Chemistry Paper 3 Questions and Answers with Confidential - Momaliche Post Mock 2020 Exam

Questions

1. You are provided with
   • Aqueous sulphuric acid labelled solution
   • Solution B containing 8.0g per litre of Sodium Carbonate
   • An aqueous solution of substance C, labelled solution C.
     
     You are required to determine the:
   • Concentration of solution
   • Enthalpy of reaction between sulphuric acid and substance C.
     
     Procedure
   • Using a pipette place 25.0cm³ of solution A into a 250ml volumetric flask. Add distilled water to make 250cm³ of solution. Label the solution D. Place solution D in a burette. Clean the pipette and use it to place 25.0cm³ of solution B into a conical flask. Add 2 drops of methyl orange indicator provided and titrate with solution D. Record your results in table 1. Repeat the titration two more times and complete the table.
     
     Table 1
     

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

     (4mks)
     
     Calculate the
     1. Average volume of solution D used. (1mk)
     2. Concentration of Sodium Carbonate in solution B (Na = 23.0, O =16.0, C = 12.0) (1mk)
     3. Concentration of sulphuric acid in solution D. (2 mks)
     4. The concentration of sulphuric acid in solution A. (1mk)
        
        B. Label six test tubes 1, 2,3,4,5 and 6. Empty the burette and fill it with solution A. From the burette place 2cm³ of solution A into test tube number 1. From the same burette place 4cm³ of solution A in test tube 2. Repeat the procedure for test tubes number 3, 4,5 and 6 as shown in table 2.
        
        Clean the burette and fill it with solution C. From the burette, place 14cm³ of solution C into a boiling tube. Measure the initial temperature of solution C to the nearest 0.5°C and record it in table 2. Add the contents of test tube number 1 to the boiling tube containing solution C. Stir the mixture with thermometer. Note and record the highest temperature reached in table 2. Repeat the process with the other volumes of solution C given in table 2 and complete the table.
        
        Table 2
        

        Test tube number	1	2	3	4	5	6
        Volume of solution A (cm3)	2	4	6	8	10	12
        Volume of solution C (cm3)	14	12	10	8	6	4
        Initial temperature of solution C (oC)
        Highest temperature of mixture (oC)
        Change in temperature, ΔT(oC)

        (6mks)
        
        
        1. On the graph paper provided, draw a graph of ΔT (vertical axis) against volumes of solution A used. (3mks)
        2. From the graph, determine
           1. The maximum change in temperature (1mk)
           2. The volume of solution A required giving the maximum change in temperature. (1mk)
           3. Calculate the
              1. Number of moles of sulphuric acid required to give the maximum change in temperature. (1mk)
              2. Molar enthalpy of reaction between sulphuric acid and substance C (kilojoules per mole of sulphuric acid). Assume the specific heat capacity of solution is 4.2Jg^-1K^-1 and density of solution is 1gcm^-3. (2mks)
2. You are provided with solid E. Carry out the following test and write your observations and inference in the space provided.
   1. Place about one –half of solid E in a dry test tube. Heat it strongly and test any gas produced using hydrochloric acid, solution K on a glass rod.

      Observations	Inferences
      (2mks)	(1mk)

   2. Place the rest of solid E in a boiling tube. Add about 10cm³ of distilled water. Shake well and use 2cm³ portions of the mixture for each of the test below.
      1. To one portion, add aqueous ammonia drop wise until in excess.
         

         Observations	Inferences
         (1mk)	(1mk)

      2. To a second portion, add about 1cm³ of hydrochloride acid.
         

         Observations	Inferences
         (2mk)	(1mk)

      3. To the third portion, add three drops of aqueous lead (II) nitrate and heat the mixture to boiling.
         

         Observations	Inferences
         (2mk)	(1mk)

3. You are provide with liquid F carry out the following tests and record your observation and inferences in the spaces provided.
   1. Place five drops of liquid F on a CLEAN DRY watch glass and ignite it
      

      Observations	Inferences
      (1mk)	(1mk)

   2. Place 3cm³ of liquid F into a test tube followed by 3cm³ of distilled water and shake the mixture
      

      Observations	Inferences
      (1mk)	(1mk)

   3. Place about 2cm³ of liquid F into a test, add about 1cm³ of acidified potassium magnate (VII) and warm the mixture.
      

      Observations	Inferences
      (1mk)	(1mk)

   4. Place about 2cm³ of liquid F into a test tube add about 1cm³ of acidified potassium dichromate (VI) and warm the mixture.
      

      Observations	Inferences
      (1mk)	(1mk)

Marking Scheme

Question 1

Procedure A

Table 1……………………5mks distributed as follows

A: COMPLETE TABLE ………………….1mk

Conditions

1. Complete table with 3 titrations done ………1mk
2. Incomplete table with 2 titrations done……..¹/₂ mk
3. Incomplete table with 1 titration done ………0mk

PENALTIES

1. Wrong arithmetic (subtraction)
2. Inverted table
3. Burette reading ˃50cm³
4. Unrealistic titre values (less than 1cm³ or hundreds)

NB Penalise ¹/₂mk each to a maximum of ¹/₂mk i.e. penalise ONCE

B. USE OF DECIMALS ………….1mk (tied to 1st and 2nd rows only.)

CONDITIONS AND PENALTIES

1. Accept 1 or two decimal places used consistently otherwise penalise fully (i.e. award 0marks)
2. If two decimals places are used the 2nd decimal place must be a ‘o’ or a ‘5’, otherwise penalise fully.
3. Accept INCONSISTENCY in the use of zeros as initial burette readings e.g. 0,0.0,0.00

C. ACCURACY …………………1mk

Compare the candidates correct titre values with the school value (s.v) i.e. the teachers correct average titre and award as follows.

1. If at least one is within ±0.1 of s.v award 1mk
2. If none is within ±0.1 of s.v but at least one is within ±0.2 of s.v award ….1/2mk
3. If no value is within ±0.2 of s.v award ….0mk
4. If there was wrong arithmetic or no subtraction done in the table compare correctly worked out value (s) with s.v and award accordingly.

D PRINCIPLES OF AVERAGING……………..1MK

CONDITIONS

1. 3 consistent titrations done and averaged ……1mk
2. 3 done but 2 are consistent and averaged …….1mk
3. Only 2 done are consistent and averaged ……..1mk
4. 3 done, are inconsistent and averaged ………..0mk
5. 2 done are inconsistent and averaged …………0mk
6. 3 consistent done but only 2 averaged ……….0mk
7. Only 2 done are inconsistent and averaged ……0mk
8. Only one titration done ………..0mk

PENALTIES

1. Penalise ¹/₂mk for wrong arithmetic if error is outside 2 units in the 2nd decimal place.
2. Penalise ¹/₂mk for no working shown even if answer is correct.
3. Correct answer from wrong working – 0
   e.g. 20 +20 +20 = 20, 20+20+20=60/3 =20
                2   

NB.

1. Accept rounding off/truncation of answer to 2d.places e.g. 21.666 as 21.66 or 21.67 otherwise penalise 1/2mk for rounding off to 1d.place or whole number.
2. Accept answer if it works out exactly to 1 place or a whole number.

E. FINAL ACCURACY ……………….1mk (Tied to correct average titre)

Compare the candidate’s correct average titre with the school value (s.v) and award as follows;

1. If there are two possible correct values for average titre from the candidate’s tables use the one closest to the s.v and credit accordingly.
   Table 1 post marks as
2. Na₂CO₃ = 46 + 12 + 48 = 106
   Conc. = ⁸/₁₀₆ = 0.075

   Or
   Conc. = ⁸/₁₀₆ =0.075

NOTES

1. Answer tied to correct arithmetic, accept reading to 3rd or 4th decimal place if not exact.
2. Accept arithmetic error if within ±2 units in the 3rd decimal place, otherwise penalise ¹/₂mk.
3. Units may not be shown, but if shown must be correct, otherwise penalise ¹/₂mk for wrong units.
4. If a candidate works beyond the expected answer penalise FULLY.

Confidential

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

1. About 120cm³ of solution A
2. About 120cm³ of solution B
3. About 100cm³ of solution C
4. One pipette 25ml
5. One volumetric flask 250ml
6. One burette 0 – 50ml
7. 8 clean dry test tubes
8. Two conical flasks
9. One test tube rack
10. One thermometer -10^oC to 110^oC
11. Two boiling tubes
12. Six labels
13. One glass rod
14. 10cm³ of liquid F (absolute ethanol)
15. 0.5g of solid E (hydrated aluminium ammonium sulphate) in a stopper container.
16. One watch glass
17. One clean metallic/wooden spatula
18. Two clean droppers
19. One 10ml measuring cylinder
20. 500ml distilled water
21. Wall clock

Access to

1. methyl orange indicator supplied with a dropper
2. Bunsen burner
3. 2M aqueous ammonia + dropper
4. 2M hydrochloric acid
5. 0.2M lead (II) nitrate
6. 0.2M acidified potassium manganite (VII)
7. 0.2M acidified potassium dichromate (VI)

CONFIDENTIAL INSTRUCTIONS NOTES

1. Solution A is prepared by dissolving 50cm³ of 1.84g/cm³ (98%) concentrated sulphur (vi) acid in about 600cm³ of distilled water and diluting to one litre of solution.
2. Solution B is prepared by dissolving 8g of solid B (hydrated sodium carbonate) in about 500cm3 of distilled water and diluting to one litre.
3. Solution C is prepared by dissolving 60g of sodium hydroxide pellets (not pearls) in about 700cm³ of distilled water and diluting to one litre of solution.