second)  150 190  225  295  430  500 
       Reciprocal of time 1/t(s-1)  0.00667 0.00526  0.00444  0.0033  0.00233  0.002 
      Marking points
      Complete Table – 1
      Decimal point – 1
      Accuracy – 1
      Trend – 1

    2. Graph:
      Straight line graph of best fit
      Label of axis = ½
      Scale = ½
      Plotting = 1
      Line = 1
      1. 1 = 0.00510 √½ From the graph and must be shown. Showing. √½
        t
        t =         1 √½    = 196.5 seconds. √½
               0.00510
      2. Mg(s) + H2SO4(aq) → MgSO4(s) + H2(g) √½
        1 : 1 With correct physical state.
      3. Moles of Mg = 0.12/24 √½ = 0.005 moles √½ 1mk
        Moles of H2SO4 used = 0.005 moles (1 : 1)
      4. Increase in length of M of ribbon results in decrease in (1/t )√½
        This is done to gradual decrease in the concentration of the acid. √½

        Table II
         Titration II  III 
         Final burette reading (cm3)  15.3 30.5  45.7 
         Initial burette reading (cm3)  0.0 15.3  30.5 
         Volume of solution B (cm3) used  15.3 15.2  15.2 
    3.            
      1. T1 + T2 + T3 √½ = C.A √½ 1 fall are consistent
                 3
        OR
        i.e 15..3 + 15.2 + 15.2 √½ = 15.233 cm3 √½
                       3
      2. Moles of sodium hydroxide = 15.233 x 0.5 = 0.007617
                                                        1000
        i.e. Ans in c (i) x 0.5 √½ = C.A. √½
                          1000 1 mk
    4.  
      1. Ans in c (ii) √½ = C.A. √½ i.e. 0.007617 = 0.003809 moles
                     2                                 1 mk
      2. Ans. in d (i) x 4 = C.A.
        i.e o.003809 x 4 = 0.015236 moles. 1 mk
    5. Ans in b (iv) + Ans. d(ii) √½ = C.A
      0.005 + Ans. d (ii) = C.A
      i.e. 0.005 + 0.015235 = 0.020236 moles. 1 mk
    6. Ans. in e x 1000 cm3 = C.A.
                      50 cm3
      i.e. 0.020236 x 1000 = 0.40472 M
                     50
 2     
  a     Observations   Inferences
 Dissolves to form colourless solution  Soluble salt or absence of coloured irons
i.e Fe3+, Fe2+, Cu2+
b i  Observations   Inferences
     No white ppt.  Pb2+, Al3+ or Mg2+ absent
NH+4, Na+, or K+ may be present. √½
ii  Observations   Inferences
   No white ppt.   NH+4, Na+ √½ or K+ possibly present. √½
Or (1 mk)
Pb2+ Al3+, Zn2+ absent 
iii  Observations   Inferences
  White ppt. formed. CO32-, SO42- Or Cl- present.
iv  Observations  Inferences
  White ppt. √ ½ dissolves in excess 
ammonia √ ½ solution to form
colourless solution. (1mk)
Cl-1 present. √1
v Observations Inferences
  Golden yellow flame. Na+ present.
3     
  Observations Inferences
  Burns with yellow flame
sooty /smoky flame. √½ 
- Long chain hydrocarbon
- Unsaturated organic compound. √½
organic compound with high C – H ratio
   
   Chemcsp3qa3a

b   Observations Inferences
     Dissolves to form 
colourless solution. √1
 Polar organic compound/ soluble salt/ soluble compound √1
c i Observations Inferences
     Effervescence /bubbles /fizzing. √½  Presence of H+ / H3O+ , R- COOH. √½
ii Observations Inferences
   Orange colour remains  the same / persists i.e
does not change green. √½
 Absence of R –OH. √½
iii Observations Inferences
   KMnO4 decolourized i.e
changes from C ═ C Or - C ≡ C −
purple to colourless√1 Or
   
   Chemcsp3qa3a 
 Unsaturated organic compound. √1

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