CHEMISTRY PAPER 3 - 2020 KCSE PREDICTION SET 1 (QUESTIONS AND ANSWERS)

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INSTRUCTIONS TO CANDIDATES

  1. Answer ALL questions in this question paper
  2. Mathematical tables and electronic calculators may be used for calculations.
  3. All working MUST be clearly shown where necessary.
  1. You are provided with
    - 2.0m NaOH solution labelled B
    - H2 SOacid solution labelled A `
    You are to:
    1. Prepare a dilute solution of NaoH solution.                                                                                   
    2. Determine the concentration of H2SO4in moles per litre.

    PROCEDURE I

    1. Using a pipette 25.0cm3 of solution B and place it into 250cm3 volumetric flask.
    2. Add about 200cm3 of distilled water and shake well.
    3. Add more water to make up to 250cm3mark. Label this solution C.
      1. Calculate the concentration of the dilute solution C in moles per litres [2mks]

    PROCEDURE II

    1. Fill the burette with solution A and record the readings in the table below.
    2. Pipette 25cm3 of dilute solution C and place it into 250ml conical flask.
    3. Add 2 - 3 drops of phenolphthalein indicator.
    4. Titrate with solution A.
    5. Record your results in the table below.
    6. Repeat the titration two or more times and complete the table.   [4mks]

      Titration no

      I

      II

      III

      Final burette reading (cm3)

           

      Initial burette reading(cm3)

           

      Volume of solution A(cm3)

           
      1. Determine average volume of the acid (solution A) used.  [1mk]
      2. Determine moles of dilute solution C in the volume used.     [2mks]
      3. Write an equation for the reaction between NaoH and H2SO4 acid.    [2mks]
      4. Determine the number of moles of A (H2SO4) used.      [2mks]
      5. Determine the concentration of A (H2SO4) in moles per litre.      [2mks]
  1. You are provided with the following
    - 2M sodium hydroxide solution
    - 2M hydrochloric acid
    You are required to determine the molar enthalpy of neutralization of the acid using sodium hydroxide.

    PROCEDURE

    1. Measure out 20cm3 of acid into a clean plastic beaker.
    2. Record the temperature of this solution in the table below
    3. Measure 5cmof sodium hydroxide and add it to the hydrochloric acid.
    4. Stir with the thermometer and record the maximum temperature reached.
    5. Repeat the above procedure adding 5cmportions of sodium hydroxide until the total volume of the solution is 50cm3. [3mks]
       Volume of acid(cm3) 20  20   20 20  20 20  20 
       Volume of NaOH added cm3  0  5  10  15  20  25  30
       Temperature(oC)of solution              
      You are required to:
      1. Plot a graph of temperature rise against sodium hydroxide added.                                 [4mks]
      2. From your graph
        1. Determine the expected temperature rise.       [2mks]
        2. Calculate the molar enthalpy of neutralization for this reaction. (C = 4.2J/g/oC) assume density of solution is 1gcm-3)
        3. The theoretical molar heat of neutralization is -57.2kj/mol-1.Compare your value in [ii] above with the theoretical value. Give the reasons for any differences noted between these two values. [2mks]
  1. You are provided with solid N carry out the tests below and record your observations and inferences.
    1. Place a spatular of N in a test tube and add 5cm3 of water and shake well divide the solution in to three portions.       [1mk]

      OBSERVATION

      INFERENCE

         
    2. Addsodium hydroxide to the first portion drop wise while observing till in excess.      [2mks]

      OBSERVATION

      INFERENCE

         
    3. Add ammonia solution to the second portion drop wise until in excess.   [2mks]

      OBSERVATION

      INFERENCE

         
    4. Add four drops of potassium iodide solution to the third portion.        [2mks]

      OBSERVATION

      INFERENCE

         
    5. Put spatular of solid P in a test tube and add 5cm3 of water and shake well. Add three drops of acid bariumnitrate followed by 5 drops of nitric acid.   [4mks]

      OBSERVATION

      INFERENCE

         


MARKING SCHEME

  1. PROCEDURE I
    1.  2 moles →1000cm3     2×25= 0.05 moles
      ?           →   25cm3       1000
      0.05 moles → 250cm3         1000×0.05 = 0.2M
                       →1000cm3            250

    PROCEDURE II
    Complete table  1
    Decimal            1
    Arithmetic         1
    Accuracy           1 ± 0.2
    1. = 12.5cm3
    2. 0.2 moles → 1000cm3 = 0.005 moles
           ?        →  25cm3
    3. 2NaoH [aq]+H2SO4[aq]→NaSO4[aq]+2H2O[l]
    4. Mole ratio
      A:C                         C=0.0025moles
      1:2                          A= =0.0025 moles
    5. 0.0025 → answer b           1000×0.0025 =_____
                 →   1000                answer b
  1. Table
    Complete table 1
    Decimal point    1
    Trend                1
    1. Graph
      chemp3graph
      Axis        - Mk each
      Scale- Mk each
      Plotting
      Curve
    2.  
      1. answer from the graph
      2. Use MCΔθ
        Mass=50 1=50g.
        From MCΔθ→50g J/g/ oC answer in b [i] above.
      3. Theoretical value is higher than the obtained value
        Heat lost to the surrounding
        Heat absorbed by the apparatus
  2.  
    1.  

      Observation

      Inference

      It dissolves into a colourless solution [1mk]

      Soluble salt [No Cu2+, Fe2+ or Fe3+] [1mk]

    2.  

      Observation

      Inference

      White precipitate soluble in excess [1mk]

      Pb2+, Zn2+, Al3+ ions present[1mk]

    3.  

      Observation

      Inference

      White precipitate insoluble in excess

      Pb2+, Al3+ ions present

    4.  

      Observation

      Inference

      Yellow precipitate formed

      Pb2+ ions present

    5.  

      Observation

      Inference

      White precipitate [1mk]

      SO42- ions

      Don’t dissolve on adding nitric acid

       Presence of NO, SO32- ions 

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