Chemistry Paper 3 Questions and Answers With Confidential - Kapsabet Mocks 2020/2021

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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 -10oC to 110oC
  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.

Questions

1. You are provided with

  • Aqueous sulphuric acid labelled solution A
  • 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 A
  • 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 0.50C 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.

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