Chemistry Paper 3 Questions and Answers - Catholic Diocese of Kakamega Mock Exams 2023

INSTRUCTIONS TO CANDIDATES:

• Answer all the questions in the spaces provided.
• Mathematical tables and electronic calculators may be used for calculations.
• All workings must be clearly shown where necessary

1.        You are provided with:
   
   • Solution P of Potassium manganate (VII).
   •  0.05M solution Q of oxalic acid.
   • Solution R containing 4.9g of ammonium iron (II) Sulphate, (NH₄)₂SO₄.FeSO₄.6H₂O, in 250cm³ of solution.
     
     You are required to:
     
     1. Determine the rate of reaction between oxalic acid and Potassium manganate (VII).
     2.  Standardize the solution P.
        PROCEDURE I:
        Using a 10ml measuring cylinder, place 1 cm³ of solution P into each of the five (5) test-tubes in a rack. Clean the measuring cylinder and use it to place 19 cm³ of solution Q into a boiling tube. Prepare a water bath by placing about 200 cm³ of water into a beaker and start to heat. Place a thermometer into solution Q and place it in the warm water until it attains a temperature of 40°C. Remove the boiling tube from the water - bath and place it in the test-tube rack. Add the first portion of solution P immediately and at the same time start a stop watch. Record the time taken for solution P to be decolourised in table I below. Repeat the procedure at temperatures of 50°C, 60°C, 70°C and 80°C to complete the table.
        

        Temperature of solution Q (°C)	40	50	60	70	80
        Time taken for decolourisation (seconds)
        1/t (sec-1)

        (5 marks)
        
        1. Plot a graph of 1/t against temperature (X-axis).
           (3marks)
        2. From the graph determine the time taken for the mixture to decolourise at 65°C
           (2marks)
        3. How does the rate of reaction between oxalic acid and Potassium manganate (VII) vary with temperature?
           (1mark)

PROCEDURE II
Fill a burette with solution P. Pipette 25cm³ of solution R into a conical flask and titrate the solution P against solution R until a permanent pink colour just appears. Record your results in table II below and repeat the procedure to fill the table.

(4marks)

1. Determine the average volume of P used
   (1mark)
2. Calculate the concentration of solution R in moles per litre. (Fe=56, S=32, O=16, N=14, H=1).
   (2marks)
3.  Find the number of moles of solution R used
   (1mark)
4. Given the ionic equation for the reaction is
   5Fe²⁺(aq) + MnO^-₄ (aq) + 8H⁺(aq) → 5Fe³⁺(aq) + Mn²⁺(aq) + 4H₂O(1)
   Find the number of moles of solution P used.
   (1 mark)
5. Determine the concentration of the Potassium manganate (VII), solution P in moles per litre. (1 marks)

2. You are provided with solid E, carry out the tests below and record your observations an inferences in the spaces provided
   
   1. Place all the solid E provided into a test tube. Add about 6 cm³ of distilled water and shake thoroughly. Filter and rinse the residue thoroughly with distilled water. Keep the Residue for use in procedure (c). Divide the filtrate into four portions.
      
      1. To the first portion add sodium hydroxide solution drop wise until in excess.
         

         Observations	Inferences
         (1⁄2 mark)	(1 mark)

      2. To the second portion dip a clean glass rod and hold its tip in the non-luminous Bunsen burner flame.
         

         Observations	Inferences
         (1⁄2 mark)	(1 mark)

      3. To the third portion add two drops of barium nitrate solution
         

         Observations	Inferences
         (1⁄2 mark)	(1 mark)

      4. To the fourth portion add two drops of acidified potassium manganate (VII)
         Observations
         

         Observations	Inferences
         (1⁄2 mark)	(1 mark)

   2. Put the residue in a boiling tube and add about 5 cm3 of dilute nitric (V) acid provided and shake thoroughly. Keep the resulting solution for the tests below.
      

      Observations	Inferences
      (1⁄2 mark)	(1 mark)

      
      Divide the solution into two equal portions.
      
      1. To the first portion add sodium hydroxide solution dropwise until in excess.
         

         Observations	Inferences
         (1⁄2 mark)	(1 mark)

      2. To the second portion add two drops of potassium iodide solution.
         

         Observations	Inferences
         (1⁄2 mark)	(1 mark)

3. You are provided with solid F.
   Carry out the following tests and record your observations and inferences in the spaces provided.
   
   1. Using a metallic spatula, take one-third of solid F and ignite it using a Bunsen burner flame.
      

      Observations	Inferences
      (1⁄2 mark)	(1 mark)

   2. Place the remaining solid F in a boiling tube. Add about 10 cm³ of distilled water. Shake the mixture until all the solid dissolves.
      
      1. To about 2cm³ of the solution, add 2 to 3 drops of bromine water. Warm the mixture.
         

         Observations	Inferences
         (1⁄2 mark)	(1 mark)

      2. To about 2cm3 of the solution, add 2 to 3 drops of acidified potassium manganate VII.
         

         Observations	Inferences
         (1⁄2 mark)	(1 mark)

      3. To about 2cm3 of the solution, add 2 to 3 drops of acidified potassium dichromate VI. Warm the mixture.
         Observations
         

         Observations	Inferences
         (1⁄2 mark)	(1 mark)

      4. To about 2cm3 of the solution, add a small amount of solid sodium hydrogen carbonate.
         

         Observations	Inferences
         (1⁄2 mark)	(1 mark)

MARKING SCHEME

1. TABLE 1
   
   1. COMPLETE TABLE
      (1mark)
      Awarded as Follows
      • All the five experiments done and time recorded award 1mark
      • 3-4 experiments done and time recorded award 1⁄2 marks
      • Less than 3 experiments done and recorded award Omark.
   2. CORRECT USE OF DECIMALS
      (1mark)
      
      • Award 1mark for time given to WHOLE numbers otherwise penalize FULLY.
        (Award Omark)
   3. ACCURACY......
      (1mark)
      • Compare the student's 1st time reading and the school value and award as follows
      • Award Imark if the student's value is in the range of ±2 units
      • Award 1⁄2 if the student's value is in the range of± 4 units otherwise award (Omark)
   4. TREND....
      (1mark)
      
      • Award Imark for a continuous drop in temperature, otherwise Award Omark 
   5. CORRECT ARITHMATIC
      (1mark)
      • Award 1mark for a correct arithmetic of finding 1/t for the five tests
      • Award 1⁄2 marks for 3-4 arithmetic are done correctly. Otherwise award(0 mark)
        

        Temp (°C)	40	50	60	70	80
        Time (sec)	74	43	22	11	7
        1/t (sec-1)	0.0135	0.0232	0.0455	0.0909	0.1428

1. GRAPH....3MARKS
   
   1. AXES 
      Conditions
      • Award 1⁄2 mark if all axes are correctly labeled
      • Units may be included or not. If units are included must be correct, otherwise award Omark for axes
   2. SCALE
      
      • Correctly plotted points MUST cover 3/4 of the grid provided for
      • The scale MUST be linear in both axes otherwise award 0 mark for the wrong scale. 
   3. PLOTTING - 1 MARK
      
      • 5 points plotted correctly
      • 3-4 points plotted correctly
      • 1-2 points plotted ....
        NOTE
        Award 0 mark for points plotted on a wrong scale.
   4. CURVE - 1 MARK
      
      • Award 1 mark for a curve drawn to correctly plotted points.
      • It MUST be a curve otherwise award 0 mark.
        
        1. The shape is as shown below.
           
           
           
        2. I/t= 0.06 (1mk) (For showing on the graph)
           t=   1   
               0.06
           = 16.67 seconds (1/2mk) (For the working- finding the reciprocal)
        3. Rate of reaction increase with increase in temperature / rate of reaction almost double with every 10°C rise in temperature.
           
           

TABLE 2
Table II

Marking points

1. Complete table (CT)  
   (Imark)
   
   • The table should be completed.
   • Penalize 1⁄2 marks for the following errors if any occurs ONCE.
   • Arithmetic error in subtraction.
   • Values recorded beyond 50cm3
   • Inversion of table
   • Penalize 1⁄2 mk only on any one of these errors.
2. Decimal point (d.p)
   (Imk)
   • All values to be recorded to ldp or
   • All values to be recorded to 2dp second decimal value being 0 or 5 only
   • Award 0 -mark if whole numbers used or more them 3dp are used on inconsistency in the number of d.p 
3. Accuracy mark (AC)
   (Imark)
   • Consider any one candidates' titre if within ±0.10cm3 of school value award (Imk)
   • If it is ± 0.11 to 0.20 award 1⁄2 mark. If beyond 0.20cm3 award (1mark)
4. Principle of Averaging (P.A)
   (1 mark)
   
   • Three titres to be averaged if within ±0.2cm3 to one another.
   • Two titres can only be arranged if they are consistent.
   • N/B- If a student averages two titres when three are consistent award Omk.
   • If a student averages three inconsistent values, award 0 mark
5. Final answer (F. A)
   (1Mark)
   
   • If averaged titre is within 0.0 to 0.10cm3 of S. V award Imk
   • If within 0.11 to 0.2cm3 of s.v award
   • If beyond 0.20cm3 award Omk.
     Summary
     Complete table (CT) = 1 mk
     Correct use of decimals (dp) = 1mk
     Accuracy (AC) = 1mk
     Averaging (PA) = 1mk
     Final answer (FA) = 1mk
                                     5mks
     N/B -For school value (SV), teacher to perform practical to obtain school value
     CALCULATIONS
     Imk
     
     1. Marked on the table
     2. Molarity
        = ^19.6/₃₉₂  (1mk)
        = 0.05 (1mk)
     3. Moles of R used
        0.05 x 25
           1000
        = 0.00125
     4. Moles of P used
        = Ans (iii)
               5
        = Correct Ans.
     5. Molarity of solution P =  Ans(iv)  x 1000
                                              sans (i)
                                         = correct Ans 
        
        Conditions
        
        • Penalize 1⁄2 mark for correct answer not given to at least 4 dp in (iii) & (iv) to at least 4dp in (ii) & (v) unless it divides to less than stated number of decimal places.
        • Ignore units if not written. However, if written must be correct otherwise award 0 mark for correct
          answer.

QUESTION 2
You are provided with solid E

1. Add sodium hydroxide solution dropwise until in excess
   

   Observations	Inferences
   No white ppt (1⁄2mk)	Pb2+, Zn2+,Al3+ ,Ca2+ and Mg2+ions absent  Only 3 - 4correct...(1⁄2mk) Only 1-2 correct..... (0 mk) Ignore sodium ions present

   (Imk)
2. To the second position dip a clean glass rod and hold its tip in the non-luminous Bunsen burner flame.
   

   Observations	Inferences
   Yellow flame	Na+ present (Ignore unsaturation.)

3. To the third portion add two drops of barium nitrate solution
   

   Observations	Inferences
   White ppt (1mk)	SO2-4 ions, CO2-3, SO2-3 present(1mk) Only 2 correct (1⁄2mk) Only 1 correct (0 mk) Penalize 1/2 mark for any contradictory ion upto max of 1 mk.

4. To the fourth portion add two drops of acidified potassium manganate (VII)
   

   Observations	Inferences
   Purple potassium manganate (VII) is decolourised (1mk)	SO2-3 present (1mk) Penalize fully for any contradictory ion

b. Put the residue in a boiling tube and add about 5 cm3 of dilute nitric (V) acid provided and shake thoroughly.

Divide the solution into two equal portions.

1. To the first portion add sodium hydroxide solution dropwise until in excess
   

   Observations	Inferences
   White ppt soluble in excess (1mk)	Pb2+,Zn2+  (1mk) Only 2 correct (1/2mk) Only 1 correct (0mk) Penalize fully for any contradictory ion. upto a max of (1mk)

2. To the second portion add two drops of potassium iodide solution.
   

   Observations	Inferences
   Yellow ppt (1/2mk)	Pb2+ present (1/2mk) Penalize fully for any contradictory ion. upto a max of (1mk)

QUESTION 3
You are provided with solid F. 
Carry out the following tests and record your observations and inferences in the spaces provided.

1. Using a metallic spatula, take one-third of solid F and ignite it using a Bunsen burner flame.
   

   Observations	Inferences
   Burns with a yellow sooty/smoky flame (1/2mk)	Unsaturated organic compound/hydrocarbon      present

2. Place the remaining solid F in a boiling tube. Add about 10 cm³ of distilled water. Shake the mixture until all the solid dissolves.
   
   1. To about 2cm³ of the solution, add 2 to 3 drops of bromine water. Warm the mixture.
      

      Observations	Inferences
      Yellow bromine water is decolourised (1/2 mk)

   2. To about 2cm³ of the solution, add 2 to 3 drops of acidified potassium manganate VII. Warm the mixture.
      

      Observations	Inferences
      Acidified potassium manganate (vii) changes from purple to colourless (decolourised) (1mk)	R - OH

   3. To about 2cm³ of the solution, add 2 to 3 drops of acidified potassium dichromate VI. Warm the mixture.
      

      Observations	Inferences
      Orange acidified potassium dichromate remains orange (1mk)	R - OH

   4. To about 2cm³ of the solution, add a small amount of solid sodium hydrogen, carbonate.
      

      Observations	Inferences
      Effervescence/bubbles (1mk)	R - COOH