Chemistry Paper 3 Questions and Answers - KCSE 2022 Prediction

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

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)

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) + 5O^2-_(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

(6marks)

1. On the grid provided plot a graph of solubility of solid F (vertical axis) against temperature (horizontal axis). (3marks)
   
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 1^st portion, add 2M sodium hydroxide solution drop wise until in excess.
         

         Observations	inferences
         (1mark)	(1mark)

      2. To the 2^nd 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

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

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)

(b)

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.