Chemistry Paper 3 Questions and Answers - Form 4 Term 2 Opener Exams 2022

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INSTRUCTIONS

  1. Sign and write the data of the examination.
  2. Answer all the questions in the spaces provided.
  3. 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.
  4. All working must be clearly shown
  5. KNEC mathematical tables and silent electronic calculations may be used.
  6. All questions should be answered in English

FOR EXAMINERS USE ONLY

QUESTIONS

MAXIMUM

CANDIDATE’S SCORE

1

21

 

2

11

 

3

08

 

TOTAL SCORE

40

 

QUESTIONS

QUESTION 1
You are provided with:

  • Solid A 5.0g (COOH) 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 800
    • 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 aburrete add 2cm3more 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

Volume of water in the boiling tube (cm3)

Temperature at which crystals of solid A appear (0C)

Solubility o solid A g/100g of water

4

6

8

10

12

   
  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)
      graph paper ahgdad

PROCEDURE II

  1.                            
    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 600C
      • 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)

     

II)

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

Observations

Inferences

(1mk)

(1½mks)

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

Observations

Inferences

1mk)

1mk

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

Observations

Inferences

(1mk)

(1mk)

d) To the forth portion add Lead (II) Nitrate solution

Observations

Inferences

(½mk)

(2mks)

e)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
To the 1st portion add sodium hydrogen carbonate

Observations

Inferences

(1mk)

(1mk)

  1. To the 2nd portion add acidified potassium manganite (VII) (KmnO4)

Observations

Inferences

(1mk)

(1mk)

  1. To the 3rd portion add Bromine water

Observations

Inferences

(1mk)

(1mk)

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

Observations

Inferences

(1mk)

(1mk)

MARKING SCHEME

QUESTION 1
You are provided with:

  • Solid A 5.0g (COOH) 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 800
    • 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 aburrete add 2cm3more 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

VOLUME OF WATER IN THE BOILING TUBE

TEMPERATURE AT WHICH CRYSTALS OF A APPEAR

SOLUBILITY OF SOLID A g/100g of water

4

70.0

125.0

6

59.0

83.3

8

54.0

62.5

10

47.0

50.0

12

40.0

41.7


    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. Draw a graph of solubility of solid A (vertical axis) against temperature (3mks)
      • Penalize ½ mk for each wrong value of solubility
    2. From your graph determine the solubility of solid A at 600C  (1mk)
      • Penalize ½ one for unrealistic temperature readings ½ above 900C and below 100C
  1. DECIMALS TIED TO TEMPARATURE -1Mk

    Accept;

    1. i) Whole numbers
    2. Idecimal place where the decimal should be 0 or 5 i.e 70.0 or 70.5

    iii. Accept 2 decimal places where it should be .00, .25,.50 or .75

    NB: If no consistence penalize fully.

    1. TREND – Tied to temperature i.e it should be decreasing (1mk)
    2. Accuracy – tied to temperature when 4cm3 of water was added

    Accept 20C of the school value.

    1a) GRAPH – award a total of 3mks distributed as follows.

    1. Labeling (½mks)
    • Both axis should be labeled if one is not or wrongly labeled award O
    • Ignore units but if indicated they should be right otherwise penalize fully
    1. Scale – (½mks)
    • The actual plotting should be half of the page
    • Paralyze 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

    1. Line – a smooth curve passing through 3 or more correct plots award 1mk

    Otherwise award 0

    1. b) – 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.
    graph answered

PROCEDURE II

  1.                            
    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 600C
      • 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.

II)
TABLE 2

  • 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
    • Unrealistic values i.e burette reading with more than 50cm3 and less than 1cm3
  3. Unrealistic titre values

NB: PENETICE ½ once
b) 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

C) Accuracy (1mk)
Complete the candidate value with the school value (S.V)

  1. If within 1 of the school value award 1mk
  2. If within 2 of S.V ward ½mk otherwise award 0

NB: Tick the candidate value that deserves a credit

D) 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

PERALTIES

  1. Penalize ½ for wrong arithmetic
  2. penalize ½mk if no working is shown and answer is correct
    • penalize fully if no working is shown and answer given is wrong
  3. Accept rounding off or truncation to the 2ndd.p
    e.g 12.666      12.67
    or
    12.66              12.66

NB:

  • The working of average must be marked before the mark for averaging is award in table 2.
  • Accept the average volume if it work out exactly to a whole number.

FINAL ACCURACY ----------------------(1MK) Tiled to correct average time.
Compare the candidate average time to the school value.

  1. If within 1 award 1mks
  2. If with n 2 award ½ mk

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

 

1

11

111

FINAL BURET READING

12.5

12.5

12.5

INITIAL BURET READING

0.0

0.0

0.0

VOLUME OF SOLUTION B USED

12.5

12.5

12.5

  1. Calculate the average volume of solution B used   (1mk

    1000-0.13
    12.5 ?
    = 12.5 x 0.13    ½
            1000
    0.001625 moles    ½
    = 0.0040625

  2. Calculate the number of moles of B used (1mk)
    ans = b x 5
                 2
    = 0.0040625
  3. Given 2 moles of Kmno4 react with 5 moles of A, calculate the number of moles of A in 25cm3  (1mk)
    ans c x 1000
    = 0.1626m      
  4. Calculate the molarity of A (1mk)
  5. Determine the molar mass of A (1mk)

    5g  250cm3
    20g – 1000  
    (1×20)/0.1625
    = 123.07

  6. Determine the value of X   (1mk)
    (C=12, O=16 H=1)
    1. 90 + 18 =123

           18  = 33
                = 33
                   18
                = 1.83
                = 2

QUESTION 2

A)    OBSERVATION

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

   

B)    OBSERVATION

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)

C)    OBSERVATION

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

D)   OBSERVATION

INFERENCES

A white    precipitate  is formed ½ mks        

-           

C1-,   So32- , SO4 2-  and CO32- present

 – ½ mk each

Penalize ½mk for any contradictory ion to a maximum of (2mks)

E)    OBSERVATION

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

a)    OBSERVATION

INFERENCES

No fizzing/bubbling /hissing (1mk)

Reject

-fissiling

-Sizzling

R- CooH Absent (1mk)

NB: Ignore H3O+ & H+

b)    OBSERVATION

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 1mks

c)     OBSERVATION

INFERENCES

Yellow /orange bromine water does not get decolonized
Accept

It remains yellow or orange  

C= C   or –C  C-
Absent
Penalize 1mk for any contradictory    group to a maximum of 1mk

d)    OBSERVATION

INFERENCES

K2CrO7 turns from orange to green(1mk) 

R-OH Present (1mk)

Penalize 1mk for any contradictory   group to a maximum of 1mk


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