Chemistry Paper 3 Questions and Answers with Confidentials - KCSE 2021 Westlands Mock Exams

Questions

Instructions to Candidates:

• You are required to spend the first 15 minutes of the 2 1/4 hours allowed for this Paper reading the whole paper carefully before commencing your work.

1. You are provided with;
   • 3.6 g of solid P in a boiling tube. Solid P is a hydrated dibasic acid with the formula H2C2O4.nH2O
   • Solution L which is a 0.2 M sodium hydroxide solution.
     You are required to determine;
     1. Solubility of solid P
     2. The value of n in the formula H2C2O4.nH2O
   • Procedure I
     1. Using a burette, add 4cm3 of distilled water to solid P in the boiling tube. Heat the mixture while stirring with the thermometer to about 70 ⁰C. When all of solid has dissolved, allow the solution to cool while stirring with the thermometer. Note the temperature at which crystals of solid P first appear. Record the temperature in table 1 below.
     2. Using the burette, add 2 cm3 of distilled water to the contents of the boiling tube. Warm the mixture while stirring with the thermometer until all the solid dissolves again. Allow the mixture to cool while stirring. Note the temperature at which crystals of solid P appears.
     3. Repeat procedure (II) three more times and record the temperatures in table 1. RETAIN the contents of the boiling tube for use in PROCEDURE II
        
        TABLE 1
        

        Volume of water in the boiling tube (cm3)	Crystallization temperature (⁰C)	Solubility of solid P in g/100 g of water
        4
        6
        8
        10
        12

        (4marks)
        1. Complete table 1 above by working out the solubility. (1 mark)
        2. On the graph provided, plot a graph of solubility of solid P against crystallization temperature. (3marks)
        3. From the graph, determine;
           1. The solubility of solid P at 50 ˚C (1 mark)
           2. The temperature at which 65 g of solid P would dissolve in 100 g of water (1 mark)
              
              
   • Procedure II
     1. Transfer the contents of the boiling tube from PROCEDURE I into a clean 250 ml volumetric flask.
     2. Add distilled water up to the mark
     3. Label the resulting solution as solution P
     4. Fill the burette with solution P
     5. Pipette 25cm³ of solution L into a conical flask. Add three drops of phenolphthalein indicator
     6. Titrate solution P against solution L to an accurate end point. Record your results in table 2 below.
        

        TABLE 2	1	2	3
        Final burette reading
        Initial burette reading
        Volume of solution P used (cm3)

        (4 marks)
        1. Calculate Average volume of solution P used. (1mark)
        2.        
           1. Moles of solution L used. (1 mark)
           2. Moles of solution P used. (1 mark)
           3. Moles of solution P in 250 cm3 of solution P (1 mark)
           4. The relative formula mass of P. (1 mark)
2. Determine the value of n in the formula; H2C2O4.nH2O (H = 1, C = 12, O = 16)(2marks)
   You are provided with solid E. Carry out the experiments below. Write your observations and inferences in the spaces provided.
   Place all solid E into a boiling tube. Add about 20cm3 of distilled water and shake. Retain the contents of the boiling tube.
   

   Observations	inferences

   Use 2cm3 of solution E, in a test tube in each experiment i, ii,iii,iv and v
   1. To experiment i, Add two drops of aqueous Sulphuric vi acid
      

      Observations	inferences

   2. To experiment ii, add NaOH solution drop wise till in excess.
      

      Observations	inferences

   3. To experiment iii, dip a stirring rod into the solution, place the rod in a non-luminous flame.
      

      Observations	inferences

   4. To experiment iv, add two drops of lead (ii) nitrate solution.
      

      Observations	inferences

   5. To experiment v, add a piece of aluminium foil followed by sodium hydroxide solution and warm . Test the gas given out with litmus papers.
      

      Observations	inferences

3. You are provided with liquid Q. carry out the tests below. Write your observations and inferences in the spaces provided.
   1. To 2 cm3 of liquid Q in a test tube, add universal indicator
      

      Observations	inferences

   2. Place 3 drops of liquid Q on a watch glass and ignite.
      

      Observations	inferences

   3. To 2 cm3 of liquid Q in a test tube , add two or three drops of acidified potassium dichromate VI and warm
      

      Observations	inferences

Confidential

Each candidate requires:

1. About 100 ml of solution L- 0.2 M NaOH
2. Distilled water in a wash bottle
3. Burette
4. Pipette and pipette filler
5. 2 conical flasks
6. 10 ml measuring cylinder
7. 250 ml volumetric flask
8. Exactly 3.6g of solid P- oxalic acid
9. Means of labelling- 1 piece
10. Test-tube holder
11. Thermometer(-10 to 110oC)
12. Boiling tube
13. 5 Test tubes
14. Glass rod
15. Aluminium Foil
16. Red and Blue litmus papers
17. Ethanol labelled Liquid Q
18. Universal indicator and universal chart paper
19. Watch glass
20. Wooden splint
21. Solid Barium Nitrate labelled solid E

ACCESS TO:

• Source of heat.
• Phenolphthalein indicator.
• NaOH.
• Dilute H₂SO₄.
• Pb(NO₃)₂ Solution.
• Acidified K₂Cr₂O₇.

Marking Scheme

1. You are provided with;
   • 3.6 g of solid P in a boiling tube. Solid P is a hydrated dibasic acid with the formula H2C2O4.nH2O
   • Solution L which is a 0.2 M sodium hydroxide solution.
     You are required to determine;
     1. Solubility of solid P
     2. The value of n in the formula H2C2O4.nH2O
   • Procedure I
     1. Using a burette, add 4cm3 of distilled water to solid P in the boiling tube. Heat the mixture while stirring with the thermometer to about 70 ⁰C. When all of solid has dissolved, allow the solution to cool while stirring with the thermometer. Note the temperature at which crystals of solid P first appear. Record the temperature in table 1 below.
     2. Using the burette, add 2 cm3 of distilled water to the contents of the boiling tube. Warm the mixture while stirring with the thermometer until all the solid dissolves again. Allow the mixture to cool while stirring. Note the temperature at which crystals of solid P appears.
     3. Repeat procedure (II) three more times and record the temperatures in table 1. RETAIN the contents of the boiling tube for use in PROCEDURE II
        
        TABLE 1
        

        Volume of water in the boiling tube (cm3)	Crystallization temperature (⁰C)	Solubility of solid P in g/100 g of water
        4
        6
        8
        10
        12

        (4marks)
        1. Complete table 1 above by working out the solubility. (1 mark)
        2. On the graph provided, plot a graph of solubility of solid P against crystallization temperature. (3marks)
        3. From the graph, determine;
           1. The solubility of solid P at 50 ˚C (1 mark)
           2. The temperature at which 65 g of solid P would dissolve in 100 g of water (1 mark)
              
              
   • Procedure II
     1. Transfer the contents of the boiling tube from PROCEDURE I into a clean 250 ml volumetric flask.
     2. Add distilled water up to the mark
     3. Label the resulting solution as solution P
     4. Fill the burette with solution P
     5. Pipette 25cm³ of solution L into a conical flask. Add three drops of phenolphthalein indicator
     6. Titrate solution P against solution L to an accurate end point. Record your results in table 2 below.
        

        TABLE 2	1	2	3
        Final burette reading
        Initial burette reading
        Volume of solution P used (cm3)

        (4 marks)
        1. Calculate Average volume of solution P used. (1mark)
        2.        
           1. Moles of solution L used. (1 mark)
           2. Moles of solution P used. (1 mark)
           3. Moles of solution P in 250 cm3 of solution P (1 mark)
           4. The relative formula mass of P. (1 mark)
2. Determine the value of n in the formula; H2C2O4.nH2O (H = 1, C = 12, O = 16)(2marks)
   You are provided with solid E. Carry out the experiments below. Write your observations and inferences in the spaces provided.
   Place all solid E into a boiling tube. Add about 20cm3 of distilled water and shake. Retain the contents of the boiling tube.
   

   Observations	inferences
   Dissolves	Soluble salt

   Use 2cm3 of solution E, in a test tube in each experiment i, ii,iii,iv and v
   1. To experiment i, Add two drops of aqueous Sulphuric vi acid
      

      Observations	inferences
      White precipitate No effervesence	Ba2+, Ca2+, Pb2+ ions present (reject any other) SO2-3, CO2-3

   2. To experiment ii, add NaOH solution drop wise till in excess.
      

      Observations	inferences
      White ppt insoluble in excess	Ba2+, Ca2+ present (penalize for any other cation introduced)

   3. To experiment iii, dip a stirring rod into the solution, place the rod in a non-luminous flame.
      

      Observations	inferences
      Green-yellow flame	Ba2+ ions present

   4. To experiment iv, add two drops of lead (ii) nitrate solution.
      

      Observations	inferences
      NO white ppt	Cl- absent

   5. To experiment v, add a piece of aluminium foil followed by sodium hydroxide solution and warm . Test the gas given out with litmus papers.
      

      Observations	inferences
      Colourless gas which turns pink Litmus red turns blue and blue remains blue	NO2-3 confirmed

3. You are provided with liquid Q. carry out the tests below. Write your observations and inferences in the spaces provided.
   1. To 2 cm³ of liquid Q in a test tube, add universal indicator
      

      Observations	inferences
      5.0 - 6.5 Accept any value within the range	Weakly acidic

   2. Place 3 drops of liquid Q on a watch glass and ignite.
      

      Observations	inferences
      Blue flame

   3. To 2 cm³ of liquid Q in a test tube , add two or three drops of acidified potassium dichromate VI and warm
      

      Observations	inferences
      CncO2-7 turns from orage to green	ROH presernt