Chemistry Paper 3 Questions and Answers - Moi Kabarak High School Mock 2020/2021

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Instructions to candidates

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

QUESTION 1
You are provided with the following

  • Metal P, Magnesium ribbon.
  • Solution Q, Hydrochloric acid.
  • Solution R, 0.50M sodium hydroxide,

Procedure I.

  1. Place five test tubes in a test tube rack and label them as 1, 2, 3, 4 and 5.
  2. Using a measuring cylinder, measure out volumes of hydrochloric acid as shown in the table and pour into the test tubes.
  3. Using a measuring cylinder add 1 cm3, 2 cm3, 3 cm3 and 4cm3 into test tube 2, 3, 4, and 5 respectively.
  4. Cut out five pieces of 1 cm long of metal P.
  5. Place one piece of magnesium into test-tube one and start a stopwatch immediately.
  6. Record the time taken for the metal to react completely in the table below.
  7. Do not pour the solutions in the test-tubes away for use in procedure II.
    Table I
    1.  Test tube  1   2   3   4   5 
       Volume of solution Q  10   9  8  7  6
       Volume of water added   0  1  2  3  4
       Time taken (sec)          
       Rate of reaction 1/t (sec−1)          
      (7 marks)
    2. Plot a graph of rate of reaction against volume of solution Q used. (3 marks)
    3. From the graph, determine the time taken for 1 cm long magnesium ribbon to react completely if volume of acid used is 7.5 cm3.    (1 mark)
    4. In terms of rate, explain the shape of the graph.     (1 mark)

Procedure II

  1. Transfer all the contents in the five test-tubes into a volumetric flask. Rinse the test tubes and transfer the contents to the 250ml flask. Add water up to the 250ml mark. Label the solution T.
  2. Pipette 25cm3 of solution T. Add 2-3 drops of phenolphthalein indicator. Fill the burette with solution T. Titrate and record your results in the table below.
    Table II
       I   II    III 
     Final burette reading (cm3)      
     Initial burette reading (cm3      
     Volume of solution T used (cm3)      
    (4 marks)
    1. Calculate the average volume of solution R used.   (1 mark)
    2. Calculate the number of moles of solution R used.   (1 mark)
    3. Calculate the number of moles of solution T used.      (1 mark)
    4. Calculate molarity of solution T.      (1 mark)
    5. Calculate the mass of magnesium ribbon used in the experiment.     (1 mark)

QUESTION 2
You are provided with solid D. Carry out the following tests on D and record your observations and inferences in the spaces provided.

  1.  
    1. Place a spatula full of solid D into a boiling tube and add about 10cm3 of distilled water. Shake the mixture thoroughly. Filter the mixture and divide the filtrate into four portions. Keep the residue for use in part (b).
        Observations      Inferences 
                    (½mark)          (½mark)
    2. To the first portion, add sodium hydroxide solution dropwise until in excess.
        Observations      Inferences 
                    (½mark)          (½mark)
    3. To the second, add Ammonia solution dropwise until in excess.
        Observations      Inferences 
                    (½mark)          (½mark)
    4. To the third portion add a few drops of Lead (II) nitrate solution.
        Observations      Inferences 
                    (½mark)          (½mark)
    5. To the fourth portion add drops of barium Nitrate solution followed by drops of nitrate acid solution.
        Observations      Inferences 
                    (½mark)          (1 mark)
  2. Place the residue in a boiling tube and add dilute nitric (V) acid little by little until all the solid dissolve. Divide the solution into two parts.
      Observations      Inferences 
                  (½mark)          (½mark)

    1. To the first portion, add sodium hydroxide dropwise until in excess.
        Observations      Inferences 
                    (1 mark)          (1 mark)
    2. To the second portion add ammonia solution dropwise until in excess.
        Observations      Inferences 
                    (1 mark)          (1 mark)

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

  1. Scoop a little of solid A with a clean metallic spatula and place it a non-luminous flame.
      Observations      Inferences 
                  (1 mark)          (1 mark)
  2. Add about 10cm3 of distilled water to the remaining solid Z in a boiling tube. Divide the resulting solution into 4 portions.
    1. To the first portion add 3 drops of acidified K2Cr2O7 solution and warm.
        Observations      Inferences 
                    (1 mark)          (1 mark)
    2. To the second portion, add 3 drops of bromine water and warm.
        Observations      Inferences 
                    (1 mark)          (1 mark)
    3. To the third portion, add 2 - 3 drops of universal indicator and determine the pH of the solution.
        Observations      Inferences 
                    (1 mark)          (1 mark)


Marking Scheme

QUESTION 1
You are provided with the following

  • Metal P, Magnesium ribbon.
  • Solution Q, Hydrochloric acid.
  • Solution R, 0.50M sodium hydroxide,

Procedure I.

  1. Place five test tubes in a test tube rack and label them as 1, 2, 3, 4 and 5.
  2. Using a measuring cylinder, measure out volumes of hydrochloric acid as shown in the table and pour into the test tubes.
  3. Using a measuring cylinder add 1 cm3, 2 cm3, 3 cm3 and 4cm3 into test tube 2, 3, 4, and 5 respectively.
  4. Cut out five pieces of 1 cm long of metal P.
  5. Place one piece of magnesium into test-tube one and start a stopwatch immediately.
  6. Record the time taken for the metal to react completely in the table below.
  7. Do not pour the solutions in the test-tubes away for use in procedure II.
    Table I
    1.  Test tube  1   2   3   4   5 
       Volume of solution Q  10   9  8  7  6
       Volume of water added   0  1  2  3  4
       Time taken (sec)  52.70   150.03   165.70   254.90   420.5 
       Rate of reaction 1/t (sec−1)  0.01923   0.006667   0.006061   0.003937   0.002381 
      (7 marks)
    2. Plot a graph of rate of reaction against volume of solution Q used. (3 marks)
      graph on rate of reaction against volume of solution Q
    3. From the graph, determine the time taken for 1 cm long magnesium ribbon to react completely if volume of acid used is 7.5 cm3.    (1 mark)
      1/t =0.56 × 10−2
         t=178.57seconds
    4. In terms of rate, explain the shape of the graph.     (1 mark)
      • An increase in the volume of acid leads to a consequent incraese in the rate of reaction  (shape must be curved directly propotional to concentration.

Procedure II

  1. Transfer all the contents in the five test-tubes into a volumetric flask. Rinse the test tubes and transfer the contents to the 250ml flask. Add water up to the 250ml mark. Label the solution T.
  2. Pipette 25cm3 of solution T. Add 2-3 drops of phenolphthalein indicator. Fill the burette with solution T. Titrate and record your results in the table below.
    Table II
       I   II    III 
     Final burette reading (cm3)  15.5   15.6   15.5 
     Initial burette reading (cm3  0.0  0.0  0.0
     Volume of solution T used (cm3)  15.5  15.6  15.5
    16 ± 5                                                      (4 marks)
    1. Calculate the average volume of solution R used.   (1 mark)
      15.5 +15.5 =15.5cm3
             2
    2. Calculate the number of moles of solution R used.   (1 mark)
      0.5moles→1000cm3
         ?        →15.5cm3
      15.5 × 0.5
          1000
        =0.00775moles
    3. Calculate the number of moles of solution T used.      (1 mark)
      MgCl2:NaOH
             1:1
      0.00775  =0.003875moles
            2
    4. Calculate molarity of solution T.      (1 mark)
      0.003875 → 2.5cm3
            ?         1000cm3
      1000 × 0.003875
                 2.5
        =0.155M
    5. Calculate the mass of magnesium ribbon used in the experiment.     (1 mark)

QUESTION 2
You are provided with solid D. Carry out the following tests on D and record your observations and inferences in the spaces provided.

  1.  
    1. Place a spatula full of solid D into a boiling tube and add about 10cm3 of distilled water. Shake the mixture thoroughly. Filter the mixture and divide the filtrate into four portions. Keep the residue for use in part (b).
        Observations      Inferences 
      Colourless filtrate
      Black              (1mark)
      Fe2+,Cu2+,Fe3+ absent
      Cu2+        (1mark)
    2. To the first portion, add sodium hydroxide solution dropwise until in excess.
        Observations      Inferences 
      White precipitate,soluble in excess (1mark) Zn2+,Al3+,Pb2+         (1mark)
    3. To the second, add Ammonia solution dropwise until in excess.
        Observations      Inferences 
      White precipitate,soluble in excess (1mark)   Zn2+       (½mark)
    4. To the third portion add a few drops of Lead (II) nitrate solution.
        Observations      Inferences 
      White precipitate   (½mark) SO42−, SO32−, Cl− ,Br       (1mark)
    5. To the fourth portion add drops of barium Nitrate solution followed by drops of nitrate acid solution.
        Observations      Inferences 
      White PPT Insoluble  (1 mark) SO42−         (½ mark)
  2. Place the residue in a boiling tube and add dilute nitric (V) acid little by little until all the solid dissolve. Divide the solution into two parts.
      Observations      Inferences 
    Black and dissolves forming blue solution      (½mark) Cu2+         (½mark)

    1. To the first portion, add sodium hydroxide dropwise until in excess.
        Observations      Inferences 
      Blue PPT insoluble in excess              (1 mark) Cu2+         (½ mark)
    2. To the second portion add ammonia solution dropwise until in excess.
        Observations      Inferences 
      Blue PPT dissolves to form deep blue solution      (1 mark) Cu2+         (½ mark)

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

  1. Scoop a little of solid A with a clean metallic spatula and place it a non-luminous flame.
      Observations      Inferences 
    Burna yellow flame            (1 mark) dehydrocarbon
    Long chain hydrocarbon         (1 mark)
  2. Add about 10cm3 of distilled water to the remaining solid Z in a boiling tube. Divide the resulting solution into 4 portions.
    1. To the first portion add 3 drops of acidified K2Cr2O7 solution and warm.
        Observations      Inferences 
      Orange persists              (1 mark) R-OH absent         (1 mark)
    2. To the second portion, add 3 drops of bromine water and warm.
        Observations      Inferences 
      yellow bromine
      Yellow persist             (1 mark)
       dehydrocarbon
        Absent       (1 mark)
    3. To the third portion, add 2 - 3 drops of universal indicator and determine the pH of the solution.
        Observations      Inferences 
       1, 2, 3              (1 mark) Strongly acidic         (1 mark)

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