Chemistry Paper 3 Questions and Answers with Confidential - Kapsabet Pre Mock Exams 2021/2022

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CHEMISTRY 
PAPER 3:PRACTICAL

INSTRUCTIONS

  • Write your name and index number in the spaces provided above.
  • Sign and write the data of the examination.
  • Answer all the questions in the spaces provided.
  • You are not supposed to start working with the apparatus for the first 15 minutes of 2¼ hours allowed for this paper. This time is meant to read through the paper and ensure you have all the chemicals and apparatus require.
  • All working must be clearly shown
  • KNEC mathematical tables and silent electronic calculations may be used.
  • All questions should be answered in English

 

QUESTION 1

You are provided with:

  • Solid A 5.0g (COOH)2·H2O
  • Solution B 0.13M KMnO4

Task

  1. You are supposed to determine the solubility of A at different temperatures.
  2. Determine the number of moles of water of crystallization in solid A.

PROCEDURE 1

  1. Using a burette, add 4cm3 of distilled water to solid A in a boiling tube.
    • Head the mixture while stirring with the thermometer to about 800C.
    • When the whole solid dissolves, allow the solution to cool while stirring with the thermometer
    • Note the temperature at which crystals first appear and record this temperature in the table 1 below.
  2. Using a burrete add 2cm3 more into the content of the boiling tube and warm until the solid dissolve.
    • Remove from the flame and allow the solution to cool in air while stirring.
    • Record the temperature at which crystal first appear in table 1.
    • Repeat procedure (b) 3 more times and complete table 1 below.
    • Retain the content of the boiling tube for procedure II

Table 1

Volume of water in
the boiling tube (cm3)
Temperature at which crystals
of solid A appear (0C)
Solubility of solid A
g/100g of water
 4
6
8
10
12
   

 

  1.          
    1. Draw a graph of solubility of solid A (vertical axis) against temperature (3mks)
    2. From your graph determine the solubility of solid A at 600C (1mk)

      PROCEDURE II     
      1.              
        • Transfer the contents of the boiling tube into a 250ml volumetric flask.
        • Add distilled water up to the mark
        • Label this solution A
      2.           
        • Using a clean pipette and a pipette filler, transfer 25ml of solution A into a conical flask.
        • Warm the mixture up to 60ºC
        • Fill a burette with solution B
        • Titrate B against the hot solution A until a permanent pink colour persist
        • Read your results in Table 2 below
      3. Repeat (b) 2 more times are record your results in the table 2 below.

        Table 2
          I II III
        FINAL BURETTE READING                                         
        INITIAL BURETTE READING      
        VOLUME OF SOLUTION B USED (CM3)      
  2.           
    1. Calculate the average volume of solution B used (1mk)
    2. Calculate the number of moles of B used (1mk)
    3. Given 2 moles of KmNO4 react with 5 moles of A, calculate the number of moles of A in 25cm3 (1mk)
    4. Calculate the molarity of A (1mk)
    5. Determine the molar mass of A (1mk)
    6. Determine the value of X (1mk)
      (C=12, O=16 H=1)

QUESTION 2
You are provided with solid C. Use it to carry the test below. Dissolve the whole of C into 10cm3 of water and divide it into five portions.

  1. To the 1st portion add sodium sulphate solution.
    Observations Inferences
                                               


    (1mk)

                                              



    (1½mk)

  2. To the 2nd portion add Ammonia solution dropwise until in Excess.
    Observations Inferences
                                               


    (1mk)

                                              



    (1mk)

  3. To the 3rd portion add sodium Hydroxide dropwise until in Excess.
    Observations Inferences
                                               


    (1mk)

                                              



    (1mk)

  4. To the forth portion add Lead (II) Nitrate solution
    Observations Inferences
                                               


    (½mk)

                                              



    (2mk)

  5. To the last portion add Barium Nitrate solution
    Observations Inferences
                                               


    (1mk)

                                              



    (1mk)

QUESTION 3

You are provided with liquid D use it to carry the test below. Divide liquid D into four equal portions

  1. To the 1st portion add sodium hydrogen carbonate
    Observations Inferences
                                               


    (1mk)

                                              



    (1mk)

  2. To the 2nd portion add acidified potassium manganite (VII) (KmNO4)
    Observations Inferences
                                               


    (1mk)

                                              



    (1mk)

  3. To the 3rd portion add Bromine water
    Observations Inferences
                                               


    (1mk)

                                              



    (1mk)

  4. To the last portion add potassium dichromate(VI) and warm.
    Observations Inferences
                                               


    (1mk)

                                              



    (1mk)

 

CONFIDENTIAL
In addition to the equipment and fittings found in a chemistry laboratory. Each candidate should be provided with;

  1. Solid A 5.0g measured accurately
  2. About 80cm3 of solution B
  3. About 0.5g solid C
  4. About 10cm3 of liquid D
  5. A thermometer (-10-110ºC)
  6. A burette
  7. A complete retort stand
  8. A pipette and a pipette filler
  9. 2 conical flasks
  10. A 250ml volumetric flask
  11. One boiling tube
  12. Five (5) test tubes
  13. 0.5g sodium hydrogen carbonate
  14. Two labels

ACCESS TO:

  1. Means of heating (Tripond stand and wire gauze)
  2. Sodium sulphate solution (NaSO4)
  3. Ammonia solution 2m
  4. 2m Sodium Hydroxide
  5. Lead Nitrate solution
  6. Barium Nitrate solution
  7. Acidified potassium manganite (VII) solution
  8. Bromine water
  9. Acidified potassium dichromate(VI) solution

NB:

  1. Solid A is 5.0g of oxalic acid (COOH)2 2H2O
  2. Solution B is KmnNO4
  3. Solid C is magnesium chloride MgCl2
  4.  Liquid D is absolute ethanol

Preparations

  1. Solution B is made by dissolving 20g of solid KmNO4 in 200cm3 of 2.0m H2SO4 and toping to 1000cm3 by distilled water.
  2. Sodium Hydroxide is prepared by dissolving 80g of NaOH pellets in 600cm3 of distilled water and top to 1000cm3 with distilled water.
  3. Ammonia solution is prepared by dissolving 150ml of conc ammonia to 600cm3 of distilled water then top to the mark.
  4. Barium Nitrate is prepared by dissolving 26g of solid Barium Nitrate in 600cm3 of water then topping to 1000cm3 with distilled water.
  5. Lead nitrate is prepared by dissolving 30g of solid Lead Nitrate in 600cm3 of water then topping to 1000cm3 with distilled water.
  6. Sodium Sulphate is prepared by dissolving 14.2g of solid sodium sulphate in 600cm3 of distilled water then topping up to 1000cm3 with distilled water.
  7. Acidified Kmno4 is prepared by dissolving 3.2g of solid Kmno4 in 200cm3 of 2.0m H2SO4 acid then topping with distilled water to 1000cm3.
  8. Acidified K2Cr2O7 is prepared by dissolving 25g of solid K2Cr2O7 in 200cm3 of 2.0m H2SO4 then topping to 1000cm3 with distilled water.

 

MARKING SCHEME

QUESTION1

TABLE 1

Volume of water in
the boiling tube (cm3)
Temperature at which crystals
of solid A appear (0C)
Solubility of solid A
g/100g of water
4 70.0   125.0
6 59.0 83.3
8 54.0 62.5
10 47.0 50.0
12 40.0 41.7

 AWARD a total of 7 MKS Distributed as follows.

  1. Complete table – 4mks
    Condition and penalties
    • A table with 8-10 values award 4
    • A table with 6-7 values a ward 3
    • A table with 4- 5 values award 2
    • A table with 2-3 values award 1
    • A table with 1 value award ½
    • A table with no value a ward 0

      Penalties
      1. Penalize ½ mk for each wrong value of solubility
      2. Penalize ½ one for unrealistic temperature readings ½ above 90ºC and below 10ºC

  2. DECIMALS TIED TO TEMPARATURE -1Mk
    Accept;
    1. Whole numbers
    2. 1 decimal place where the decimal should be 0 or 5 i.e 70.0 or 70.5
    3. Accept 2 decimal places where it should be .00, .25,.50 or .75

      NB: If no consistence penalize fully.

  3. TREND – Tied to temperature i.e it should be decreasing (1mk)
  4. Accuracy – tied to temperature when 4cm3 of water was added
    Accept 20C of the school value.
  5. GRAPH – award a total of 3mks distributed as follows.
    1. Labeling (½mks)
      • Both axis should be labeled if one is not or wronglylabeled award O
      • Ignore units but if indicated they should be right otherwise penalize fully
    2. Scale – (½mks)
      • The actual plotting should be halfof the page
      • Peralize fully if scale changes on the way both axis must be correct.
      • Plots (1mk)
      • 4-5 correct plots award (1mk)
      • 3 correct plots award (½mks)
      • Less than 3 correct plots award 0
    3. Line – a smooth curve passing through 3 or more correct plots award 1mk
      • Otherwise award 0
    4.            
      •  award ½ mk for sloping or the graph
      • award ½ mk for calculating
        NB:
        1. Award fully for calculation from correct graph even if not shown on the graph
        2. Reject any value from a wrong graph.
          KapPreM Chem p3qa4

TABLE 2

  I II III
FINAL BURETTE READING 12.5 12.5 12.5
INITIAL BURETTE READING  0.0  0.0 0.0 
VOLUME OF SOLUTION B USED (CM3)  12.5  12.5  12.5

 
Award a total of 5mks distributed as follows

  1. Complete table (1mk)
    • Complete table with 3 titration 1mk
    • Incomplete table with 2 titrations ½ mk
    • Incomplete table with 1 titration 0 mks

      Penalties
      1. Wrong arithmetic
      2. Invented table
      3. Unrealistic values i.e burette reading with more than 50cm3 and less than 1cm3 withoutexplanation.
      4. Unrealistic titre values

        NB: penalize ½ once

  2. Use of decimals (1mk) (Tied to the 1st and 2nd row only)
    Accept 1 or 2 decimal places used consistently otherwise penalize fully
    • If 2 dp are used the 2nd should be a “O” OR “5” e.g 20.10 or 20.15 otherwise penalize fully
    • Accept the use of Zero as the initial burette reading i.e 0,0.0 or 0.0
  3. Accuracy (1mk)
    • Complete the candidate value with the school value (S.V)
      1. If within 0.1 of the school value award 1mk
      2. If within 0.2 of S.V ward ½mk otherwise award 0

        NB: Tick the candidate value that deserves a credit

  4. PRINCIPLE OF AVERAGING ------------------------1MK
    Conditions
    1. If 3 titration done but only two are consistence and averaged award 1mk
    2. If 3 titration are done and consistency and averaged award 1mk
    3. If two titration are done and are consistency and averaged award 1mk
    4. If three consistency titration one done but 2 are averaged award 0
    5. If three 3 titration are done and are inconsistence and are done averaged award zero
    6. If two titration are done and are inconsistence and are averaged award 0

PENALTIES

  1. Penalize ½ for wrong arithmetic
  2. penalize ½mk if no working is shown and answer is correct
  3. penalize fully if no working is shown and answer given is wrong
  4. Accept rounding off or truncation to the 2nd d.p
    e.g 12.666 12.67
    or
    12.66 12.66

    NB:
    1. The working of average must be marked before the mark for averaging is award in table 2.
    2. Accept the average volume if it work out exactly to a whole number.

  1. FINAL ACCURACY ----------------------(1MK) Tiled to correct average time.
    Compare the candidate average time to the school value.
    1. If within0.1 award 1mks
    2. If with n 0.2 award ½ mk
    3. Otherwise award 0

      NB:
      • If there are 2 possible correct average titre, use the one the one close to the school value and award accordingly.
      • If wrong value are averaged, pick the correct values average for the candidate and award accordingly.
      • Record the marks as follows besides the table to the right.
        CT – 1mk
        D - 1mk
        A -1mk
        PA -1mk
        FA - 1mk
        Total 05 mks

  2. Calculate the number of moles of B used
    1000 - 0.13
    12.5 ?
    = 12.5×0.131000 √ ½
    0.001625 moles √ ½
    = 0.0040625 √ ½

  3. Ans b×52
    = 0.0040625 √ ½

  4. Ans c×100025 √ ½
    = 0.1626m √ ½

  5. 5g 250cm3
    20g – 1000
    1×200.1625
    = 123.07 √ ½

  6. 90 + 18x =123
    18x = 33
    x= 3318
    x= 1.83
    = 2

 

QUESTION 2

  1. To the 1st portion add sodium sulphate solution.
    Observations Inferences
    No white precipitate formed (1mk)
                                 

    Ba2+ Ca2+ and Pb2+ Absent
    Each ½ mks
    Penalize ½mk to a maximum of 1 ½ mks for any contradictory ion                                       

  2. To the 2nd portion add Ammonia solution dropwise until in Excess.
    Observations Inferences
                                               

    No white precipitate insoluble in excess
    NB: White precipitate ½ mk
    Insoluble in excess ½ mk

    Zn2+ absent (1mk)
    Penalize 1mk for each contradicting ion to a maximum of (1mk)                               

     

  3. To the 3rd portion add sodium Hydroxide dropwise until in Excess.
    Observations Inferences
    A white precipitate ½
    Insoluble in excess ½
                                       

    Mg2+ present (1mk)
    Accept Al3+ absent for ½mk
    Panelize 1mks for any contracting ion to a Maximum of 1mks                                            

  4. To the forth portion add Lead (II) Nitrate solution
    Observations Inferences

    A white precipitate is formed ½ mks

     Cl-, So32- , SO4 2- and CO32- present
    – ½ mk each
    Penalize ½mk for any contradictory ion to a maximum of 

  5. To the last portion add Barium Nitrate solution
    Observations Inferences
             

    No white precipitate formed

     

     

                                 

    CI- Present (1mks)
    -accept SO42- AND SO32- OR CO32- absent for fully marks
    penalize 1mks for any contradictory ion to a maximum of 1mks
    Three anions given – 1mk
    Two anions given -½mk
    One onion given – 0mk                                   

QUESTION 3

  1. To the 1st portion add sodium hydrogen carbonate
    Observations Inferences
                                               

    No fizzing/bubbling /hissing (1mk)
    Reject
    -fissiling
    -Sizzling

    R- CooH Absent (1mk)
    NB: Ignore H3O+ & H+                                  



  2. To the 2nd portion add acidified potassium manganite (VII) (KmNO4)
    Observations Inferences
                                               

    KMNO4 get decolorized
    Or
    KMNO4 turns from purple to colorless (1mk)
    Reject
    The solution turns colourless
    It turns colourless

    C= C OR –C= C- (½mk)
    Present

    R- OH Present (½mks)

    NB: Penalize ½mk for any contradictory group to a maximum of  



  3. To the 3rd portion add Bromine water
    Observations Inferences
     Yellow /orange bromine water does not get decolonized


    Accept
    It remains yellow or orange

    Chem qa2dfgfd
     or –C C-

    Absent
    Penalize 1mk for any contradictory group to a maximum of         



  4. To the last portion add potassium dichromate(VI) and warm.
    Observations Inferences
    K2CrO7 turns from orange to green

     R-OH Present (1mk)
    Penalize 1mk for any contradictory group to a maximum of

 


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