Chemistry Paper 3 Questions and Answers with Confidential - Form 4 End Term 2 2021

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CHEMISTRY
PAPER 3
TIME: 2  HOURS

INSTRUCTIONS

  • Answer ALL the questions 
  1. You are provided with:
    • Solution A, Dilute hydrochloric acid
    • Solution B, made by dissolving 0.5g of sodium hydroxide in water and made to 250cm3 of solution
    • Solid C, Magnesium ribbon
    • Phenolphthalein in indicator

      You are required to:

      1. Standardize solution A
      2. Determine the rate of reaction between solution A and magnesium
    • PROCEDURE
      1. Measure exactly 10cm3 of solution A using a burette and transfer into a 250ml volumetric flask. Top up to the mark using distilled water. Label this solution D.
      2. Drain the remaining solution A in the burette, rinse the burette thoroughly and fill the burette with solution D.
      3. Pipette 25cm3 of solution B into a conical flask. Add three drops of phenolphthalein indicator
      4. Titrate solution D with solution B. Record your results in the table below. Repeat procedure (i) to (iv) to complete the table. (3 marks)
          1 2 3
        Final burette reading (cm3)      
        Initial burette reading (cm3      
        Volume of solution D used (cm3)      

        1. Calculate the average volume of solution D used (1 mark)
        2. Calculate:
          1. Number of moles of solution B used (1½ marks)
          2. Number of moles of solution D in 250cm3 of solution (1½ marks)
          3. Morality of solution A (1 mark)
    • PROCEDURE II
      1. Cut solid C into equal pieces, each 2cm long
      2. Using a burette, measure 12cm3 of solution A, into a clean boiling tube.
      3. Drop one piece of solid C into the boiling tube containing solution A and start stopwatch immediately. Stop the stopwatch when all solid C has just reacted. Record your results in the table below.
      4. Repeat steps (ii) and (iii) above using 10cm3, 8cm3, 6cm3 and 4cm3 of solution A. Top up each with distilled water to make 12cm3 of solution and complete the table below. (4 marks)
        Volume of solution A (cm3) Volume of distilled water (cm3) Concentration of solution a (moles/l) Time(s) I/t (s-1)
                 
                 
                 
                 

        1. Plot a graph of I/t (y-axis) against the concentration of solution A (3 marks)
        2. From the graph, determine the time taken for the reaction to reach completion when 1.5 moles of solution A are used (2 marks)
        3. Comment on the shape of the graph (1 mark)
  2. You are provided with solid Q. Carry out the tests below and record your observations and inferences in the spaces provided.
    1. Strongly heat a spatula-end full of solid Q in a dry test tube (1 mark)
      Observation Inference
         
    2.    
      1. Place the remaining solid Q in a boiling tube. Add 10cm3 of distilled water. Divide the solution into five portions. (2 marks)
        Observation Inference
           
      2. To the first portion, add aqueous lead (II) nitrate solution (1 mark)
        Observation Inference
           
      3. To the second portion add dilute nitric (V) acid, followed by barium nitrate solution (2marks)
        Observation Inference
           
      4. To the third portion add a few drops of sodium hydroxide until excess observation (2marks)
        Observation Inference
           
      5. To the fourth portion, add a few drops of aqueous ammonia until is excess. (2 marks)
        Observation Inference
           
      6. To the fifth portion, add a few drops of hydrochloric acid (1½ marks)
        Observation Inference
           
  3. You are provided with solid R. carry out the tests below and record your observations and inferences.
    1. Place a spatula-end full of solid R in a dry boiling tube and add about 10cm3 of distilled water. Shake thoroughly and heat to boil. Divide the solution into five portions. (1½ marks)
      Observation Inference
         
    2.    
      1. Test the first portion with the universal indicator solution provided. (1½ marks)
        Observation Inference
           
      2. To the second portion, add a few drops of acidified potassium manganite (VII) solution(2 marks)
        Observation Inference
           
      3. To the third portion, add a few drops of bromine water (2 marks)
        Observation Inference
           
      4. To the fourth portion, add half spatula of sodium hydrogen carbonate (1 mark)
        Observation Inference
           
      5. To the fifth portion in a boiling tube, add 5cm3 of ethanol followed by a few drops of concentrated sulphuric (VI) acid. Warm the mixture. (1 ½ Marks)
        Observation Inference
           

CONFIDENTIAL

Each candidate requires

  1. Solution A, 60cm3 of 2mHcl.
  2. Solution B, 100cm3 of 0.05MNaOH
  3. Solid C, 10cm magnesium ribbon.
  4. 10ml measuring cylinder
  5. 25ml pipette
  6. 50ml Burette
  7. Complete stand
  8. Stopwatch
  9. 2 labels
  10. Distilled water
  11. 6 test tubes.
  12. 0.5g sodium hydrogen carbonate.
  13. 5cm3 ethanol.
  14. 1 – 14 PH chart.
  15. Solid R, 1g Oxalic acid.
  16. Solid Q, Mixture of (NH4)2SO4 and Al(SO4)3 (ration 1:1)
  17. Pipette filler
  18. Phenolphthalein indicator
  19. 250ml conical flask (2)
  20. 250ml volumetric flask
  21. 1 boiling tube
  22. 1 spatula

Access to:

  1. Universal indicator solution
  2. Acidified potassium manganite (VII) solutio
  3. Bromine water
  4. Conc. Sulphuric (VII) acid with a dropper.
  5. Means of heating.
  6. 2M Lead (II) nitrate solution.
  7. 2M dilute nitric (V) acid solution
  8. 0.5m Barium nitrate solution
  9. 2M sodium hydroxide solution.
  10. 2M Aqueous ammonia.

MARKING SCHEME

  1 2 3
Final burette reading (cm3) 12.5 25.0 12.5
Initial burette reading (cm3 0.2 12.5  0.0
Volume of solution D used (cm3) 12.5 12.5 12.5

Marks distributed as follows:

  1. Complete table (1mk)
    • 3 tiration done (1 mk)
    • Incomplete table with 2 titration (½ mk)
    • Incomplete table with one titration done (0 mk)

      Penalties

    • Wrong arithmetic
    • Inverted table
    • Unrealistic title values (unless explained)
    • Penalize ( ½ mk) for each to a miximum of (½ mk)
  2. Decimal place (1mk)
    • Accept only 1 or 2 d.p used consistently, otherwise penalize fully.
    • Accept inconsistency in the use of zeros as initial burette reading e.g. 0.0, 0.00 or 0.000.
      NB decimal place tied to 1st and 2nd rows only.
  3. Accuracy (1 mk)
    • Compare candidate’s title value with school value S.V. if one value within ± 0.1 of S.V (1mk)
    • No value within ± 0.1 of S.V butat least 1 value within ± 0.2 or S.V (0 mk)
  4. Averaging (1 mk)
    • Values averaged must be sown
    • If 3 consistent titrations done and averaged = (1 mk)
    • If 3 titration done, but only 2 are consistent and averaged = (1 mk)
    • If only 2 titrations done, are consistent and averaged = (1 mk)
    • Otherwise penalize fully

CALCULATIONS

  1. 12.5+12.5+12.5 (1/2 mk)=12.5cm3 (1/2 mk)
           3
  2.    
    1. Moles in 250cm3 = 0.5/40=0.0125 moles (1/2 mk)
      Moles used =(0.0125 ×25)/250 (1/2 Mk)=0.00125 moles
    2. Moles of acid reacting = 0.00125(1/2 mk) mole ratio 1:1
      250 → ?
      (0.0125 ×25)/250 (1/2 mk)=moles (1/2 mk)
    3. Molarity of solution A
      10cm3 = 0.25 moles
      1000 =?
      (1000 ×0.25)/10 (1/2 mk)=25 moles(1/2 Mk)
    • PROCEDURE II
      1. Cut solid C into equal pieces, each 2cm long
      2. Using a burette, measure 12cm3 of solution A, into a clean boiling tube.
      3. Drop one piece of solid C into the boiling tube containing solution A and start stopwatch immediately. Stop the stopwatch when all solid C has just reacted. Record your results in the table below.
      4. Repeat steps (ii) and (iii) above using 10cm3, 8cm3, 6cm3 and 4cm3 of solution A. Top up each with distilled water to make 12cm3 of solution and complete the table below. (4 marks)
        Volume of solution A (cm3) Volume of distilled water (cm3) Concentration of solution a (moles/l) Time(s) I/t (s-1)
         12  0  2.5  25.5  0.0363
         10  2  2.08  34.25  0.0292
         8  4  1.67  45.45  0.0220
         6  6  1.25  69.44  0.0144
        4 8 0.83 120.01 0.0083

        1. Plot a graph of I/t (y-axis) against the concentration of solution A (3 marks)
        2. From the graph, determine the time taken for the reaction to reach completion when 1.5 moles of solution A are used (2 marks)
        3. Comment on the shape of the graph (1 mark)
  1. You are provided with solid Q. Carry out the tests below and record your observations and inferences in the spaces provided.
    1. Strongly heat a spatula-end full of solid Q in a dry test tube (1 mark)
      Observation Inference

      Colourless liquid condenses at coller parts of test tube
      Gas evolved turns red litmus blue
      A white residue remain

      Hydrated salt
      NH+4 ions

    2.    
      1. Place the remaining solid Q in a boiling tube. Add 10cm3 of distilled water. Divide the solution into five portions. (2 marks)
        Observation Inference

        Solid dissolves (½ mk) to form a colourless 
        solution

        Mg2+Al3+,Zn2+ Present

        Any One
        Or Cu2+, Fe2+, Fe3+ absent

      2. To the first portion, add aqueous lead (II) nitrate solution (1 mark)
        Observation Inference
        A white precipitate form ( ½ mk)

        CO2-3,SO-24.Cl-,SO2-3 present
        Any two ions award ( ½ mk)
        Penalize the ( ½ mk) for any contradictory ion.

      3. To the second portion add dilute nitric (V) acid, followed by barium nitrate solution (2marks)
        Observation Inference
        white precipitate is formed ( ½ mk) SO2-4 ions confirmed ( ½ mk)
      4. To the third portion add a few drops of sodium hydroxide until excess observation (2marks)
        Observation Inference

        White precipitate ( ½ mk) dissolves in excess
        to form a colourless solution ( ½ mk)

        Zn2+, Pb2+ or Al3+
        Any 3 ions – (1mk)
        2 ions ( ½ mk)
        0 mk for any only one ion

      5. To the fourth portion, add a few drops of aqueous ammonia until is excess. (2 marks)
        Observation Inference
        White precipitate ( ½ mk) in excess

        Pb2+, AI3+ present
        Both ions (1mk)
        One ion ( ½ mk)

      6. To the fifth portion, add a few drops of hydrochloric acid (1½ marks)
        Observation Inference
        No white precipitate formed (1 mk) Pb2+ absent or Al3+ present (½ mk)
  2. You are provided with solid R. carry out the tests below and record your observations and inferences.
    1. Place a spatula-end full of solid R in a dry boiling tube and add about 10cm3 of distilled water. Shake thoroughly and heat to boil. Divide the solution into five portions. (1½ marks)
      Observation Inference
      Dissolves form (½mk) a colourless homogenous solution (½ mk) Polar substance ( ½ mk)
    2.    
      1. Test the first portion with the universal indicator solution provided. (1½ marks)
        Observation Inference
        PH value 1 – 3 ( ½ mk) Strong acid present ( ½ mk)
      2. To the second portion, add a few drops of acidified potassium manganite (VII) solution(2 marks)
        Observation Inference

        Purple acidified
        KMnO4 decolorised (1 mk)


         chemerp3qa3bii
        OR –OH present
      3. To the third portion, add a few drops of bromine water (2 marks)
        Observation Inference
        Bromine water decolourised (1 mk)  
        chemerp3qa3biii

        OR –Oh. Present
        All three (1mk)
        Two only (½ mk)

      4. To the fourth portion, add half spatula of sodium hydrogen carbonate (1 mark)
        Observation Inference
        Effervescence ( ½ mk)  
        chemerp3qa3biv
        Present ( ½ mk)
      5. To the fifth portion in a boiling tube, add 5cm3 of ethanol followed by a few drops of concentrated sulphuric (VI) acid. Warm the mixture. (1 ½ Marks)
        Observation Inference

        Sweet smelling
        Compound formed (1 mk)

         
        chemerp3qa3bv
        Present ( ½ mk)

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