Chemistry Paper 3 Questions and Answers - Butere Mock Exams 2021

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You are provided with:

  • 4.5g of solid A in a boiling tube.
  • Solution B, 0.06M acidified potassium manganate (VII)

You are required to determine:-

  1. The solubility of solid A at different temperatures.
  2. The number of moles of water of crystallization in solid A.


  1. Using a burette, add 4cm3 of distilled water to solid A in a boiling tube. Heat the mixture while stirring with the thermometer to about 70ºC. When all the solid has dissolved allow the solution to cool while stirring with the thermometer. Note the temperature at which crystals of solid A first appear. Record this temperature in table I.
  2. Using the burette, add 2.0cm3 of distilled water to the contents of the boiling tube. Warm the mixture while stirring with the thermometer until all solid dissolves. Allow the mixture to cool while stirring. Note and record the temperature at which crystals of solid A first appear.
  3. Repeat procedure (b) two more times and record the temperatures in the table I. Retain the contents of the boiling tube for use in procedure (e)
    1. Complete table I by calculating the solubility of solid A at different temperatures. The solubility of a substance is the mass of that substance that dissolves in 100cm3 (100g) of water at a particular temperature.
      Table I (6mks)
      Volume of water in the boiling tube (cm3)

      Temperature at which crystals of solid A first appear

      Solubility of solid A (g/100g water)
      =   100 x mass of solid  
        Volume (mass) of water 
    2. On the grid provided, plot a graph of solubility of solid A against temperature. (3 marks)
    3. Using your graph
      1. Determine the temperature at which 100g of solid A would dissolve in 100cm3 of water. (1mark)
      2. Calculate the mass of solid A that will crystallize out when a hot solution at 55ºC cooled to 45ºC. (1 mark)
    1. Transfer the contents of the boiling tube into 250ml volumetric flask. Rinse both the boiling tube and the thermometer with distilled water and add to the volumetric flask. Add more distilled water to make up to the mark. Label this solution A. Fill burette with solution B. Using a pipette and pipette filler, place 25.0cm3 of solution A into a conical flask. Warm the mixture to about 60ºC. Titrate the hot solution A with solution B until a permanent pink colour persists. Record your readings in table 2. Repeat the titration two more times and complete table 2.

Table 2

Final Burette reading      
Initial Burette reading      
Volume of solution B used (cm3)      

(4 marks )


  1. Average volume of solution B used. (1 mark )
  2. Number of moles of potassium manganate (VII) used. (1 mark )
  3. Number of moles of A in 25.0cm3 of solution A given that 2 moles of potassium manganate (VII) react completely with 5 moles of A. (1mark)
  4. Relative formula mass of A. (2marks)
  5. The formula of A has the form H2C2O4. xH2O. Determine the value of x in the formula (C=12, H=1, O=16) (2marks)

You are provided with solid, C carry out the following tests and write your observations and inferences in the spaces provided.

  1. Place all of solid C in boiling tube. Add about 10 cm3 of distilled water and shake thoroughly. Filter the mixture into another boiling tube. Retain the filtrate for use in test 2 (b) below. Dry the residue using pieces of filter papers.
  2. Transfer about half of the dry residue into a dry test tube. Heat the residue strongly and test any gas produced using a burning splint
    Observation Inference
    (1mark)  (1mark)
  3. Place the rest of the residue in a dry test tube. Add 3cm3 of 2M Hydrochloric acid. Retain the mixture for test (iii) below.
    Observation Inference
    (1mark)  (1mark)
  4. To 2 cm3 of the solution obtained in (ii) above, add few drops of aqueous ammonia until in excess.
    Observation Inference
    (1mark)  (1mark)
  5. To 2 cm3 of the filtrate obtained in (a) above a few drops of aqueous ammonia until in excess
    Observation Inference
    (1mark)  (1mark)
  6. To 2cm3 of the filtrate, add three drops of 2M Hydrochloric acid
    Observation Inference
    (1mark)  (1mark)
  7. To 2cm3 of the filtrate add two drops of Barium nitrate solution
    Observation Inference
    (1mark)  (1mark)

You are provided with liquid K. Carry out the following tests and record your observations and inferences in the spaces provided.

  1. Place five drops of liquid K on a clean dry watch glass and ignite it.
    Observation Inference
    (1mark)  (1mark)
  2. Place about 2cm3 of K in a clean dry test tube and add sodium hydrogen carbonate provided
    Observation Inference
    (1mark)  (1mark)
  3. Place about 2cm3 of liquid K in a test tube, add acidified potassium dichromate (VI) and warm the mixture.
    Observation Inference
    (1mark)  (1mark)


QUESTION 1(22Marks)

Table 1 (6marks)

Volume of water in the boiling tube (cm3)

Temperature at which crystals of solid A first appear

Solubility of solid A (g/100g water)
=   100 x mass of solid  
  Volume (mass) of water 
4 66.0  112.5
58.0 75
52.0 56.25
10 45.0 45

Column 1 - 4 marks
Distributed as follows:

  1. Complete table 2 marks
    • Complete table with 4 readings 2 marks
    • Incomplete table with 3 readings 1 ½ marks
    • Incomplete table with 2 readings 1 mark
    • Incomplete table with 1 reading 0 mark
  2. Use of decimals 1 mark
    Accept unit if all readings are recorded consistently either as whole numbers or to
  3. 1 d. place of 0.0 or 0.5, otherwise penalize fully.
  4. Trend ½ mark
    • Award ½ mark for a continuous drop in temperature readings in column I, otherwise penalize fully.
      Column II 2 marks ( 4 x ½ mks)
    • Award ½ mark for each value of solubility correctly calculated, otherwise penalize fully.
    • Ignore units in grams if attached to correct answer, otherwise penalize if wrong units are attached.

Graph 3 marks
Distributed as follows:-

  1. Labelling of axes ½ mark
    • Penallise fully for any inversion of axis.
    • Penallise fully if wrong units are given or shown BUT ignore if not attached.
    • Penalise fully if only one axis is labeled.
  2. Scale ½ mark
    • Area covered in units should be at least ¾ of the total big square of the grid, given on both vertical and horizontal axis, otherwise penalize fully.
    • Scale intervals must be consistent, otherwise penalize fully.
    • Scale chosen must accommodate all plots, otherwise penalize fully.
  3. Plotting 1 mark
    • Award 1mark for 3 or 4 points correctly plotted.
    • If there are only 2 correctly plotted points, award ½ mark.
    • Accept plots even when axis is interchanged.
  4. Curve 1 mark
    • Award 1 mark for a smooth rising curve joining at least 3 correctly plotted points of which one must be at 112.5 / 4.0cm3 of water.
    • Reject a curve obtained from wrong calculated values in column II.
    1. 1 mark
      • Accept correct reading with or without showing on the graph.
      • If shown on the graph but reading is wrongly read, or absent award ½ mark for showing.
      • Penallise ½ mark for wrong units otherwise ignore if not shown.
      • Reject any reading and showing from a wrong graph e.g exchange of axis, wrong plotting at volume of 4.0cm3.
    2. 2 marks
      Solubility at 55ºC = correct reading  ½
      Solubility at 45ºC = correct reading
      Mass of crystals = difference in solubilities  ½

Table 2 - 5 marks

Final Burette reading 30.0 30.0 30.0
Initial Burette reading 0.0 0.0 0.0
Volume of solution B used (cm3) 30.0 30.0 30.0

Distributed as follows:-

  1. Complete table 1 mark
    1. Complete table with 3 titrations 1 mark
    2. Incomplete table with 2 titrations ½ mark
    3. Incomplete table with 1 titration 0 mark

      • Wrong arithmetic
      • Inverted tables.
      • Values beyond 50.0cm3 unless explained
      • Unrealistic values i.e values below 1.0cm3 and above hundreds
        NB: Penalize ½ mark each to a maximum of ½ mark (penalize once)
  2. Decimals 1 mark (Tied to 1st and 2nd rows only)
    • Accept 1 or 2 dp used consistently
    • Accept 2 d.p only if the 2nd place of decimal is “0” or “5”.
    • Allow inconsistency of zeros i.e 0.0, 0.00 or 0 in the initial values
      NB: Penalize fully if any of the conditions is not met.
  3. Accuracy 1 mark
    Compare any of the titre readings with school values (S.V) tick () the chosen value on the table.
    • If any value is within ± 0.1 1 mark
    • If any value is within ± 0.2 ½ mark
    • If not within ± 0.2 0 mark
      NB: If there is a wrong arithmetic or subtraction compare the S.V with the worked out correct value and award accordingly.
  4. Principles of averaging 1 mark
    Values averaged MUST be shown and must be within ± 0.2 of each other.
    • If 3 consistent values are averaged 1 mark
    • If 3 titrations alone, only 2 possible and averaged 1 mark
    • If 2 titrations alone, and are consistent and averaged 1 mark
      NB: Award 0 mark if averaging involves.
    • 3 consistent values but only 2 averaged
    • 3 inconsistent values are averaged.
    • 2 inconsistent values are averaged.
  5. Final answer 1 mark (Tied to correctly averaged titre )
    • If within ± 0.1 S.V 1 mark
    • If within ± 0.2 S.V ½ mark
    • If beyond ± 0.2 of S.V 0 marks


Moles of KMnO4 = 0.06 x titre
= correct answer 


  1. Penalize ½ mark for wrong transfer of titre, otherwise penalize fully for strange figure.
  2. 0.06 must be transferred intact otherwise penalize fully.
  3. Moles of A in 25.0cm3 = Ans in (II) x 5
    = correct ans 
  4. RFM of A 2 marks
    Moles in 250cm3 = an in III x 250 
    = correct ans
    RFM = 4.5 
    Ans above
    = Correct answer 
    Mass in 25cm3 = 0.45
    RFM =       0.45g        
              Moles in part III
    = Correct answer 
    Mass in 1000cm3= 4.5 x 4 = 18g
    Molarity of A = 1000 x ans III 
    RFM =     18     
    = correct ans 


  1. Penalize fully if 4.5 is not used intact
  2. Reject if RFM is less than 108 and greater than 162.
  3. Penalize ½ mark for any units used or attached to the final answer e.g g

V. Determining the value of X 2 marks
2 + 24 + 64 + x (2+16) = ans (IV) 
18x = ans (IV) – 90
x = ans (IV) – 90
= correct answer 
OR 90 +18x = ans (IV)
x = ans (IV) – 90 
= correct ans 
x = ans (IV) – 90 
= correct ans 1mk

Penalize ½ mark if units given or attached to final answer.
NB: For all calculations, any working beyond the expected answer penalize fully.


observations inferences
a(i)yellow residue when hot
White on cooling ½ mk
Burning splint extinguished ½ mk
Zn2+present ½ mk
CO32- present ½ mk
(ii) Bubbles of colourless gas  CO32- present 
(iii)white ppt soluble in excess  Zn2+present 
b(i)white ppt soluble in excess  Zn2+present 
(ii) No effervescence  CO32- SO32-- absent
white ppt  SO42-


observation inference
(a) Burns with a blue flame Saturated organic compound//low carbon to hydrogen ratio// double, triple bond absent
(b) No efferevescence //no fizzing RCOOH absent 
(c) Orange potassium dichromate(VI) turns green R-OH present
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