Chemistry Paper 3 Questions and Answers - KCSE 2021 Past Papers

QUESTIONS

1. You are provided with:
   • Solution A: 0.10 M solution of a monobasic acid A;
   • Solution B: Sodium hydroxide solution:
   • Solution C: containing 10.0g of acid C per litre of solution.
     You are required to:
   • Standardise solution B using solution A;
   • Determine the number of moles of sodium hydroxide that react with one mole of acid C.

PROCEDURE I
Fill the burette with solution A. Using a pipette and pipette filler, place 25.0cm of solution B into 250 ml conical flask. Titrate solution B with solution A using phenolphthalein indicator and record your results in Table 1. Repeat the titration and complete Table 1.

1. Table 1
   

   	I	II	III
   Final burette reading
   Initial burette reading
   Volume of solution A used, cm3

   (3 marks)
2. Calculate the:
   1. average volume of solution A used. (1 mark)
   2. number of moles of solution A in the average volume used. (1 mark)
   3. number of moles of sodium hydroxide (N) in 25.0cm of solution B. (1 mark)
   4. concentration of sodium hydroxide in moles per litre. (1 mark)
      
      PROCEDURE II
      Clean the burette and fill it with solution C. Using a pipette and pipette filler, place 25.0cm of solution B into a 250 ml conical flask.
      Titrate solution B with solution C using phenolphthalein indicator and record your results in Table 2. Repeat the titration and complete Table 2.
3. Table 2
   

   	I	II	III
   Final burette reading
   Initial burette reading
   Volume of solution A used, cm3

   (3 marks)
4. Calculate the:
   
   1. average volume of solution Cused. (1 mark)
   2. concentration in moles per litre, of solution C, given that the relative formula mass of acid C is 210.0. (1 mark)
   3. number of moles of acid C in the average volume used. (1 mark)
5.      
   1. Write the ratio of moles of acid Cto moles of sodium hydroxide (N) in the 25.0cm of solution B. (1 mark)
   2. Determine the number of moles of sodium hydroxide that react with one mole of acid C. (1 mark)

2. You are provided with solid D.
   You are required to determine the freezing point of solid D.
   PROCEDURE
   1. Fill a 250 ml beaker with about 200 cm³ of tap water and heat the water until it boils.
   2. Place all solid D provided in a dry test tube and insert a thermometer into the solid.
   3. Place the test tube in the boiling water and allow the solid to heat until it all melts.
   4. When the temperature of the melted solid is approximately 90°C, remove the test tube, wipe the sides with tissue paper and then place the test tube into an empty 250 ml beaker
   5. Start the stop watch or clock when the temperature of the melted solid is 85.0°C.
   6. As the solid cools, measure and record its temperature every 30 seconds and complete Table 3.
      1. Table 3
         

         Time, s	0	30	60	90	120	150	180	210	240	270	300
         Temperature, °C

         (4 marks)
      2. On the grid provided, plot a graph of temperature (vertical axis) against time. (3 marks)
      3. Using the graph in (b), determine the freezing point of solid D. (1 mark)
3. You are provided with solid E. Carry out the following tests and record your observations and inferences in the spaces provided.
   1. Place all the solid E in a boiling tube. Add about 10 cm of dilute nitric(V) acid, warm the mixture and then allow to stand until all the solid dissolves. Add about 10 cm of distilled water to the solution and shake. Retain the solution for tests (b) and (c)
      

      Observations	Inferences
      (2 marks)	(1 mark)

   2. Use about 2 cm³ portions of the solution obtained in 3(a) for each of the following tests.
      1. To the first portion add 2 or 3 drops of aqueous barium nitrate,
         

         Observations	Inferences
         (1 marks)	(1 mark)

      2. To the second portion add 2 or 3 drops of aqueous lead(II) nitrate.
         

         Observations	Inferences
         (1 marks)	(1 mark)

      3. To the third portion add aqueous sodium hydroxide dropwise until in excess. 
         

         Observations	Inferences
         (1 marks)	(1 mark)

      4. Place about 3 cm of aqueous ammonia in a test tube. To the fourth portion, add all the aqueous ammonia from the test tube dropwise.
         

         Observations	Inferences
         (1 marks)	(1 mark)

   3. To the remaining solution of solid in the boiling tube, add all the solid providedn
      Shake the mixture for about 2 minutes. Filter the mixture into a boiling tube. Retain the filtrate for tests (i) and (ii) below.
      

      Observations	Inferences
      (1 marks)	(1 mark)

      1. To about 2 cm portion of the filtrate, add aqueous ammonia dropwise until in excess
         

         Observations	Inferences
         (1 marks)	(1 mark)

      2. To about 2 cm portion of the filtrate add 2 or 3 drops of dilute hydrogen peroxide solution.
         

         Observations	Inferences
         (1 marks)	(1 mark)

MARKING SCHEME

1.      
   1.      
      

      		24.0		CT 1
      	I	II	III	D1
      Final burette reading				A1
      Initial burette reading				PA1
      Volume of solution A used, cm3				FA1

   2.    
      1. Correct working 
         correct ans (b)(i)
      2. =correct ans (b)(i) x 0.1 = correct ans (b)(ii)
                       1000
      3. Ratio A:N b1:1 
         ans (b) (iii)
      4. =1000 x ans (b)(iii)  = ans (b)(iv)
                      25

PROCEDURE II

3. Table 2
   

   		16.5		CT1
   	I	II	III	D1
   Final burette reading				A1
   Initial burette reading				PA1
   Volume of solution A used, cm3				FA1

4.        
   1. Correct working 
      correct ans (d)(i)
   2. = ¹⁰/₂₁₀ = 0.0476
   3. =correct ans (d)(i) x 0.0476 = correct ans (d)(iii)
                        1000
5.      
   1. ans (d)(iii) : ans(b)(iii)
   2. =ans(b)(iii)  = ans (e)(ii)
        ans(d)(iii)

2. Table 3
   1.      
      

      Time, s	0	30	60	90	120	150	180	210	240	270	300
      Temperature, °C	85.0	82.0	80.5	78.0	75.5	72.0	69.0	69.0	69.0	67.5	65.0

   2.      
      
   3. showing
      correct reading
3.    
   1.     
      

      Observations	Inferences
      No effervescence E dissolves to form a blue solution (1 marks)	CO32-/ SO32- absent Cu2+ present (1 mark)

   2.      
      1.      
         

         Observations	Inferences
         No white ppt formed	SO42- absent

      2.    
         

         Observations	Inferences
         No yellow ppt formed No white ppt formed	I- absent Cl-/Br- absent

      3.     
         

         Observations	Inferences
         Blue ppt formed insoluble in excess	Cu2+ present

      4.      
         

         Observations	Inferences
         Blue ppt formed that dissolves in excess to form a deep blue solution	Cu2+ present

   3.    
      

      Observations	Inferences
      Blue solution changes to green Brown residue Green filtrate Boiling tube becomes warm	Cu2+ displaced by G/G is more reactive than Cu/G is oxidized by Cu2+ /Cu2+ are reduced by G/ Cu2+ are displaced by Fe

      1.  
         

         Observations	Inferences
         Green ppt formed insoluble in excess	Fe2+

             
      2.    
         

         Observations	Inferences
         Green solution changes to brown/Yellow Effervescence	Fe2+ oxidized to Fe3+  Fe3+ formed