Chemistry Paper 3 Questions and Answers - KCSE 2022 Prediction

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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 not the chemicals you require
  • Mathematical tables and electronic calculators may be used.
  • All working MUST be clearly shown where necessary.

Questions

  1. You are provided with;
    • Solution A containing 6.95g of Iron II Sulphate heptahydrate R.F.M = 278 in 250cm3 of solution
    •  Solution B of potassium manganate (VII)
    •  Solution C of hydrogen peroxide.
      You are required to
      1. Standardize the potassium manganate (VII) solution C
      2. Determine the concentration of hydrogen peroxide solution C.

PROCEDURE I
Pipette 25cm3 of solution A into a conical flask.
Fill the burette with solution B. Titrate this solution against solution A until the first permanent pink colour appears. Record your results in table I and repeat the procedure to fill the table 1 below.
Table I 4 marks

                   

 I

 II

 III

 Final burette reading (cm3)

     

 Initial  burette reading (cm3)

     

 Volume of solution B used (cm3)

     
  1. Calculate the average volume of solution B used (1 marks)
  2. Given that the equation for the reaction is
    Mno-4(aq) + 5Fe2+(aq) + 8H+(aq) → Mn2+(aq) + 5Fe3+(aq) + 4H2O(l)
    Calculate
    1. The number of moles of Iron II sulphate solution A used (1mark)
    2. The number of moles of solution B that reacted. (1mark)
    3. The concentration of the potassium manganate (VII) solution B in moles per litre. (1mark)

PROCEDURE II
Pipette 25cm3 of hydrogen peroxide, solution C into a conical flask. Fill the burette with solution B. Titrate this solution against solution C until the first permanent pink colour appears. Record results in table II.

TABLE II (4marks)

 Titre  number

 I

 II

 III

 Final burette reading cm3

     

 Initial burette reading cm3

     

 Volume solution B used cm3

     
  1.  Work out average volume of potassium manganate (VII) solution B used. (1mark)
  2. Given that the equation for the reaction is
    2MnO-4 (aq) + 5H2O(aq) + 6H+(aq) → 2Mn2+(aq) + 8H2O(l) (aq) + 5O2-(aq)
    Calculate
    1. The number of moles of Potassium Manganate (VII) solution B that reacted. (1mark)
    2. The number of moles of hydrogen peroxide solution C that reacted. (1mark)
    3. The concentration of hydrogen peroxide solution C in moles per dm3(mol dm-3) (1mk)

2. You are provided with 4g of Solid F.
You are required to determine the solubility of solid F at different temperatures.
PROCEDURE

  1. Carefully transfer all solid F in a clean boiling test tube and using a burette, add 15cm3 of distilled water. Heat the mixture while stirring with a thermometer to about 85ºC. when all the solid has dissolved, allow the solution to cool while stirring with the thermometer. Note the temperature at which the crystals of solid F first appear. Record this temperature in Table III.
  2. Transfer 5cm3 of distilled water to the contents in the boiling tube. Warm the mixture while stirring with the thermometer until the solid dissolve. Allow the mixture to cool while stirring. Note and record the temperature at which crystals first appear.
  3. Repeat procedure (b) two or more times and record the temperatures in table III.
  4. Complete table III by calculating the solubility of solid F at the different temperatures.

TABLE III

 Volume of water in the boiling tube (cm3)

 Temperature at which crystals of solid F first appear.

 Solubility of solid F in g/100g of water.

 15

   

 20

   

 25

   

 35

   

 40

   

(6marks)

  1. On the grid provided plot a graph of solubility of solid F (vertical axis) against temperature (horizontal axis). (3marks)
    gridp2chemkcse2022
  2. Using your graph, determine the temperature at which 15g of solid F, would dissolve in 100cm3 of water. (1mark)

3. You are provided with solid D. carry out the following tests and write down all the observations and inferences.

  1.                      
    1. Place half spatula end full of solid D in a dry test tube. Heat gently then strongly until there is no further change.

      Observations

      inferences

      (1mark)

      (1mark)

    2. Place the remaining solid D in a test tube, add about 10cm3 of distilled water and shake vigorously. Divide the mixture into four portions.
      1. To the 1st portion, add 2M sodium hydroxide solution drop wise until in excess.

        Observations

        inferences

        (1mark)

        (1mark)

      2. To the 2nd portion, add ammonia solution drop wise till in excess.

        Observations

        inferences

        (1mark)

        (1mark)

      3. To the fourth portion add 4 drops of sodium chloride.

        Observations

        inferences

        (1mark)

        (1mark)

  2. You are provided with liquid E, Carry out the following tests on it.
    1. Place about one spatula end full of liquid E on a metallic spatula and ignite it in a Bunsen burner flame.

      Observations

      inferences

      (1mark)

      (1mark)

    2. To 2cm3 of liquid E add 3 drops of acidified KMnO4. Solution B.

      Observations

      inferences

      (1mark)

      (1mark)

    3. To 2cm3 of liquid E add 3 drops of acidified K2Cr2O7.

      Observations

      inferences

      (1mark)

      (1mark)

Marking Scheme

Question 1
Notes to award marks on the table.
TABLE 1

Titre number

I

II

III

CT = 1

DP = 1

Acc = 1

Ap = 1

Final burette reading cm3

28.0

28.0

28.0

Initial burette reading cm3

0.0

0.0

0.0

Titre volume cm3

28.0

28.0

28.0

  1. CT – complete table 1mk
    The table should be filled completely
    Errors like
    • Values recorded beyond 50.0cm3
    • wrong arithmetic between final and initial
    • inversion of the table
    • titre values less than 1
      penalize ½ mk only for any point of the errors mentioned above.
  2. D.p. = Decimal point 1mk
    Either all values recorded to 1 d.p OR all recorded to 2 d.p, second figure of the d.p being 0 or 5 only e.g. 23.60 , 24.75 not 21.36 , 22.57
    A student should not record same values to 1 d.p other to 2 d.p in the same table. If this happens award 0mk for d.p
  3.  Acc. = Accuracy …………1mk
    Consider any one of the candidates’ titre if within 0.0 to 0.1cm3 of school value (s.v) award 1 mk
    ±0.11 to ± 0.20cm3 of school value (s.v) award ½ mk
    If beyond ± 0.20cm3 award 0mk
  4. PA = Principles of Averaging ……1mk
    1. Candidates to average 3titres if they are within ± 0.1cm3 to one another i.e. maximum deviation between smallest and largest being 0.20cm3, if beyond this limit award 0mk
      Or
    2. Candidates to average 2 titres if only 2 titres are possible
    3. If a candidate averages 2 titres when three are possible award 0mk for averaging.
    4. Averaged titre to be recorded to 2d.p if it is recurring e.g. 26.333 to 26.33 not 26.3 or 26.35
      Round off or truncate allowed e.g 24.6666 to 24.67 or 24.66
  5. F.A – Final Answer ………………1mk
    This is correctly averaged titre compared to school value.
    Award marks as for accuracy.
    0.0 to ± 0.10 ½ mk
    0.11 to ± 0.20 ½ mk
    Summary of award of marks to be written beside the table
    N/B
    School value is obtained by the teacher performing the experiment and calculating the average titre.

CALCULATIONS PROCEDURE 1

  1. Molarity of solution A = 6.95 x 1000
                                         278      250
    = 0.1 molar ½ mk
    No. of moles = 0.1 x 25 = 0.0025moles ½ mk
                              1000
  2. Mole ration A:B is 5 : 1 ½ mk
    So moles of B = 1/5moles of A
    = 1/5 x 0.0025 = 0.0005moles ½ mk
  3.  Av. Titre had 0.005mol.
    Therefore 100cm3 of solution B has = 1000 x 0.005
                                                                  Average titre      ½ mk
    e.g. 1000 x 0.0005 = 0.0179
                   28                   NB: use candidate’s titre value in part (i) above

PROCEDURE II
TABLE II 5mks
Award marks as shown in table I
Calculations Procedure II

  1. 1000cm3 of solution B has
    Therefore Average titre has
    = Av, Titre table II x (ans. in c above) ½ mk
                        1000
    = ans ½ mk
  2. Mole ration B: C is 2:5
    No. of mole of C = 5/2 moles of B ……………… ½ mk
    = 5/2 x ans. in (a) above.
    e.g. 5/2 x 0.00429 = 0.001073 ½ mk
  3. 25cm3 of solution. C has (ans in (b) above)
    1000cm3 of solution C. has 1000 x ans. (b) above ½ mk
                                                             25
    = ans.
    e.g. 24 x 0.0179 = 0.000429
                 1000                     ½ mk
    Grand total marks for Q1 = 17mks

Question 2
Table III

Volume of water in

 boiling tube cm3

Temp. at which crystals of F

 first appear 0C

Solubility of solid F

(g/100g of water)

CT2

D1

AC1/2

Td1/2

15

80

26.5

20

71

20

25

65

16

30

52

13.3

35

48

11.4

40

45

10

Notes on marking table
CT =Complete table …………….2mk (temp. column only)

2mks for all temp. values filled, 1 ½ mkfor only 4 or 5 entries made, 1mk for 3 entries and 0mk for only 2 or 1 values entered
Penalize ½ mk for all temp. readings above 84.50 and below 100C to a maximum of 1mk

D.P = Decimal point ………1mk
All temp. values to be recorded as whole numbers OR with a decimal as 0 or 0.5 only if any other figure is used award 0 mk for d.p
Accuracy ……… ½ mk
Compare the candidates temp. reading at volume 15cm3 with that of the teacher.
If ±20C from the school value award ½ mk
Trend ………………. ½ mk
Award ½ mk for continuous temp. drop, otherwise penalize fully.
Solubility column …………….. 2mks
For 6 correct entries ……………………2mks
For 4 or 5 correct entries …………….. 1 ½ mks
For 3 correct entries …………………. 1mk
Below 3 entries ………………………0mk

  1. Graph …………………3mks
    Labelled axis ……………… ½ mk
    Axis to be labeled with quantities and unit i.e. Temp in 0C and solubility ( g / 100g of water).
    Scale ………………. ½ mk
    Plot to cover 2/3 of the given grid.
    Plots …………..1mk
    To be plotted accurately
    Curve ……………….1mk
    Plots to be joined to give a smooth curve increasing with increase in temp. award 1mk smooth curve passing through any experimental values one MUST be through 26.5g/100g of water.
  2. Showing on graph at solubility 15g/100g of water (only from a correct curve) …….½ mk correct reading…….1/2mark
    Total marks for Question 2 = 10mks
    Question 3. (13mks)

(a)

Observation

Inference

  • Colourless liquid forms on cooler sides of tests tube
  • Colourless gas produced which relights a glowing splint
  • Brown gas with a pungent smell produced

Each ½ mk to a max. of 1mk

  • Solid D is hydrated  ½ mk
  • NO3- ions present ½ mk

(b)

Observation

Inference

(i)   White ppt  ½ mk
Soluble in excess  ½ mk

 

AL3+, Zn2+, or Pb2+ present

  • 1mk for 3 cations
  • ½ mk for 2 cations
  • 0mk for 1 cation

(ii)White ppt   ½ mk
Insoluble in excess  ½ mk

 Al3+ ½ mk or Pb2+  ½ mk  present

(iii)No white ppt formed  1mk`

 Al3+,  present  1mk

Or

Pb2+  absent  ½ mk

(iv) Burns with a blue smokeless flame  ½ mk

Saturated organic compound
Or
Hydrocarbon with low C:H ratio {any point ½ mk}
or
hydrocarbon kjhauygda

(v)Purple KMnO4 turns colourless 1mk

R-OH     ½ mk
or 
roh uagda


(vi) Orange K2Cr2O7 turns green   1mk

R – OH   present   1mk

 

Confidential 

Each candidate will require

  1. Solution A 100cm3
  2. Solution B 150 cm3
  3. Solution C 100cm3
  4. Burette
  5. 25ml pipette
  6. 2 conical flasks
  7. Retort stand
  8. Filter funnel
  9. Pipette filler
  10. 100ml of distilled water
  11. Thermometer
  12. 1 Spatula – full of solid D
  13. About 10ml liquid E
  14. 4g solid F ( weighed exactly)
  15. 2 boiling tubes
  16. Six test tubes in a rack
  17. Test tube holder
  18. Metallic spatula

ACCESS TO

  • 1M NaOH
  • 1M NH4OH
  • 0.1M NaCl
    Acidified K2Cr2O7
  • KMnO4- use some amount of solution B
  • Source of heating
  • NOTES
  1. Solid A
    Dissolve 7.0g Ferrous Sulphate (FeSO4.7H2O) in 50ml of 1MH2SO4, dilute to 1dm3 with water. (Should be prepared in the morning of the exam day)
  2. Solution B
    Dissolve 0.8g of KMnO4 in 50cm3 of 1MH2SO4. Dilute to 1 dm3 with water.
  3. Solution C
    Measure 3cm3 of 20vol. H2O2
    Dissolve in 1dm3 of solution.
  4. 1M H2SO4
    Measure 55cm3 of conc. H2SO4 add to about 200cm3 of water, stir, dilute to 1 dm3
  5. Solid D
    Aluminium Nitrate
  6. Solid F
    Potassium Chlorate (KClO3)
  7. Acidified K2Cr2O7
    Dissolve 0.3g of K2Cr2O7 in 50cm3 of 1MH2SO4. Dilute to 1 dm3 with water.
  8. Liquid E
    Ethanol.

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