Chemistry Paper 3 Questions - Mokasa II Mock Exams 2022

Questions

Instructions to students:

•  Answer all questions in the spaces provided

1. You are provided with:
   •  60 cm3 Solution L, Hydrochloric acid solution.
   • 120 cm3 of Solution M containing 12.6g of a dibasic acid (H2C2O4
   • 2H2O) per litre. (Retain part of this solution to be used in question 2.)
   • 200 cm3 of solution K, Sodium hydroxide solution.
   • 3 pieces of Metal Z each 2cm long.
     
     You are required to:
   • Standardize sodium hydroxide solution K.
   • Use the standard solution K to determine the concentration of L.
   • React the hydrochloric acid solution L with metal Z and determine the mass per unit length of metal Z.
     
     Procedure I
   • Fill the burette with solution M. Pipette 25cm3 of solution K into a conical flask. Titrate using phenolphthalein indicator. Record your results in the table below.
     

     	I	II	III
     Final burette reading (cm3 )
     Initial burette reading (cm3)
     Volume of M used (cm3 )

     (3 marks)
     1. Calculate the average volume of solution M used. (1 mark)
     2. Calculate the concentration of the dibasic solution M in moles per litre. (C=12, H=1, O=16)(1 mark)
     3. Calculate the concentration of the sodium hydroxide in moles per litre. (2 marks)
        
        
   • Procedure II
   • Using a 100cm³ measuring cylinder, measure 90cm3 of distilled water and place it in a 250cm3 beaker. Add 10cm³ of solution L. Mix the solution well and label it W. Fill the burette with solution W. Pipette 25cm³ of solution K into a conical flask and titrate it with W using phenolphthalein indicator.
     

     	I	II	III
     Final burette reading (cm3 )
     Initial burette reading (cm3)
     Volume of M used (cm3 )

     (3 marks)
     1. Determine the average volume of W used (1 mark)
     2. Calculate the concentration of the dilute hydrochloric acid solution W in moles per litre. (2 marks)
     3. Determine the concentration of the original hydrochloric acid solution L in moles per litre. (1 mark)
        
        
   • Procedure III
     Measure 10cm³ of solution L into a boiling tube. Wrap the boiling tube with a tissue paper, measure the temperature of the solution and record it in the table below. Place one of the 2cm pieces of metal Z into the hydrochloric acid solution L in the boiling tube. Stir with a thermometer and record the highest temperature attained. Repeat the procedure using the other pieces of metal Z.
     

     Pieces of Metal Z	1st	2nd	3rd
     Highest Temperature(°C)
     Initial Temperature (°C)
     Change in temperature, DT (°C)

     (2 marks)
     1. Calculate the average change in temperature, DT (1 mark)
     2. Calculate the heat change for the reaction between Z and hydrochloric acid. (1 mark)
        (c= 4.2 KJ/kg/K).
     3. Given that the heat of the reaction is 440 kJ per mole of Z, calculate the number of moles of Z used in this reaction. (2 marks)
     4. Calculate the mass per unit length of metal Z. (Z = 24). (2 marks)
2. You are provided with:
   • About 80 cm³ of acidified potassium manganate (VII), solution A
   • Solution M (Retained from question 1).
     
     You are required to determine the effect of temperature on the reaction between potassium manganate (VII) with oxalic acid.
   • Procedure
     Transfer 10cm³ of solution A into five separate test-tubes in a rack. Label the test-tubes 1,2,3,4,5 respectively. Clean the measuring cylinder and use it to measure 10cm³ of solution M into a clean boiling tube. Place the boiling tube in a water bath and heat it to a temperature of 400C. Add the contents of test- tube 1. Start the stop-watch and shake the mixture thoroughly. Record the time taken for the purple colour of the mixture to decolourise.
     Repeat the procedure using solution A from test-tubes 2, 3, 4 and 5 at temperatures of 50°C, 60°C, 70°C and 80°C respectively. Fill the table below.
     

     Temperature of solution M (°C)	40	50	60	70	80
     Time taken for A to decolourise t(secs)
     1/t (sec-1)

     (2 marks)
     1. Plot a graph of 1/t against temperature on the grid below. (3 marks)
     2. From the graph, determine the time taken for decolourisation of the mixture when the temperature of solution M was 65°C. (1 mark)
     3. How does the rate of reaction of potassium manganate (VII) with solution M vary with temperature? (1 mark)
3. You are provided with solids P and Q. Carry out the tests below and write your observations and inferences in the spaces provided.
   1. Place all solid P in a boiling tube. Add about 8cm3 of distilled water.
      1. Dip a glass rod into the boiling tube containing the solution formed. Place it in a non- luminous flame.
         

         Observation	Inference
         (1 mark)	(1 mark)

      2. Describe how you can confirm that the solution contains sulphate ions, using barium chloride solution and dilute nitric (V) acid consecutively.
         

         Test 1	Expected Observation
         (1 mark)	(½ mark)

         
         
         

         Test 2	Expected Observation
         (1 mark)	(½ mark)

      3. Using a portion of the solution, carry out the tests you described in (ii) above.
         

         Observation	Inference
         (1 mark)	(1 mark)

      4. Using about 2 cm³ portion of the solution, add acidified potassium dichromate (VI).
         

         Observation	Inference
         (1 mark)	(½ mark)

      5. Give the possible identity of the anion present in solution of P. (½ mark)
   2.    
      1. Scoop a third of solid Q using a metallic spatula. Ignite it in a non-luminous flame
         

         Observation	Inference
         (1 mark)	(1 mark)

      2. Place the remaining solid Q in a boiling tube. Add 8cm3 of distilled water.. Place about 2cm3 of the solution in a test-tube. Add 2 -3 drops of acidified Potassium Manganate (VII) and warm.
         

         Observation	Inference
         (1 mark)	(½ mark)