Chemistry Paper 3 Questions and Answers - Form 4 Term 1 Opener Exams 2021

CHEMISTRY
PAPER 3
(PRACTICAL)
FORM 4 TERM 1 OPENER EXAMS
TIME: 2¼ HOURS

INSTRUCTIONS TO CANDIDATES

• Answer ALL the questions in the paper
• Mathematical tables and silent electronic calculators may be used.
• All questions should be answered in English.
• Spend the first 15 minutes of the 2 ¼ hours to read the questions paper and ensure you have all the chemicals and apparatus that you may need.

1. Question one   
   1. You are provided with:-
      1. 2.86g of solid E, Na₂CO₃ x H₂O
      2. Solution W, 0.1M hydrochloric acid
   2. You are required to-
      1. Prepare a dilute solution of Na₂CO₃ . XH₂O
      2. Determine the value of X in Na₂CO₃ . XH₂O
   3. Procedure:
      • Step I
        Place all the solid E in a 250ml volumetric flask. Add about 150ml of distilled water. Shake the mixture well until the solid dissolves. Add distilled water up to mark. Label this solution D.
      • Step II
        Using a pipette and pipette filler place 25.0cm³ of solution D into a conical flask. Add two drops of the indicator provided. Fill the burette with solution W and titrate solution D. with solution W. Record your results in the table below. Repeat the titration two more times and complete the table below.
        

        	I.	II	III
        Final burette reading(cm3)
        Initial burette reading(cm3)
        Volume of solution W used (cm3)

        
        
        1. Calculate
           1. Average volume of solution W used (1mk)
           2. Moles of solution W, hydrochloric acid used. (2mks)
        2. The equation for the reaction that took place is:
           Na₂CO_3(aq) + 2HCl_(aq)→ 2 NaCl_(aq) + CO_2(g) + H₂O_(l)
           Calculate:
           1. Moles of solutionD, Na₂CO₃.XH₂O used (2mks)
           2. Moles of solutionD, Na₂CO₃.XH₂O in 250cm³ of solution (2mks)
           3. Molarity of solutionD, Na₂CO₃.XH₂O (2mks)
           4. Relative formula mass of Na₂CO₃.XH₂O (3mks)
           5. The mass in grams of the water of crystallization in a litre of solution ∆ (Na= 23, C=12, O = 16, H = 1) (2mks)
           6. The value of X in Na₂CO₃.XH₂O (2mks)
2.  Question two  
   1. You are provided with solid Y. Carry out the tests below and identify the ions.
      1. Add all solid Y in a boiling tube. Add about 10cm³ of distilled water. Divide the solution into 4 portions.
         

         Observation	Inferences
         (1mk)	(1mk)

      2. To the first portion add three drops of sodium hydroxide then in excess.
         

         Observation	Inferences
         (1mk)	(1mk)

      3. To the second portion add three drops of ammonium hydroxide then in excess
         

         Observation	Inferences
         (1mk)	(1mk)

      4. To the third portion add three drops of lead(II) nitrate and warm
         

         Observation	Inferences
         (1mk)	(1mk)

      5. To the forth portion add three drops of hydrochloric acid.
         

         Observation	Inferences
         (1mk)	(1mk)

   2. You are providedwith solid X. Carry out the tests below.
      1. Scoop half of solid X using a spatula and heat over non-luminous flame.
         

         Observation	Inferences
         (1mk)	(1mk)

      2. Put the remaining solid X in a boiling tube and add about 10cm³ of distilled water. Divide the solution into 3 portions.
         

         Observation	Inferences
         (1mk)	(1mk)

      3. To the first portion add 3 drops of universal indicator
         

         Observation	Inferences
         (1mk)	(1mk)

      4. To the second portion add sodium carbonate provided.
         

         Observation	Inferences
         (1mk)	(1mk)

      5. To the third portion add three drops of acidified potassium manganate (VII) solution.
         

         Observation	Inferences
         (1mk)	(1mk)

CONFIDENTIAL

• Each student is provided with:
  1. 100cm³ solution W, 0.1M HCL
  2. Solid E
  3. Solid Y
  4. 250ml volumetric flask
  5. 500 ml Distilled water
  6. 1 pipette
  7. 1 burette
  8. 2 conical flasks
  9. 1pipette filler
  10. 1 label
  11. 1 stand clamp and boss
  12. 1 white tile
  13. 1 filter funnel
  14. 2 boiling tubes
  15. 6 test tubes in a test tube rack
  16. 10cm³ measuring cylinder
  17. About0.5g of sodium carbonate
      
      
• Access to:
  1. Means of heating
  2. 2M NaOH solution with a dropper
  3. 2M NH4OH solution with a dropper
  4. 0.1M lead (II) nitrate solution with a dropper
  5. Acidified potassium manganate (VII) solution with a dropper
  6. 1M HCl solution with a dropper
  7. Universal indicator
  8. pH chart.
     
     
• NB: methyl orange indicator
  1. Solid E is 2.86g of Na₂CO₃ . 10H2O weighed accurately.
  2. Solid Y is about 2.0g of ZnSO₄ crystals
  3. Solid X is about 2.0g of oxalic acid
  4. 2M ammonia solution is prepared by dissolving 112cm³ of concentrated ammonia solution in 1 litre.
  5. 2M sodium hydroxide solution is prepared by dissolving 80g of sodium hydroxide in 1 litre.
  6. Acidified potassium manganate (VII) is prepared by dissolving 3.2g of potassium manganate (VII) in 600cm³ of 2M sulphuric (VI) acid and diluting to a litre.
  7. 0.1M hydrochloric acid is prepared by dissolving 8.6cm3 of concentrated hydrochloric acid in 1000cm³ of water.

MARKING SCHEME

1. Question one   
   1. You are provided with:-
      1. 2.86g of solid E, Na₂CO₃ x H₂O
      2. Solution W, 0.1M hydrochloric acid
   2. You are required to-
      1. Prepare a dilute solution of Na₂CO₃ . XH₂O
      2. Determine the value of X in Na₂CO₃ . XH₂O
   3. Procedure:
      • Step I
        Place all the solid E in a 250ml volumetric flask. Add about 150ml of distilled water. Shake the mixture well until the solid dissolves. Add distilled water up to mark. Label this solution D.
      • Step II
        Using a pipette and pipette filler place 25.0cm³ of solution D into a conical flask. Add two drops of the indicator provided. Fill the burette with solution W and titrate solution D. with solution W. Record your results in the table below. Repeat the titration two more times and complete the table below.
        

        	I.	II	III
        Final burette reading(cm3)	20.5	20.5	20.5
        Initial burette reading(cm3)	0.0	0.0	0.0
        Volume of solution W used (cm3)	20.5	20.5	20.5

        
        
        1. Calculate
           1. Average volume of solution W used (1mk)
              • Average volume = 20.5 + 20.5 + 20.5
                                                      3
                = 20.5cm³ 
           2. Moles of solution W, hydrochloric acid used. (2mks)
              • 0.1 moles in 1000cm³
                     ?             20.5cm³
                1.1 x 20.5 (1mk)
                   1000
                = 0.00205 moles
        2. The equation for the reaction that took place is:
           Na₂CO_3(aq) + 2HCl_(aq)→ 2 NaCl_(aq) + CO_2(g) + H₂O_(l)
           Calculate:
           1. Moles of solutionD, Na₂CO₃.XH₂O used (2mks)
              • Mole ratio HCl : Na₂CO₃ . XH2O
                                  2 : 1
                Moles of D = 0.00205 (1mk)
                                         2
                = 0.00103 moles
           2. Moles of solutionD, Na₂CO₃.XH₂O in 250cm³ of solution (2mks)
              • 0.00103 moles in 25cm³
                          ?              250cm³
                0.00103 x 250 (1mk)
                25
                = 0.0103 moles
           3. Molarity of solutionD, Na₂CO₃.XH₂O (2mks)
              • 0.0103 moles in 250cm³
                                ?       1000cm³
                = 0.0103 x 1000 (1mk)
                          250
                = 0.0412M
           4. Relative formula mass of Na₂CO₃.XH₂O (3mks)
              • 2.86g in 250cm³
                     ?      1000cm³ (1mk)
                = 11.4 g/litre
                0.04 = 11.4 (1mk)
                           mm
                mm = 11.4
                          0.04
                = 286 (1mk)
           5. The mass in grams of the water of crystallization in a litre of solution ∆ (Na= 23, C=12, O = 16, H = 1) (2mks)
              • Rmm of Na₂CO₃ = (23 x 2) + 12 + (16 x 3) = 106 (1mk)
                Mass of water = 286 – 106 = 180g (1mk)
           6. The value of X in Na₂CO₃.XH₂O (2mks)
              • XH₂O = 180
                Rmm of H₂O = (1 x 2) + 16 (1mk)
                = 18
                18 x =180
                18       18
                X = 10 (1mk)
                
2.  Question two  
   1. You are provided with solid Y. Carry out the tests below and identify the ions.
      1. Add all solid Y in a boiling tube. Add about 10cm³ of distilled water. Divide the solution into 4 portions.
         

         Observation	Inferences
         The solid dissolved to form a colourless solution     (1mk)	Soluble salt      (1mk)

      2. To the first portion add three drops of sodium hydroxide then in excess.
         

         Observation	Inferences
         In few drops white precipitate is formed which dissolves in excess         (1mk)	Zn2+Pb2+ and Al3+ present      (1mk)

      3. To the second portion add three drops of ammonium hydroxide then in excess
         

         Observation	Inferences
         In few drops white precipitate is formed which dissolves in excess         (1mk)	Zn2+ present       (1mk)

      4. To the third portion add three drops of lead(II) nitrate and warm
         

         Observation	Inferences
         White precipitate is formed which does not dissolve on warming         (1mk)	CO32- , SO32- or SO42- present      (1mk)

      5. To the forth portion add three drops of hydrochloric acid.
         

         Observation	Inferences
         The white precipitate remains        (1mk)	SO42-  present      (1mk)

   2. You are providedwith solid X. Carry out the tests below.
      1. Scoop half of solid X using a spatula and heat over non-luminous flame.
         

         Observation	Inferences
         It burns with a yellow sooty flame        (1mk)	C = C or -C = C- present       (1mk)

      2. Put the remaining solid X in a boiling tube and add about 10cm³ of distilled water. Divide the solution into 3 portions.
         

         Observation	Inferences
         Dissolves to form a colourless solution         (1mk)	Polar compound       (1mk)

      3. To the first portion add 3 drops of universal indicator
         

         Observation	Inferences
         pH ( 4or 5)        (1mk)	Weakly acidic       (1mk)

      4. To the second portion add sodium carbonate provided.
         

         Observation	Inferences
         There was effervescence        (1mk)	Presence of H+       (1mk)

      5. To the third portion add three drops of acidified potassium manganate (VII) solution.
         

         Observation	Inferences
         The purple colour of the solution persists         (1mk)	-C=C- or -C =  C-  absent      (1mk)