Chemistry Paper 3 Questions And Answers with Confidentials - Form 3 Term 2 Opener Exams 2021

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CHEMISTRY
PAPER 3
TERM 2 OPENER EXAM

INSTRUCTIONS

  • Answer all the questions

 

  1. You are provided with:
    • Solution X1 which is 0.1 M NaOH
    • Solution X2 which is a dibasic acid ,H2Z
    • Phenolphthalein indicator.

      You are required to determine the concentration of the dibasic acid and the mass of the anion Z in the acid

    • Procedure
      Fill the burette with solution X2 .pipette 25cm3 of solution X1in to a conical flask and add two to three drops of phenolphthalein indicator .titrate X1 against X2 .repeat this procedure two more times and complete the table below .
        I II III
      Final burette reading cm3      
      Initial burette reading cm3      
      Volume of solution X2 used cm3      

      1. Determine the average volume of solution X2 used. 2mks
      2. Workout the number of moles of NaOH solution X1 used. 3mk
      3. Given that the equation for the reaction is :
        H2Z(aq) + 2NaOH(aq) → Na2Z(aq) + 2H2O(l)
      4. Determine the number of moles of the dibasic acid H2Z used 3mks
      5. Work out the concentration of solution X2 in moles per dm3. 3mks
      6. Given that the concentration of solution X2 is 8.17g/dm3 determine the relative formula mass of H2Z. 3mks
      7. Find the molar mass of the anion Z-2 in the acid H2Z(H= 1.0) 3mks

  2. You are provided with solid M .carry out the test below on M and fill the observation and inferences.
    Test Observation Inferences
    Observe solid M and describe its appearance            2mks          1mk
    Take half of solid M in a clean dry boiling tube
    and heat it gently
             
               2mks
     
             1mk
    Take the remaining solid M in to a boiling tube
    and add about 5cm3 of distilled water and shake
    divide the solution formed in to two portions.
     

               2mks
     

             1mk
    To the first potion add a few drops of ammonia
    solution and then in excess
     
               2mks

             1mk 

    To the second portion add a few drops of barium
    chloride followed by a few drops of dilute
    hydrochloric acid.

     



               2mks




              1mk

    Hence write down the cation and anion in solid M(3mks)

MARKING SCHEME

  1.     
      I II III
    Final burette reading cm3 15.0 14.9 15.0
    Initial burette reading cm3  0.0  0.0  0.0
    Volume of solution X2 used cm3  15 14.9 15.0

    complete table
    Consistency in use of decimal places
    Accuracy( titre should be around 15.0cm3
    1.  15.0 + 14.9/2= 14.95cm(correct principle of averageing)
    2. no. of moles of NaOH= 25/100 x 0.1=0.0025
    3. Mole ratio H2Z: NaOH=1:2
      moles of H2Z=0.0025/2 = 0.00125moles
    4. 14.95cm3 H2Z contains 0.00125moles
      1000cm3 H2Z contains 1000/14.95 x 0.00125 = 0.084m
    5. Molar mass=concentration/ molarity = 8.17/0.084=97
    6. (1x2)+Z=97
      2+Z=97
      Z=95

  2.  You are provided with solid M .carry out the test below on M and fill the observation and inferences.
    Test Observation Inferences
    Observe solid M and describe its appearance  Blue crystals          2mks M is likely to be a copper(ii) salt         1mk
    Take half of solid M in a clean dry boiling tube
    and heat it gently
    White powder formed colourless liquid formed
               2mks
     M is a hydrated salte
             1mk
    Take the remaining solid M in to a boiling tube
    and add about 5cm3 of distilled water and shake
    divide the solution formed in to two portions.
    Solid dissolve forming pale-blue solution

               2mks

    M is likely to be a copper(ii) salt 

             1mk

    To the first potion add a few drops of ammonia
    solution and then in excess
    Pale-blue precipitate formed which dissolves in excess to form deep-blue solution
               2mks
    Cu2+ present
             1mk 

    To the second portion add a few drops of barium
    chloride followed by a few drops of dilute
    hydrochloric acid.

    White ppt formed insoluble in dilute HCl

               2mks

    Cu2+ present
             1mk

    Cations - Cu2+ 
    Anions -  SO42-

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