KCSE 2011 Chemistry Paper 3 Questions with Marking Scheme

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  1. You are provided with:
    • 1.6g of solid A , a dibasic acid.
    • Solution B containing 4.75g per litre of salt B.
    • Aqueous sodium hydroxide , solution C.
    • Phenolphthalein indicator.

      You are required to prepare a solution of solid A and use it determine the:

    • Concentration of sodium hydroxide , solution C
    • React salt B with excess sodium hydroxide and then determine the relative molecular mass of salt B.

      Procedure I

      1. Using a burette , place 25.0cm3 of solution B in each of two 250ml conical flasks. Using a pipette and pipette filler, add 25.0cm3 of solution C to each of the two conical flasks. (The sodium hydroxide added is in excess). Label the conical flasks 1 and 2.

      2. Heat the contents of the first conical flask to boiling and then let the mixture boil for 5 minutes. Allow the mixture to cool.

      3. Repeat procedure (b) with the second conical flask.

        While the mixture are cooling, proceed with procedure II

    • Procedure II
      1. Place all of solid A in 250 ml volumetric flask. Add about 150cm3 of distilled water shake well to dissolve the solid and then add water to make up to mark. Label this as solution A.

      2. Place solution A in clean burette. Using a pipette filler. Place 25.0cm3 of solution C in a 250ml conical flask. Add 2 drops of phenolphthalein indicator and titrate with solution A. Record your results in table 1 . Repeat the titration two more times and complete the table.


        titration kcse 2011

        Calculate the;
        1. Average volume of solution A used;(1/2 marks)
        2. Concentration in moles per litre of the dibasic acid in solution A; (Relative molecular mass of A is 126) (2 marks)
        3. Moles of the dibasic acid used; (1 mark)
        4. Moles of sodium hydroxide in 25.0cm3 of solution C. (1 mark)
        5. Concentration of sodium hydroxide in moles per liter. (2 marks)


    • Procedure III

      Add 2 drops of phenolphthalein indicator to the contents of the first conical flask prepared in procedure I and titrate with solution A. Record you results in Table 2. Repeat the procedure with the contents of the second conical flask and complete the table.

      titration kcse 2011

      Calculate the
      1. Average volume of solution A used: (1 ½ marks)
      2. Moles of the dibasic acid used: (1 mark)
      3. Moles of sodium hydroxide that reacted with dibasic acid.(1 mark)
      4. Moles of sodium hydroxide that reacted with 25.0cm3 of salt B in solution B; (2 marks)
      5. Given that 1 mole of salt B reacts with 2 moles of sodium hydroxide ,calculate the:
        1. Number of moles of salt B in 25.0cm3 of solution B; (1 mark)
        2. Concentration in moles per litre of salt B in solution B; (1 mark)
        3. Relative molecular mass of salt B; (2 marks)


    1. You are provided with solid D .Carry out the following tests and write your observations and interference in the space provided.

      1. Place about one half of solid D in a test tube and heat it strongly. Test any gas produced with both red and blue litmus papers.

        obsinference

      2. Place the rest of solid D in a boiling tube . Add about 10cm3 of distilled water. Shake well.

        To a 2 cm3 portion of the solution, add about 1 cm3 of hydrogen peroxide and shake well. To the resulting mixture, add aqueous sodium hydroxide drop wise until in excess.

        obsinference

    2. You are provided with solution E, carry out the following tests and write your observations and inferences in the spaces provided.

      Divide solution E into two portions.
      1. To one portion of solution E in attest tube add 3 drops of barium nitrate. Retain the mixture for use in test tube (ii) below.

        obsinference

      2. To the mixture obtained in (i) above, add about 5 cm3 of 2M nitric (V) acid.

        obsinference


      3. To portion two of solution E in a test tube, add 2 drops of acidified potassium dichromate (VI) and warm the mixture.

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

    1. Place five drops of liquid F on a clean dry watch glass and ignite it.

      obsinference

    2. Place about 2cm3 of liquid F in a clean dry test tube,add all the sodium hydrogen carbonate provided.

      obsinference

    3. Place about 2cm3 of liquid F in attest tube ,add about 1cm3 of acidified potassium dichromate(VI) and warm the mixture.

      obsinference

MARKING SCHEME

  1.      
    1. Table I
        I II III
      Final burette reading 29.70 33.40 44.60
      Initial burette reading  0.00 4.00 15.30
      Volume of solution A used(cm3)  29.70 29.40 29.30

      1. average volume
        =29.4+ 29.3
                 2
        29.35cm3
      2. concentration of the dibasic acid A;(2 marks)
        conc= 1.6/126 = 0.01269; 0.01269 x 4 = 0.05M
      3. moles of the dibasic acid used;
        29.35  x 0.05
         1000
        0.0014675 moles (1 mark)
      4. moles of NaOH in 25.0cm3.
        = (0.0014657 x 2) = 0.002935 moles(1 mark)
      5. The concentration of NaOH in moles per litre.
        = 25.0 cm3 of NaOH  = 0.002935
        1000cm3  = 0.1174 M (2 marks)
    2. Table II
      1. average volume;
        11.4 + 11.5
             2
        11.45 cm3
      2. moles of the dibasic acid
        =0.05 x 11.45
               1000
        0.0005725 moles
      3. moles of NaOH that reacted with the dibasic acid.
        = (0.0005725 x 2)
        =0.001145 moles
      4. moles of NaOH that reacted with 25.0cm of salt B in solution B;
        =0.0029314 -0.001145
        =0.0017864 moles
      5.      
        1. moles of salt B in 25.0cm3 of solution B;
          0.0017884 x 1/2
          6=  0.00089 moles

        2. concentration in moles per litre of salt B in solution B;
          = 0.00089 x 1000/ 25
          =0.0357 M
        3. relative molecular mass of salt B;
          = 4.73/ 0.0357
          =133.0
  2.                 
    1.          
      1.      
        Observations Inferences
        -Gas which turns red litmus paper blue
        -Brown solid formed
        NH+4 present
      2.  
        Observation Inferences
        Yellow / brown solution
        Brown ppt
        Fe3+ formed
             
    2.               
      1.      
        Observation Inferences
        White ppt formed  CO2-3, SO2-3, SO2-4
      2.        
        1.  
          Observation Inferences
          White ppt dissolved/disappears
          Effervescence occurs
          SO2-3, CO2-3 
              
        2.      
          Observation Inferences
          Changes from orange to green  SO2-3
           
  3.              
    1.      
      Observation Inferences
      Burns with a blue flame  Saturated compound or
      Short-chain hydrocarbon
        .
    2.      
      Observation Inferences
      No effervescence Not acidic
        
    3.    
      Observation Inferences
      colour changes from orange to green R-OH present

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