Chemistry Paper 3 Questions and Answers - Form 3 End Term 2 Exams 2022

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QUESTIONS
Q1 You are provided with

  • Metal carbonate M2CO3 solid Q which weighs exactly 5.0g
  • 2M hydrochloric acid solution P
  • Sodium hydroxide solution R made by dissolving 40g of the solid in a litre of solution.

You are required to determine
The relative formula mass (RFM) of M2CO3 and hence the relative atomic mass (RAM) of M.

Procedure

  • Measure accurately using a measuring cylinder 100cm3 of solution P into a clean 250cm3 conical flask.
  • Add all the 5.0g of solid Q,shake well and wait for effervescence to stop.
  • Put the solution into a 250ml clean beaker.
  • Label the resulting solution S.
  • Pipette 25cm3 of solution R into a clean conical flask and add 2-3 drops of phenolphthalein indicator
  • Fill the burette with solution S and titrate against solution R.
  • Record the results in the table below
  • Repeat the titration two more times and record in the table. (4mks)
  II III
Final burette reading      
Initial burette reading      
Volume of S used(cm3)      
  1. Calculate the average volume of solution S used (1mk)
  2. Calculate:
    1. Moles of sodium hydroxide solution R used (2mks)
    2. Moles of hydrochloric acid solution S in the average volume used (2mks)
    3. Moles of HCl solution S in 100cm the of solution. (2mks)
    4. Moles of hydrochloric acid solution P in the 100cm2 of the original solution (2mks)
    5. Moles of HCl solution P that reacted with solid Q (2mks)
    6. Moles of M2CO3 solid Q that reacted (2mks)
    7. The RFM of M2CO3 (2mks)
    8. The RAM of metal M (1mk)
      (Na=23 O=16, H=1)

Q2 Carry out the tests on the substances given and record your observations and inferences in the spaces provided.

  1.      
    1. Put half of solid A in a boiling tube, add about 3cm3 of distilled water shake and retain the solution
      Observation Inference
         
    2. To the solution above, add a few drops of ammonia solution then add in excess
      Observation Inference
         
    3. Scoop the rest of solid A in a clean metallic spatula and burn on a non-luminous flame.
      Observation Inference
         
  2.          
    1. Put ½ of solid B in a test tube and add about 1cm3 of distilled water
      Observation Inference
         
    2. Put the remaining solid in a test tube and add about 1cm3 of dilute hydrochloric acid.
      Observation Inference
         
  3.        
    1. Put ½ of solid C in a test tube,heat gently then strongly observing the colour changes in the solid when heating and after cooling. Test any gas produced with wet litmus papers and a glowing splint.
      Observation Inference
         
    2. Put the remaining solid C in a test tube and add about 2cm3 of distilled water,shake well. Add ammonia solution drop wise then in excess
      Observation Inference
         
  4. Put solid D in a test tube and add 2cm3 of distilled water. Shake well then add ammonia solution drop wise then in excess.
    Observation Inference
       
  5. Repeat procedure in (d) above with solid E.
    Observation Inference
       
  6. Divide solution F into 3 portions.
    1. Test the PH of the first portion with universal indicator.
      Observation Inference
         
    2. In the second portion drop a piece of magnesium ribbon and test for any gas produced with a burning splint.
      Observation Inference
         
    3. To the third portion add a ¼ spatula of sodium hydrogen carbonate.


MARKING SCHEME

Q1

  1. Table 3 titrations - 1mk
    2 titrations - 1/2mk
    1 titration - 0mk
  2. Decimals 1dp or 2dp used consistently - 1mk
    (if 2dp,the 2nd should be a 0 or 5)
  3. Accuracy -Compare with any of SV within + 0.1 - 1mk
    +0.2 - 1/2mk
  4. Principles of averaging
    2 0r 3 consistent values averaged - 1mk
    :- penalize 1/2mk for arithmetic error
  5. Final answer ____ 1mk
    Within + 0.1 of SV - 1mk
    + 0.2 of SV ______ 1/2mk
    1. See 4 and 5 above (≈ 23.5cm3)
    2.      
      1. 40g/40 = 1M
        No of moles = 1 x 25 = 0.025 moles of NaOH
                               1000
      2. NaOH + HCL → NaCl + H2O
        Mole ratio
        NaOH : HCl
        1 : 1
        0.025 : ? = 0.025 moles of HCl
      3. 0.025 moles = X (volm from (a) above)
        ? = 100cm3
        0.025 x 100 = 2.5moles of HCl
                 X             X 
      4. 2 moles - 1000cm3
        ?- 100cm3
        = 100 x 2 = 0.2 moles of P
             1000
      5. (0.2 - 2.5/X) moles of HCl
      6. M2CO3 + 2HCl → 2MCl + CO2 + H2O
        Mole ratio
        M2CO3 : HCl
        1 : 2
        ? - 0.2 –  2.5/X
        = (0.2 – 2.5/X) x ½ = moles of Q
      7. RFM =     5.0     
                   ( 0.2- 2.5/x) x ½
      8. 2M + 60 =     5.0      
                        (0.2- 2.5/x)x ½
        M = ½(         5.0        - 60)
                    (0.2- 2.5/x)x ½

Q2

Observation Inference
  1.         
    1. Dissolves to form a Colourless solution
    2. No white ppt formed
    3. Burns with a yellow flame
  2.      
    1. Dissolves to form a colourless solution
    2. Effervescence is produced
  3.       
    1.      
      • solid changes from white to Yellow and back to white On cooling
      • Brown gas is produced
      • Turns wet blue litmus paper red and red remains red
      • Relights a glowing splint
    2. Dissolves to form a colourless solution
      White ppt formed,soluble in excess
  4. White ppt formed insoluble in excess
  5. Blue ppt formed,soluble in excess to form a deep blue solution
  6.      
    1. PH 1 or 2
    2. Produces effervescence and a gas that burns with a pop sound
    3. Produces effervescence and a colourless gas




  1.        
    1. presence of a soluble salt
       Award absence of coloured ions
    2. Absence of Pb2+, Al3+,Zn2+
    3. Presence of Na+
  2.    
    1. As (a)(i) above
    2. Presence of a CO3-2
  3.    
    1.      
      • Presence of Zn2+
      • Presence of NO3-
      • Presence of an acidic gas
      • Presence of O2 ½ each for any 3
    2.    
      • Presence of a soluble salt
      • Presence of Zn2+
  4. Presence of Pb 2+ ,Al3+
  5. Presence of Cu2+
  6.        
    1. Presence of a strong acid
    2. Presence of an acidic solution
      Allow presence of H+
    3. Presence of an acidic solution
      Allow presence of H+

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