Chemistry Paper 3 Questions with Confidential - Mokasa 1 Joint Pre Mock Exams 2023

• Answer all the questions in the spaces provided below each question.
• KNEC Mathematical tables and silent electronic calculators may be used.
• All working must be clearly shown where necessary.
• Candidates should answer the questions in English.
• Candidates should take the first 15 minutes to go through the instructions.

1. You are provided with:
   5.0g of solid G
   You are required to determine:
   The solubility of solid G at different temperatures
   PROCEDURE I
   1. Fill the burette with distilled water. Put all solid G in a clean boiling tube and add 4cm³ of distilled water using the burette. Heat the mixture using a non-luminous flame while stirring with the thermometer to about 78°C until all the solid dissolves.
   2. Allow the solution to cool while stirring with the thermometer and note the temperature at which crystals of G first appear. To ensure faster cooling, carefully run a continuous stream of tap water over the wall of the boiling tube. Record the temperature at which crystals of G start to appear in Table 1 below.
   3. Using the burette, add 2cm³ of distilled water to the contents of the boiling tube. Heat the mixture again until all solid G dissolves. Allow the solution to cool while stirring with the thermometer and note the temperature at which crystals first start to appear. Record the temperature in Table 1 below.
   4. Repeat procedure III three more times with different volumes of distilled water as indicated in Table 1 and complete the table.
      
      Table 1
      

      Volume of distilled water in the boiling tube (cm3)	Temperature at which crystals of solid G appear (°C)	Solubility of solid G in g/100g of water
      4
      6
      8
      10
      12

      (3½ marks)
      1. Complete the table by calculating the solubility of solid G in g/100g of water. (2½ marks)
      2. On the grid provided, plot a graph of solubility of G against temperature. (3 marks)
      3. Using the graph, determine:
         1. The temperature at which 100g of solid G would dissolve in 100cm³ of water. (1 mark)
         2. The solubility of solid G at 55°C. (1 mark)
         3. Concentration of the solution in moles per litre at 60°C. (Relative Molecular Mass of solid G is 126) (2 marks)
2. You are provided with:
   Solid P - 2.6g of anhydrous metal carbonate, X₂CO₃
   Solution Q – 0.5M hydrochloric acid
   You are required to determine:
   The Relative Atomic Mass of X
   PROCEDURE
   1. Using a measuring cylinder, measure 100cm³ of distilled water into a 200ml beaker. Add all solid P and stir until it dissolves. Label this solution P.
   2. Fill the burette with solution Q.
   3. Pipette 25cm³ of solution P into a clean 250ml conical flask. Add 2 drops of methyl orange indicator. Titrate against solution Q until a permanent pink colour.
   4. Repeat procedure III and complete table 2 below.
      
      Table 2
      

      Experiment	1	2	3
      Final burette reading (cm3)
      Initial burette reading (cm3)
      Volume of solution Q (cm3)

      (4 marks)
      1. Determine the average volume of solution Q used in cm3. (1 mark)
      2. Calculate:
         1. The number of moles of solution Q used. (1 mark)
         2. The number of moles of solution P in 25cm³ of solution. (2 marks)
         3. The number of moles of solution P in 100cm³ of solution. (1 mark)
         4. The relative formula mass of solid P, X₂CO₃.                        (1 mark)
         5. The relative atomic mass of X. (C = 12, O = 16) (1 mark)
3.  
   1. You are provided with solid R. Carry out the tests below. Write your observations and inferences in the spaces provided.
      1. Scoop a spatula endful of solid R and place in a test tube. Heat the solid gently.
         

         Observation	Inferences
         (1 mark)	(1 mark)

      2. Place the remaining solid in a boiling tube and add about 10cm³ of distilled water. Shake the solution. Divide the solution into four portions.
         1. To the 1^st portion, dip both blue and red litmus papers.
            
            

            Observation	Inferences
            (1 mark)	(1 mark)

         2. To the 2^nd portion, add 2 – 3 drops of barium nitrate and shake.
            
            

            Observation	Inferences
            (1 mark)	(1 mark)

         3. To the 3^rd portion, add sodium hydroxide dropwise till in excess.
            
            

            Observation	Inferences
            (1 mark)	(1 mark)

         4. To the 4th portion, add ammonia solution dropwise till in excess.
            
            

            Observation	Inferences
            (1 mark)	(1 mark)

   2. You are provided with solid T. Carry out the tests below and record your observations and  inferences in the spaces provided.
      1. Scoop a third of solid T using a clean spatula and ignite using a Bunsen burner.
         
         

         Observation	Inferences
         (1 mark)	(1 mark)

      2. Place the remaining solid in a boiling tube and add about 5cm³ of distilled water. 
         1. To about 2cm³ of the solution in a test tube, add 2 drops acidified potassium manganate (VII) and warm.
            
            

            Observation	Inferences
            (1 mark)	(1 mark)

         2. To about 2cm³ of the solution in a test tube, add a spatula endful of sodium hydrogen carbonate.
            
            

            Observation	Inferences
            (1 mark)	(1 mark)

CONFIDENTIAL

Together with the usual laboratory fittings, each student should be provided with;

• Solid G – 5.0g weighed accurately in a stoppered container
• Solid P – 2.6g weighed accurately in a stoppered container
• Solid R – About 2.0g in a stoppered container
• Solid T – About 2.0g in a stoppered container
• Solution Q – About 120cm³
• One 50 ml burette with a clamp and stand
• One 25 ml pipette
• Two 250 ml conical flasks
• One 200 ml beaker
• Distilled water in a wash bottle
• Two boiling tubes
• Six test tubes in a test tube rack
• One thermometer (-10°C to 100°C)
• One label
• Source of heat
• One metallic spatula
• One red and one blue litmus paper

Access to;

• Methyl orange indicator
• 0.1M Barium nitrate solution
• 2M Sodium hydroxide solution
• 2M Ammonia solution
• Acidified potassium manganate (VII)
• Sodium hydrogen carbonate

NOTES:

• Solid G is Oxalic acid
• Solid P is anhydrous Sodium carbonate
• Solid R is Aluminium sulphate
• Solid T is Maleic acid
• Solution Q is 0.5M Hydrochloric acid