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

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

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QUESTIONS

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 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)
      

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 1^st portion add sodium sulphate solution.

Observations	Inferences
(1mk)	(1½mks)

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

Observations	Inferences
1mk)	1mk

c) To the 3^rd 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 1^st portion add sodium hydrogen carbonate

Observations	Inferences
(1mk)	(1mk)

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

Observations	Inferences
(1mk)	(1mk)

1. To the 3^rd portion add Bromine water

Observations	Inferences
(1mk)	(1mk)

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

Observations	Inferences
(1mk)	(1mk)

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MARKING SCHEME

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 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. 
   
   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
2. 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.

   2. TREND – Tied to temperature i.e it should be decreasing (1mk)
   3. 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.
   

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 1^st and 2^nd row only)
Accept 1 or 2 decimal places used consistently otherwise penalize fully

• If 2 dp are used the 2^nd 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 2^ndd.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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