CHEMISTRY PAPER 3 - KCSE 2019 JOINT PRE MOCK EXAMINATION NAMBALE

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  1. You are provided with:
    -Solid A, 6 cm length of magnesium ribbon.
    -Solution B, hydrochloric acid solution.
    -Solution C, 0.2M sodium hydroxide solution.

    You are required to determine:
    -the concentration of solution B in moles per litre
    -molar heat of reaction of magnesium metal with hydrochloric acid.

    Procedure I
    Measure 25cm3 of solution Band put into a 100 ml plastic beaker. Determine its temperature and record in table 1 below. Cut six 1cm length of magnesium ribbon. Put the first 1cm length of A into the hydrochloric acid solution B in the plastic beaker and simultaneously start a stop watch. Record the time taken in seconds for the piece of magnesium ribbon to disappear in table II below. Put the second 1cm piece of magnesium ribbon to the solution left and immediately start a stop clock and record the time taken for the piece to disappear. Repeat the procedure using the third, fourth and fifth 1cm magnesium ribbon each time recording the time taken for the piece to disappear in tableII. Retain the sixth piece for question 3. Take the final temperature of the mixture after the fifth piece has disappeared and record in table II below. Label the resulting mixture as D and keep it for procedure II

    Final temperature of solution B (0C)

     

    Initial temperature of solution B (0C)

     

    Change in temperature,         (0C)

     
    Table 1I

    Pieceof magnesium added

    1st

    2nd

    3rd

    4th

    5th

    Time taken for the piece to disappear (seconds)

             

    Total time taken (seconds)

             

    Total length of magnesium ribbon added (cm)

             
    (4 marks)
    1. Plot a graphof total time taken for magnesium ribbon to disappear against total length of magnesium ribbon added.(3 marks)
    2. From the graph determine the time taken for 4.5cm length of magnesium solid A to react completelywith hydrochloric acid solution (1mark)

      Procedure III
      Transfer solution D into a 250 cm3 volumetric flask. Top with distilled water while shaking up to the mark and label the resulting solution as solution E. Fill the burette with solution E. Pipette 25cm3 of solution C into a conical flask, add three drops of phenolphthalein indicator and titrate with solutionE from the burette and record your results in table III below. Repeat the procedure two more times

      Table III(4 marks)
       

      I

      II

      III

      Final burette reading (cm³)

           

      Initial burette reading (cm³)

           

      Volume of E used (cm³)

           

      1. Calculatethe average volume of E (1 mark)
      2.  
        1. Calculate thenumber of moles of sodium hydroxide, solution C (1 mark)  
        2. Calculate the number moles of hydrochloric acid solution E used.                       (1 mark)
      3. Calculate the number moles of hydrochloric acid in solution in 250cm3 of solution (1mark
      4.  
        1. Given that 5cm of magnesium ribbon weighs 0.17g, calculate the number of moles magnesium ribbon that reacted with 25cm3 of hydrochloric acid, solution (Mg = 24)(1 mark)
        2. Hence calculate the number of moles of hydrochloric acid that reacted with 25cm3of magnesium ribbon.                                                                                                            (1 mark)
        3. Using b(iii) and c(i) above, determine the total number of moles ofhydrochloric acidin 25cm3 of solution B.      (1 mark)
        4. Calculate the concentration of hydrochloric acidsolution B in moles per litre.    (2 marks)
      5. Find the molar heat of reaction when magnesium reacts with hydrochloric acid.(take specific heat capacity of solution as 4.2J/g/k and density of solution as 1.0 g/cm3(2 marks)
      6. You are provided with solid F. Carry out the following tests and record your observations and inferences, in the spaces provided.
  2. Place all of solid F into boiling tube and add 10cm3 of distilled water. Shake to dissolve the solid. Divide the resulting solution into five portions.
    1. To the first portion add about 1cm3 of aqueous sodium sulphate solution. 

      Observations

      Inference

        (1 mark)

      (1 mark)

           
    2. To the second portion. Add about 1cm3 of aqueous lead nitrate solution. 

      Observations

      Inference

        (1 mark)

      (1 mark)

       
    3. To the fourth portion, add aqueous sodium hydroxide solution drop wise until excess.

      Observations

      Inference

        (1 mark)

      (1 mark)

    4. To the fifth portion, add aqueous ammonia solution drop wise until excess.

      Observations

      Inference

        (1 mark)

      (1 mark)

  1. You are provided with solidG. Carry out the tests in (a) and (b) and write your observations and inferences in the spaces provided.
    1. Place the rest of solidG in a boiling tube. Add about 8cm3 of distilled water. Shake the mixture well. Retain the mixture to use it in the tests in (b) (i) to (iii)

      Observations

      Inference

        (1 mark)

      (1 mark)

    2. To 2 cm3 of the mixture in a test tube add about 1 cm3 of acidified potassium dichromate (VI) solution and warm.

      Observations

      Inference

        (1 mark)

      (1 mark)

    3. To 2 cm3 of the mixture in a test tube add the remaining 1 cm magnesium ribbon.

      Observations

      Inference

        (1 mark)

      (1 mark)

    4. Determine the PH of the mixture obtained in (b)

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