KCSE 2011 Chemistry Paper 3 Questions with Marking Scheme

1. You are provided with:
   • 1.6g of solid A , a dibasic acid.
   • Solution B containing 4.75g per litre of salt B.
   • Aqueous sodium hydroxide , solution C.
   • Phenolphthalein indicator.
     
     You are required to prepare a solution of solid A and use it determine the:
     
     
   • Concentration of sodium hydroxide , solution C
   • React salt B with excess sodium hydroxide and then determine the relative molecular mass of salt B.
     
     Procedure I
     
     
     1. Using a burette , place 25.0cm³ of solution B in each of two 250ml conical flasks. Using a pipette and pipette filler, add 25.0cm³ of solution C to each of the two conical flasks. (The sodium hydroxide added is in excess). Label the conical flasks 1 and 2.
        
        
     2. Heat the contents of the first conical flask to boiling and then let the mixture boil for 5 minutes. Allow the mixture to cool.
        
        
     3. Repeat procedure (b) with the second conical flask.
        
        While the mixture are cooling, proceed with procedure II
        
        
   • Procedure II
     1. Place all of solid A in 250 ml volumetric flask. Add about 150cm³ of distilled water shake well to dissolve the solid and then add water to make up to mark. Label this as solution A.
        
        
     2. Place solution A in clean burette. Using a pipette filler. Place 25.0cm³ of solution C in a 250ml conical flask. Add 2 drops of phenolphthalein indicator and titrate with solution A. Record your results in table 1 . Repeat the titration two more times and complete the table.
        
        
        
        
        Calculate the;
        
        1. Average volume of solution A used;(1/2 marks)
        2. Concentration in moles per litre of the dibasic acid in solution A; (Relative molecular mass of A is 126) (2 marks)
        3. Moles of the dibasic acid used; (1 mark)
        4. Moles of sodium hydroxide in 25.0cm³ of solution C. (1 mark)
        5. Concentration of sodium hydroxide in moles per liter. (2 marks)
           
           
           
   • Procedure III
     
     Add 2 drops of phenolphthalein indicator to the contents of the first conical flask prepared in procedure I and titrate with solution A. Record you results in Table 2. Repeat the procedure with the contents of the second conical flask and complete the table.
     
     
     
     Calculate the
     1. Average volume of solution A used: (1 ½ marks)
     2. Moles of the dibasic acid used: (1 mark)
     3. Moles of sodium hydroxide that reacted with dibasic acid.(1 mark)
     4. Moles of sodium hydroxide that reacted with 25.0cm³ of salt B in solution B; (2 marks)
     5. Given that 1 mole of salt B reacts with 2 moles of sodium hydroxide ,calculate the:
        1. Number of moles of salt B in 25.0cm³ of solution B; (1 mark)
        2. Concentration in moles per litre of salt B in solution B; (1 mark)
        3. Relative molecular mass of salt B; (2 marks)
           
           
           
2. 1. You are provided with solid D .Carry out the following tests and write your observations and interference in the space provided.
      
      
      1. Place about one half of solid D in a test tube and heat it strongly. Test any gas produced with both red and blue litmus papers.
         
         
         
         
      2. Place the rest of solid D in a boiling tube . Add about 10cm³ of distilled water. Shake well.
         
         To a 2 cm³ portion of the solution, add about 1 cm³ of hydrogen peroxide and shake well. To the resulting mixture, add aqueous sodium hydroxide drop wise until in excess.
         
         
         
         
   2. You are provided with solution E, carry out the following tests and write your observations and inferences in the spaces provided.
      
      Divide solution E into two portions.
      1. To one portion of solution E in attest tube add 3 drops of barium nitrate. Retain the mixture for use in test tube (ii) below.
         
         
         
         
      2. To the mixture obtained in (i) above, add about 5 cm³ of 2M nitric (V) acid.
         
         
         
         
         
      3. To portion two of solution E in a test tube, add 2 drops of acidified potassium dichromate (VI) and warm the mixture.
         
         
3. You are provided with liquid F. Carry out the following tests and record your observations and inferences in the spaces provided.
   
   
   1. Place five drops of liquid F on a clean dry watch glass and ignite it.
      
      
      
      
   2. Place about 2cm³ of liquid F in a clean dry test tube,add all the sodium hydrogen carbonate provided.
      
      
      
      
   3. Place about 2cm³ of liquid F in attest tube ,add about 1cm³ of acidified potassium dichromate(VI) and warm the mixture.
      
      

MARKING SCHEME

1.      
   1. Table I
      

      	I	II	III
      Final burette reading	29.70	33.40	44.60
      Initial burette reading	0.00	4.00	15.30
      Volume of solution A used(cm3)	29.70	29.40	29.30

      
      
      1. average volume
         =29.4+ 29.3
                  2
         29.35cm³
      2. concentration of the dibasic acid A;(2 marks)
         conc= ^1.6/₁₂₆ = 0.01269; 0.01269 x 4 = 0.05M
      3. moles of the dibasic acid used;
         29.35  x 0.05
          1000
         0.0014675 moles (1 mark)
      4. moles of NaOH in 25.0cm³.
         = (0.0014657 x 2) = 0.002935 moles(1 mark)
      5. The concentration of NaOH in moles per litre.
         = 25.0 cm³ of NaOH  = 0.002935
         1000cm³  = 0.1174 M (2 marks)
   2. Table II
      1. average volume;
         11.4 + 11.5
              2
         11.45 cm³
      2. moles of the dibasic acid
         =0.05 x 11.45
                1000
         0.0005725 moles
      3. moles of NaOH that reacted with the dibasic acid.
         = (0.0005725 x 2)
         =0.001145 moles
      4. moles of NaOH that reacted with 25.0cm of salt B in solution B;
         =0.0029314 -0.001145
         =0.0017864 moles
      5.      
         1. moles of salt B in 25.0cm3 of solution B;
            0.0017884 x 1/2
            6=  0.00089 moles
            
            
         2. concentration in moles per litre of salt B in solution B;
            = 0.00089 x 1000/ 25
            =0.0357 M
         3. relative molecular mass of salt B;
            = 4.73/ 0.0357
            =133.0
2.                 
   1.          
      1.      
         

         Observations	Inferences
         -Gas which turns red litmus paper blue -Brown solid formed	NH+4 present

      2.  
         

         Observation	Inferences
         Yellow / brown solution Brown ppt	Fe3+ formed

              
   2.               
      1.      
         

         Observation	Inferences
         White ppt formed	CO2-3, SO2-3, SO2-4

      2.        
         
         1.  
            

            Observation	Inferences
            White ppt dissolved/disappears Effervescence occurs	SO2-3, CO2-3

                
         2.      
            

            Observation	Inferences
            Changes from orange to green	SO2-3

             
3.              
   1.      
      

      Observation	Inferences
      Burns with a blue flame	Saturated compound or Short-chain hydrocarbon

        .
   2.      
      

      Observation	Inferences
      No effervescence	Not acidic

        
   3.    
      

      Observation	Inferences
      colour changes from orange to green	R-OH present