Chemistry Paper 3 Questions, Answers and Confidential - Bunamfan Post Mock 2021 Exams

Instructions to Candidates

1. Mathematical tables and silent electronic calculators may be used.
2. All working MUST be clearly shown where necessary.

For Examiner’s Use Only

Question	Maximum Score	Candidate’s Score
1	20
2	10
3	10
TOTAL SCORE	40

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QUESTIONS

1. You are provided with:
   • 2 g of an alkanoic acid labelled solid A in a boiling tube.
   • 2 M sodium hydroxide solution labelled solution B.
              You are required to:
     1. Determine the solubility of solid A at different temperatures.
     2. Find the molar mass of the alkanoic acid.
                 Procedure 1
        1. Using a burette, add 10cm³ of distilled water to solid A in the boiling tube. Heat the mixture while stirring with the thermometer to about 75 ⁰C.  When the entire solid had dissolved, allow the solution to cool while stirring with the thermometer. Note the temperature at which crystals of solid M appear.  Record this temperature in table I.
        2. Using the burette, add 2cm³ of distilled water to the contents of the boiling tube.  Warm the mixture while stirring with the thermometer until all the solid dissolves.  Allow the mixture to cool while stirring.  Note the temperature at which crystals of solid A appear.
        3. Repeat procedure (ii) two more times and record the temperatures in table I. 
           Retain the contents of the boiling tube for use in procedure II.
           1.                    
              1. Complete table I by calculating the solubility of solid A at different temperatures. (6 marks)
                 Table I
                 

                 Volume of water in the boiling tube (cm3)	Temperature at which crystals of solid A appear (0C)	Solubility of A (g/100g water)
                 10   12   14   16

              2. On the grid provided, plot a graph of the solubility of solid A against temperature. (3 marks)
                 
              3. Using the graph determine the temperature at which 52 g of solid A would dissolve in 100cm³ of water. (1 mark)     

Procedure II

1. Transfer the contents of the boiling tube in procedure I into a 250ml volumetric flask.  Rinse both the boiling tube and the thermometer with distilled water and add it to the volumetric flask.  Add more distilled water to make up to the mark.  Transfer the solution into a 250ml beaker.  Label this solution E.  Rinse the volumetric flask with distilled water ready for use in step (ii).
2. Using a measuring cylinder, place 25cm³ of solution B into a 250ml volumetric flask.  Add about 200cm³ of distilled water and shake well.  Add more distilled water to make up to the mark.  Label this solution F.
3. Fill the burette with solution E. Using a pipette and a pipette filter, place 25cm³ of solution F into a conical flask.  Add 2 – 3 drops of phenolphthalein indicator and titrate with solution E.  Record your results in table II. Repeat the procedure (iii) two more times to complete the table.

Table 2

Titration	1	2	3
Final burette reading (cm3)
Initial burette reading (cm3)
Volume of solution E used (cm3)

(4 marks

Determine:

1. Average volume of solution E used. (1 mark)
2. Concentration of solution F in moles per litre (1 mark)
3. Number of moles in 25cm³ of solution B (1 mark)
4. Moles of alkanoic acid, solution E used (1 mark)
   ( 1 mole of acid reacts with 2 moles of base)
5. Concentration of solution E in moles per litre (1 mark)
6. Relative formula mass of the alkanoic acid, solid B.(1 mark)

2. You are provided with solid E, carry out the tests below and record your observations and inferences in the spaces provided

1. Place all the solid E provided into a test tube. Add about 6cm³ 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 dropwise until in excess

Observations	Inferences
( ½  mark)	(1 mark)

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

Observations	Inferences
(1 mark)	(1 mark)

iii) To the third portion add two drops of barium nitrate solution

Observations	Inferences
(1 mark)	(1 mark)

iv) To the fourth portion add two drops of acidified potassium manganite (VII)

Observations	Inferences
(1 mark)	( 1 mark)

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

Observations	Inferences
( ½  mark)	( ½  mark)

Divide the solution into two equal portions.

i) To the first portion add sodium hydroxide solution dropwise until in excess

Observations	Inferences
( 1 mark)	(1 mark)

ii) To the second portion add two drops of sodium iodide solution.

Observations	Inferences
( ½  mark)	(1 mark)

3. You are provided with liquid L in stoppered container.  Carry out the tests and record your observations and inferences.
a) Place about 3 drops of liquid L on a watch glass and ignite using a Bunsen burner flame.   

Observations	Inferences
(1 mark)	(1 mark)

b). Divide the remaining liquid L into four portions in test tubes.
i). To the first portion, add about 6cm³ of distilled water and shake well. 

Observations	Inferences
( ½   mark)	( ½   mark)

ii) To the second portion, add the sodium hydrogen carbonate solid provided.

Observations	Inferences
(½   mark)	( 1 mark)

iii). To the third portion, add two drops acidified potassium manganite (VII)

Observations	Inferences
( ½    mark)	( 1 mark)

iv). To the last portion, add two drops acidified potassium dichromate (VI) solution.

Observations	Inferences
( 1 mark)	( 1 mark)

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MARKING SCHEME

1. Procedure 1: Table 1
   1. Complete table……………………………………………………… (1mk)
      (Tied to 1st column of temperature readings only)
      • Conditions
        • Complete table with 4 readings (1mk)
        • Incomplete table with 2 – 3 readings ( ½ mk)
        • Incomplete table with less than 2 readings ( 0 mks)
      • Penalties
        • Penalise ½ mk once for unrealistic temperature readings i.e. below 20ºC as initial reading.
        • Penalize ½ mk if temperature readings are all the same.
   2. Use of decimals ………………………………………………… (1mk)
      (tied to 1st column only)
      • Conditions
        • Award 1mk for temperature readings given as whole numbers consistently.
        • Award 1mk for temperature readings given to 1 d.p of .0 or .5 consistently.
        • Award 1mk for temperature readings given to 2 d.p of .00, .25, .50 or .75 consistently
          NB: Penalise fully if none of the above conditions are met.
   3. Accuracy - (tied to 1st reading only)………………………. (1mk)
      Award 1mk if within + 20C to S.V otherwise penalize fully
   4. Trend …………………………………… …………………….………..(1mk)
      Award 1mk if continuous drop otherwise penalize fully
   5.                                  
      1. Solubility …………………………………………………………….. (2mks)
         ( tied to 2nd column)
         Award ½ mk for each reading
         Condition/penalties
         Penalise 1/2mk once for value given to less than 1 dp unless it works out exactly
      2. Graph ……………………………………. (3mks)
         • labeling of axes …………………………………………..½ mk
           penalize fully for inverted axes
         • scale ……………………………………………………( ½ mk)
           Award ½ mk if plots occupy atleast half of the grid
         • Plotting ………………………………………………..(1mk)
           3 or 4 points correctly plotted - (1mk)
           2 points correctly plotted - ( ½ mk)
           Less than 2 points - ( 0mks)
         • Curve…………………………………………….. ……….(1mk)
           Smooth curve of best fit otherwise penalize fully
      3. Correct showing on graph - ( ½ mk)
         correct reading - ( ½ mk)

Procedure II: Table II

1. Complete table ……………………………………………………………….(1mk)
   Complete table with 3 titrations done – 1mk
   In Complete table with 2 titrations done - 1mk
   incomplete table with 1 titration done – 0mks
   • Penalize ½ mk once for
     • Inverted table
     • Wrong anthmetic
     • Unrealistic titre values ( below 1 or above 50 unless explained)
2. Use of decimals………………………………………………………………..1mk
   • Accept 1 or 2 d.p uses consistently otherwise penalize fully
   • If 2 d.p used the 2nd d.p should be either 0 or 5 otherwise penalize fully
3. Accuracy ……………………………………………………………………….1mk
   Compare the candidates titre values with the S.V
   • If any value is within +- 0.1 award 1mk
   • If within + -0.2 award ½ mk
   • If beyond +- 0.2 award zero mark
4. Principles of averaging ……………………………………………………….1mk
   1f 3 consistent titrations done and averaged 1mk
   If 3 titrations done but only 2 are consistent and averaged (1mk)
   If only two titrations done, are consistent and averaged (1mk)
   If 3 titrations done and are consistent but only 2 are averaged ( 0mk)
   If 3 inconsistent titres averaged ( 0mk)
   1f 2 inconsistent titres averaged (0mk)
5. Final answer accuracy……………………………………………………………..1mk
   1. Compare the candidates correct average titre with S.V
      • If within +- 0.1 of S.V (1mk)
      • If within +- 0.2 of S.V ( ½ mk)
      • If beyond +- 0.2 of S.V ( 0mk)
        Calculations
   2. 2moles ________ 1000cm³
      ? ________ 25cm³
      25 x 2 ½ = 0.05 moles ½
       1000
   3. 0.05 moles ______ 250cm³
      ? _________ 1000cm³
      1000 x 0.05 ½ = 0.2 moles 1 litre
       250
   4. Mole of base
      0.2 moles ______ 1000cm³
      ? _______ 25cm³
      25 x 0.2 = 0.005 moles
       1000
   5. 0.0025 ______ average volume
      ? ______ 1000cm³
      0.0025 x 1000 ½ = correct answer ½
          Average
   6. 6.2g _____ 250cm³
      ? _____ 1000cm³
      6 x 1000 ( ½ ) = 24.8g ( ½ )
        250
      Answr in (v) = 24.8g
      RFM
      RFM = 24.8 ( ½ ) = correct answer in (v) ( ½ )
      Note:
      1. Answer for moles should be given to at least 4 d.p unless it works out exactly other wise penalize ½ mk for rounding off to less than 4 d.p
      2. Answer for concentration in moles per litre should be given to at least 3 d.p unless it works out exactly otherwise penalize ½ mk for rounding off to less than 3 d.p.
      3. Units may or may not be given but if given must be correct otherwise penalize ½ mk for wrong units.
      4. Average volume should be given to at least 2 d.p unless it works out exactly to less than 2 d.p otherwise penalize ½ mk for rounding off to less than 2 d.p.
      5. Answer for (vi) above should be between 121- 144 otherwise penalize ½ mk for answer outside this range.

2. You are provided with solid E.
(i) Add sodium hydroxide solution dropwise until in excess

Observations	Inferences
No white ppt  (1/2 mark)	Pb2+, ½ Zn2+ or Al3+ ions absent 1mk Only 2 correct…(1/2 mark)  Only 1 correct….. 0 mk Ignore sodium ions  (1      mark)

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

Observations	Inferences
Yellow flame  1 mark	Na+ present Ignore unsaturation. 1 mark

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

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

iv). To the fourth portion add two drops of acidified potassium manganite (VII)

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

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

Observations	Inferences
Bubbles 1/2 mark	CO3 2-          and    SO32-  present…… 1mk Only 1 correct….1/2 mk Penalize ½ mk for ay contradictory ion upto a max of 1 mk.

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

Observations	Inferences
White ppt soluble in excess	Pb2+, ½ Zn2+ or Al3+ ions present. Only 2 correct…………… 1/2 mark Only 1 correct……………..0 mk

ii). To the second portion add two drops of sodium iodide solution.

Observations	Inferences
Yellow ppt	Pb2+ present. Penalize fully for any contradictory ion.

3. You are provided with liquid L
a). Place about 3 drops of liquid L on a watch glass and ignite using a Bunsen burner flame.

Observations	Inferences
Burns with blue flame	absent… 1mk Saturated organic compound present……1 mk

b) Divide the remaining liquid L into four portions in test tubes.
i). To the first portion, add about 6cm³ of distilled water and shake well.

Observations	Inferences
Miscible to form a colourless solution                          ½     mark	Polar liquid.                       ½     mark

ii). To the second portion, add the sodium hydrogen carbonate solid provided.

Observations	Inferences
No bubbles	R – COOH  absent....1 MK H+ absent………..................½ mark

iii). To the third portion, add two drops acidified potassium manganite (VII)

Observations	Inferences
Purple potassium manganate (VII) turns colourless	….½     mark R-OH PRESENT……. ½     mark Penalize ½     mark for any contradictory ion upto max of 1 mk

iv) To the last portion, add two drops acidified potassium dichromate (VI) solution.

Observations	Inferences
Orange potassium dichromate turns green	R – OH…………1mk Penalise fully for any other contradictory ion.

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CONFIDENTIAL

In addition to the fittings and apparatus found in chemistry laboratory, each student will require the following.

1. About 6.2 g of solid A in a stoppered container.
2. About 100 cm³ of solution B.
3. Solid E, about 0.6gbin a stoppered container.
4. About 0.2 g sodium hydrogen carbonate solid in a stoppered container.
5. 10 cm³ dilute nitric (V)acid .
6. 10 cm³ of liquid L in a stoppered container.
7. Distilled water in a wash bottle.
8. One pipette and pipette filler
9. Burette.
10. Filter funnel.
11. 6 test tubes.
12. 2 Filter papers
13. Means of labeling (2 labels).
14. 50 cm³ measuring cylinder.
15. 250ml volumetric flask.
16. Thermometer.(-10ºc-100ºc)
17. Glass rod
18. 250 cm³ beaker.
19. Two conical flasks.
20. 2 boiling tubes.
21. Metallic spatula
22. A test-tube holder.
23. Bunsen burner.
24. Watch glass.

BENCH REAGENTS/ ACCESS REAGENTS

1. Phenolphthalein indicator – supplied with dropper
2. Distilled water in a 500ml wash bottle
3. 2M Sodium hydroxide solution with dropper.
4. 0.1M Barium nitrate solution with dropper
5. Acidified Potassium manganate (VII) solution with a dropper.
6. Acidified Potassium dichromate(VI) solution with a dropper
7. 0.1 M Potassium iodide solution with a dropper.

NOTES

• Solid A is 6.2 g oxalic acid.
• Solution B is 2M sodium hydroxide solution.
• Solid E is 0.5 g sodium sulphite and 0.1g lead (II)carbonate.
• Liquid L is absolute ethanol.

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