Chemistry Paper 3 Questions and Answers with Confidential - Kapsabet Pre Mock Exams 2021/2022

CHEMISTRY 
PAPER 3:PRACTICAL

INSTRUCTIONS

• Write your name and index number in the spaces provided above.
• Sign and write the data of the examination.
• Answer all the questions in the spaces provided.
• You are not supposed to start working with the apparatus for the first 15 minutes of 2¼ hours allowed for this paper. This time is meant to read through the paper and ensure you have all the chemicals and apparatus require.
• All working must be clearly shown
• KNEC mathematical tables and silent electronic calculations may be used.
• All questions should be answered in English

QUESTION 1

You are provided with:

• Solid A 5.0g (COOH)2·H2O
• Solution B 0.13M KMnO4

Task

1. You are supposed to determine the solubility of A at different temperatures.
2. Determine the number of moles of water of crystallization in solid A.

PROCEDURE 1

1. Using a burette, add 4cm3 of distilled water to solid A in a boiling tube.
   • Head the mixture while stirring with the thermometer to about 800C.
   • When the whole solid dissolves, allow the solution to cool while stirring with the thermometer
   • Note the temperature at which crystals first appear and record this temperature in the table 1 below.
2. Using a burrete add 2cm³ more into the content of the boiling tube and warm until the solid dissolve.
   • Remove from the flame and allow the solution to cool in air while stirring.
   • Record the temperature at which crystal first appear in table 1.
   • Repeat procedure (b) 3 more times and complete table 1 below.
   • Retain the content of the boiling tube for procedure II

Table 1

1.          
   1. Draw a graph of solubility of solid A (vertical axis) against temperature (3mks)
   2. From your graph determine the solubility of solid A at 600C (1mk)
      
      PROCEDURE II     
      1.              
         • Transfer the contents of the boiling tube into a 250ml volumetric flask.
         • Add distilled water up to the mark
         • Label this solution A
      2.           
         • Using a clean pipette and a pipette filler, transfer 25ml of solution A into a conical flask.
         • Warm the mixture up to 60ºC
         • Fill a burette with solution B
         • Titrate B against the hot solution A until a permanent pink colour persist
         • Read your results in Table 2 below
      3. Repeat (b) 2 more times are record your results in the table 2 below.
         
         Table 2
         

         	I	II	III
         FINAL BURETTE READING
         INITIAL BURETTE READING
         VOLUME OF SOLUTION B USED (CM3)

2.           
   1. Calculate the average volume of solution B used (1mk)
   2. Calculate the number of moles of B used (1mk)
   3. Given 2 moles of KmNO₄ react with 5 moles of A, calculate the number of moles of A in 25cm³ (1mk)
   4. Calculate the molarity of A (1mk)
   5. Determine the molar mass of A (1mk)
   6. Determine the value of X (1mk)
      (C=12, O=16 H=1)

QUESTION 2
You are provided with solid C. Use it to carry the test below. Dissolve the whole of C into 10cm3 of water and divide it into five portions.

1. To the 1st portion add sodium sulphate solution.
   

   Observations	Inferences
   (1mk)	(1½mk)

2. To the 2nd portion add Ammonia solution dropwise until in Excess.
   

   Observations	Inferences
   (1mk)	(1mk)

3. To the 3rd portion add sodium Hydroxide dropwise until in Excess.
   

   Observations	Inferences
   (1mk)	(1mk)

4. To the forth portion add Lead (II) Nitrate solution
   

   Observations	Inferences
   (½mk)	(2mk)

5. To the last portion add Barium Nitrate solution
   

   Observations	Inferences
   (1mk)	(1mk)

QUESTION 3

You are provided with liquid D use it to carry the test below. Divide liquid D into four equal portions

1. To the 1st portion add sodium hydrogen carbonate
   

   Observations	Inferences
   (1mk)	(1mk)

2. To the 2nd portion add acidified potassium manganite (VII) (KmNO₄)
   

   Observations	Inferences
   (1mk)	(1mk)

3. To the 3rd portion add Bromine water
   

   Observations	Inferences
   (1mk)	(1mk)

4. To the last portion add potassium dichromate(VI) and warm.
   

   Observations	Inferences
   (1mk)	(1mk)

CONFIDENTIAL
In addition to the equipment and fittings found in a chemistry laboratory. Each candidate should be provided with;

1. Solid A 5.0g measured accurately
2. About 80cm³ of solution B
3. About 0.5g solid C
4. About 10cm3 of liquid D
5. A thermometer (-10-110ºC)
6. A burette
7. A complete retort stand
8. A pipette and a pipette filler
9. 2 conical flasks
10. A 250ml volumetric flask
11. One boiling tube
12. Five (5) test tubes
13. 0.5g sodium hydrogen carbonate
14. Two labels

ACCESS TO:

1. Means of heating (Tripond stand and wire gauze)
2. Sodium sulphate solution (NaSO4)
3. Ammonia solution 2m
4. 2m Sodium Hydroxide
5. Lead Nitrate solution
6. Barium Nitrate solution
7. Acidified potassium manganite (VII) solution
8. Bromine water
9. Acidified potassium dichromate(VI) solution

NB:

1. Solid A is 5.0g of oxalic acid (COOH)₂ 2H₂O
2. Solution B is KmnNO₄
3. Solid C is magnesium chloride MgCl₂
4.  Liquid D is absolute ethanol

Preparations

1. Solution B is made by dissolving 20g of solid KmNO₄ in 200cm³ of 2.0m H₂SO₄ and toping to 1000cm³ by distilled water.
2. Sodium Hydroxide is prepared by dissolving 80g of NaOH pellets in 600cm³ of distilled water and top to 1000cm3 with distilled water.
3. Ammonia solution is prepared by dissolving 150ml of conc ammonia to 600cm³ of distilled water then top to the mark.
4. Barium Nitrate is prepared by dissolving 26g of solid Barium Nitrate in 600cm³ of water then topping to 1000cm³ with distilled water.
5. Lead nitrate is prepared by dissolving 30g of solid Lead Nitrate in 600cm³ of water then topping to 1000cm³ with distilled water.
6. Sodium Sulphate is prepared by dissolving 14.2g of solid sodium sulphate in 600cm³ of distilled water then topping up to 1000cm3 with distilled water.
7. Acidified Kmno4 is prepared by dissolving 3.2g of solid Kmno4 in 200cm³ of 2.0m H₂SO₄ acid then topping with distilled water to 1000cm³.
8. Acidified K2Cr₂O₇ is prepared by dissolving 25g of solid K2Cr₂O₇ in 200cm³ of 2.0m H₂SO₄ then topping to 1000cm³ with distilled water.

MARKING SCHEME

QUESTION1

TABLE 1

 AWARD a total of 7 MKS Distributed as follows.

1. Complete table – 4mks
   Condition and penalties
   • A table with 8-10 values award 4
   • A table with 6-7 values a ward 3
   • A table with 4- 5 values award 2
   • A table with 2-3 values award 1
   • A table with 1 value award ½
   • A table with no value a ward 0
     
     Penalties
     1. Penalize ½ mk for each wrong value of solubility
     2. Penalize ½ one for unrealistic temperature readings ½ above 90ºC and below 10ºC
        
        
2. DECIMALS TIED TO TEMPARATURE -1Mk
   Accept;
   1. Whole numbers
   2. 1 decimal place where the decimal should be 0 or 5 i.e 70.0 or 70.5
   3. Accept 2 decimal places where it should be .00, .25,.50 or .75
      
      NB: If no consistence penalize fully.
      
      
3. TREND – Tied to temperature i.e it should be decreasing (1mk)
4. Accuracy – tied to temperature when 4cm3 of water was added
   Accept 20C of the school value.
5. GRAPH – award a total of 3mks distributed as follows.
   1. Labeling (½mks)
      • Both axis should be labeled if one is not or wronglylabeled award O
      • Ignore units but if indicated they should be right otherwise penalize fully
   2. Scale – (½mks)
      • The actual plotting should be halfof the page
      • Peralize fully if scale changes on the way both axis must be correct.
      • Plots (1mk)
      • 4-5 correct plots award (1mk)
      • 3 correct plots award (½mks)
      • Less than 3 correct plots award 0
   3. Line – a smooth curve passing through 3 or more correct plots award 1mk
      • Otherwise award 0
   4.            
      •  award ½ mk for sloping or the graph
      • award ½ mk for calculating
        NB:
        1. Award fully for calculation from correct graph even if not shown on the graph
        2. Reject any value from a wrong graph.
           

TABLE 2

 
Award a total of 5mks distributed as follows

1. Complete table (1mk)
   • Complete table with 3 titration 1mk
   • Incomplete table with 2 titrations ½ mk
   • Incomplete table with 1 titration 0 mks
     
     Penalties
     1. Wrong arithmetic
     2. Invented table
     3. Unrealistic values i.e burette reading with more than 50cm³ and less than 1cm³ withoutexplanation.
     4. Unrealistic titre values
        
        NB: penalize ½ once
        
        
2. Use of decimals (1mk) (Tied to the 1st and 2nd row only)
   Accept 1 or 2 decimal places used consistently otherwise penalize fully
   • If 2 dp are used the 2nd should be a “O” OR “5” e.g 20.10 or 20.15 otherwise penalize fully
   • Accept the use of Zero as the initial burette reading i.e 0,0.0 or 0.0
3. Accuracy (1mk)
   • Complete the candidate value with the school value (S.V)
     
     1. If within 0.1 of the school value award 1mk
     2. If within 0.2 of S.V ward ½mk otherwise award 0
        
        NB: Tick the candidate value that deserves a credit
        
        
4. PRINCIPLE OF AVERAGING ------------------------1MK
   Conditions
   1. If 3 titration done but only two are consistence and averaged award 1mk
   2. If 3 titration are done and consistency and averaged award 1mk
   3. If two titration are done and are consistency and averaged award 1mk
   4. If three consistency titration one done but 2 are averaged award 0
   5. If three 3 titration are done and are inconsistence and are done averaged award zero
   6. If two titration are done and are inconsistence and are averaged award 0

PENALTIES

1. Penalize ½ for wrong arithmetic
2. penalize ½mk if no working is shown and answer is correct
3. penalize fully if no working is shown and answer given is wrong
4. Accept rounding off or truncation to the 2nd d.p
   e.g 12.666 12.67
   or
   12.66 12.66
   
   NB:
   1. The working of average must be marked before the mark for averaging is award in table 2.
   2. Accept the average volume if it work out exactly to a whole number.
      
      

1. FINAL ACCURACY ----------------------(1MK) Tiled to correct average time.
   Compare the candidate average time to the school value.
   1. If within0.1 award 1mks
   2. If with n 0.2 award ½ mk
   3. Otherwise award 0
      
      NB:
      • If there are 2 possible correct average titre, use the one the one close to the school value and award accordingly.
      • If wrong value are averaged, pick the correct values average for the candidate and award accordingly.
      • Record the marks as follows besides the table to the right.
        CT – 1mk
        D - 1mk
        A -1mk
        PA -1mk
        FA - 1mk
        Total 05 mks
        
        
2. Calculate the number of moles of B used
   1000 - 0.13
   12.5 ?
   = 12.5×0.131000 √ ½
   0.001625 moles √ ½
   = 0.0040625 √ ½
   
   
3. Ans b×52
   = 0.0040625 √ ½
   
   
4. Ans c×100025 √ ½
   = 0.1626m √ ½
   
   
5. 5g 250cm3
   20g – 1000
   1×200.1625
   = 123.07 √ ½
   
   
6. 90 + 18x =123
   18x = 33
   x= 3318
   x= 1.83
   = 2

QUESTION 2

1. To the 1st portion add sodium sulphate solution.
   

   Observations	Inferences
   No white precipitate formed (1mk)	Ba2+ Ca2+ and Pb2+ Absent Each ½ mks Penalize ½mk to a maximum of 1 ½ mks for any contradictory ion

2. To the 2nd portion add Ammonia solution dropwise until in Excess.
   

   Observations	Inferences
   No white precipitate insoluble in excess NB: White precipitate ½ mk Insoluble in excess ½ mk	Zn2+ absent (1mk) Penalize 1mk for each contradicting ion to a maximum of (1mk)

3. To the 3rd portion add sodium Hydroxide dropwise until in Excess.
   

   Observations	Inferences
   A white precipitate ½ Insoluble in excess ½	Mg2+ present (1mk) Accept Al3+ absent for ½mk Panelize 1mks for any contracting ion to a Maximum of 1mks

4. To the forth portion add Lead (II) Nitrate solution
   

   Observations	Inferences
   A white precipitate is formed ½ mks	Cl-, So32- , SO4 2- and CO32- present – ½ mk each Penalize ½mk for any contradictory ion to a maximum of

5. To the last portion add Barium Nitrate solution
   

   Observations	Inferences
   No white precipitate formed	CI- Present (1mks) -accept SO42- AND SO32- OR CO32- absent for fully marks penalize 1mks for any contradictory ion to a maximum of 1mks Three anions given – 1mk Two anions given -½mk One onion given – 0mk

QUESTION 3

1. To the 1st portion add sodium hydrogen carbonate
   

   Observations	Inferences
   No fizzing/bubbling /hissing (1mk) Reject -fissiling -Sizzling	R- CooH Absent (1mk) NB: Ignore H3O+ & H+

   
   
   
2. To the 2nd portion add acidified potassium manganite (VII) (KmNO₄)
   

   Observations	Inferences
   KMNO4 get decolorized Or KMNO4 turns from purple to colorless (1mk) Reject The solution turns colourless It turns colourless	C= C OR –C= C- (½mk) Present R- OH Present (½mks) NB: Penalize ½mk for any contradictory group to a maximum of

   
   
   
3. To the 3rd portion add Bromine water
   

   Observations	Inferences
   Yellow /orange bromine water does not get decolonized Accept It remains yellow or orange	or –C C- Absent Penalize 1mk for any contradictory group to a maximum of

   
   
   
4. To the last portion add potassium dichromate(VI) and warm.
   

   Observations	Inferences
   K2CrO7 turns from orange to green	R-OH Present (1mk) Penalize 1mk for any contradictory group to a maximum of