Chemistry P3 Questions and Answers - Butula Sub-County Post Mock Exams 2021/2022

INSTRUCTIONS TO CANDIDATES

1. Write your name and index number in the spaces provided above.
2. Sign and write the date of examination in the spaces provided above.
3. Answer ALL the questions in the spaces provided in the question paper.
4. You are NOT allowed to start working with the apparatus for the first 15 minutes of the 2 ¼ hours allowed for this paper. This time is to enable you to read the question paper and make sure you have all the chemicals and apparatus that you may need.
5. All working MUST be clearly shown where necessary.
6. KNEC Mathematical tables and silent non-programmable electronic calculators may be used.
7. This paper consists of 7 printed pages.
8. Candidates should check the question paper to ensure that all pages are printed as indicated and no questions are missing.
9. Candidates should answer all the questions in English.

FOR EXAMINER`S USE ONLY

QUESTIONS

 You are provided with;

• Solution A: 0.02M acidified potassium manganate (VII) solution.
• Solution B, which is prepared by dissolving 5.88g of ammonium iron (II) sulphate in 250cm³ of distilled water.
• Solution C: 0.25M oxalic acid.

You are required to:

1. Determine the number of moles of B that react with one of potassium manganate (VII)
2. Determine the rate of reaction between solution C and A at different temperatures.

Procedure I.
Fill the burette with solution A. using a clean pipette, place 25.0cm3 of solution B into a clean conical flask, and titrate with solution A until the solution becomes permanent pink. Record your results in table I below. Repeat the procedure two more times to complete table I.(RETAIN THE REMAINING SOLUTION A FOR USE IN PROCEDURE II)

1. Table I.
   

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

   (4 marks)
2. Calculate the:
   1. Average volume of solution A used. (1 mark)
   2. Number of moles of solution A used. (1 mark)
   3. Concentration of solution B in moles per litre. (RFM of B is 392) (1 mark)
3. Calculate the number of moles of B:
   1. In 25.0cm³ of solution. (1 mark)
   2. Which react with one mole of solution A( Acidified potassium manganite (VII) ) (1 mark)

Procedure II.
Place 5 test tube on a test tube rack. Fill the burette with solution A. To each test tube place 10.0cm³ of solution A from the burette. Using a clean 10ml measuring cylinder, place 10.0cm³ of solution C into a clean boiling tube. Insert a thermometer in solution C in the boiling tube. Heat solution C in the warm water bath until it attains a temperature of 40ºC.
Remove the boiling tube from the water bath and place it in a test tube rack. Add the first portion of solution A and immediately start a stop watch. Record the time taken for the purple colour of the mixture to decolourise in table 2 below. Repeat the experiment by using 10.0cm3of solution C at temperature of40ºC, 50ºC, 60ºC, 70ºCand 80ºC.Record the time taken in each case in table 2 below.
d) Table 2

(3½ marks)
e) Complete the table by computing    1   (sec^-1)  (2½ marks)
                                                            Time
f) Plot a graph of   1   (sec^-1) against temperature. (3 marks)
                           Time

g) From the graph determine the time that would be taken if the temperature was 52.0ºC. ( 1 mark)
h) How does the rate of reaction of acidified potassium manganate (VII) with oxalic vary with temperature? (1 mark)
2. You are provided with solid P which is a mixture of two salts. You are required to carry out the tests below on solid P. Write you observations and inferences in the spaces provided.

1. Place the entire solid in a boiling tube. Add about 10cm³ of distilled water and shake. Filter the mixture. Retain both the residue and the filtrate.
   

   Observation	Inference
   (1mark)	(1mark)

2. Divide the filtrate into four portions.
   1. To the first portion, add sodium hydroxide solution drop wise until in excess.
      

      Observation	Inference
      (1mark)	(1mark)

   2. Dip a nichrome wire loop into the second portion. Hold the wire in a non-luminous Bunsen burner flame.
      

      Observation	Inference
      (½ mark)	(½ mark)

   3. To the third portion, add 2 drops of Barium nitrate.
      

      Observation	Inference
      (1mark)	(1mark)

   4. To the fourth portion, add 2 drops of acidified potassium manganate (VII)
      

      Observation	Inference
      (1mark)	(1mark)

3.                      
   1. Scrap the residue from the filter paper and put it in a test tube. Add about 5cm³ of dilute nitric (V) acid.
      

      Observation	Inference
      (1mark)	(1mark)

   2. To about 2cm³ of the resultant solution in C(i), add2M ammonia solution dropwise until in excess.
      

      Observation	Inference
      (1mark)	(1mark)

3. You are provided with an organic compound labeled solid T. Carry out the tests indicated below and write your observations and inferences in the spaces provided.

1. Put half of solid T on a clean metallic spatula and ignite
   

   Observations	Inferences
   (1 mark)	(1 mark)

2. Put the remaining solid T in a boiling tube. Add 10cm³ of distilled water and shake. Divide the mixture into four portions.
   1. Using the first portion, determine the PH of solution T
      

      Observations	Inferences
      (½ mark)	(½ mark)

   2. To the second portion, add sodium hydrogen carbonate solid.
      

      Observations	Inferences
      (1 mark)	(1 mark)

   3. To the third portion, add two drops of acidified potassium manganate (VII).
      

      Observations	Inferences
      (1 mark)	(1 mark)

MARKING SCHEME

1.PROCEDURE 1
Table 1

Award a total of 5 marks distributed as follows.

A. COMPLETE TABLE …………………………………………………… (1 mark)

1. Complete table with 3 titrations done award. (1 mark)
2. Incomplete table with 2 titrations done award. (½ mark)
3. Incomplete table with 1 titration done. (0 mark)

Penalties

1. Wrong arithmetic / subtraction.
2. Inverted table.
3. Burette reading beyond 50cm² unless / explained.
4. Unrealistic value is less than 1cm or > 100cm³

Note
Penalise ½ mk each to a maximum of ½mark

B. USE OF DECIMALS ………………………………………………… 1 mark
[Tied to 1st and 2nd rows only]

1. Accept 1 or 2d.p used consistency otherwise penalise FULLY
2. If 2d.p are used the 2nd d.p digits should be 0.5 otherwise penalise FULLY.
3.  Accept inconsistently in teh use of zero is initial burette reading i.e. 0, 0.0, 0.00

C. ACCURACY.
Compare the candidates correct titre value with the S.V.
Conditions.

1. If at least the value is within ± 0.1 cm³ of S.V award.
2. If no value is within ± 0.1cm³ of S.V but within ± 0.2cm³ of S.V. award . . . (½ mark)
3. If none is within ± 0.2cm³ award zero. (0 mark)

d) PRINCIPLES OF AVERAGING . . . . . (1 mark)
Conditions.

1. If 3 consistent titration done and averaged award (1 mark)
2. If 3 titrations are done but ONLY 2 are consistent and averaged award . . . . (1 mark)
3. If only 2 titrations are done, are consistent and averaged . . . . . (1 mark)
4. 3 titrations are done are consistent but only are averaged award . . . . . (0 marks)
5. 3 titrations done, inconsistent and averaged award. . . . (0 marks)

Penalties

1. Penalise ½mk for wrong arithmetic in the answer if error is outside ± 2 units in the 2nd d.p.
2. Penalise ½ mk if no working is shown and answer given is correct.
3. Penalise FULLY if no working is shown and answer given is wrong.
4. Accept rounding off of answer to the 2nd d.p and truncation to 2dp but not to 1dp or whole number. Otherwise penalise ½mk for rounding off to 1dp or to a whole number.

NB
Accept the average volume if it works out exactly to a whole number or to 1d.p.

E. FINAL ACCURACY…………………………………………… (1 mark)
(Tied to average titre)
Compare the candidates correct average titre to the S.V.
Condition.

1. If within ± 0.1cm³ of S.V award (1 mark)
2. If not within ± 0.1cm³ of S.V but within ± 0.2cm³ award . . . . (½ mark)
3. If beyond ± 0.2 cm³ of S.V award (0 mark)

B. I.
II. Number of moles of solution A used.
= 0.02 × average titre
         1000
= correct ans
III. Concentration of solution B in moles / litre.
250cm³ = 5.88g
1000cm³ = ?
= 1000 × 5.88 g/litre
          250
=23.52g/litre
Concentration of solution B
= 23.52 mole/litre
     392
= 0.06 moles/litre
C.
I. Number of moles of B in 25.0cm³
= 2.5 × 0.06 moles
        1000
= 0.015 moles
II. Number of moles of B which react with
1 mole of solution A
=          0.0015            = correct ans
     ans in On ib III above

PROCEDURE 2
Table 2 . . . . . . . 6 marks
DISTRIBUTION OF THE MARKS.

A. Completing table . …………………………………………………………….. (3 marks)
Conditions

1. Complete table with 12 correct entries .. . 3 marks)
2. Incomplete table with 6 times with 4.5 correct rates. . . . . . . . . . . . . . . . . (2½ marks)
3. Incomplete table with 6 times with 3 correct rates. . . . . . . . . . . . . . . . . . (2 marks)
4. Incomplete table with 6 time with less than 3 correct rates . . . . . . . . . . . . . . . . (1½ marks)
5. Incomplete table with 4.5 times with 4 second correct rates. (1 mark)
6. Incomplete table with 4.5 time with 3 correct rates. . . . . . . . . . . . . . . . . . . (½ mark)
7. Incomplete table with 4.5 time with less than 3 correct rates . . . . . . . . . . . . . . (½ mark)
8. Incomplete table with 3 time with 3 correct rates. . . . . . . . . . . . . . . . . . . . . . . (½ mark)
9. Incomplete table with 3 times with less than 3 correct rates . . . . . . . . . . . . . . . . . (0 mark)
10. Where values are constant in time column award a maximum of ½ mark for complete table.

B. Use of decimals………………………………………………………………………. (1 mark)
Tied to time subject to at least 2 reading of time in each case.
2. Accept whole numbers or 1 or 2 decimal places used consistently for time column otherwise penalise FULLY (award 0 mark)

C. Accuracy ……………………………………………………………………………… ( 1 mark)
[Tied to time column only]
Compare the candidates time readings.
When temperature = 40°C with the school value.
i) If within ± 2.0 seconds of S.V award . . . . (1 mark)
otherwise penalise FULLY . . . . award. (0 mark)

D. Trend………………………………………………………………………………… (1 mark)
[Tied to time column only]
- Decrease in time from temp = 400C to temp =80°C.
Otherwise penalise FULLY..award. (0 marks)

PROCEDURE 2
b) GRAPH. . . . . . . . . (3 marks)
Awards as follows:

A. Labelling
Award ½ if both axes are correctly labelled
[1/Time sec -1 on vertical axis and temperature in °C on the horizontal axis.

Penalties.

1. Penalise FULLY for inverted axes.
2. Penalise FULLY if wrong units are used otherwise ignore if units are omitted.

B. Scale
Conditions.

1. Area covered by the actual plots must be ½ of the grid provided for each axis.
2. Scale intervals must be consistent.
3. Scale chosen must accommodate ALL plots.

Note
Penalise fully if any of the above condition not met.

c) Plotting . . . . . . . . . (1 mark)

1. If 6 to 5 plots are correctly plotted. (1 mark)
2. If only 3 - 4 are correctly plotted. (½ mark)

d) Lines / . . . . . . . . . . (1 mark)
Accept a curve passing through atleast 3 correctly plotted points as shown award. (1 mark)

1(f) 2 marks awarded as follows

• For showing 1/T currently on the graph (½ mark)
• For stating the correct reading . . . .½ mark
• for using it (correct reading . . . . . .½ mark
• Correct answer.

g) Rate of reaction is directly proportional to increase in temperature.
NB- Correct answer MUST be tied to the correct trend otherwise penalise fully. (0 mark)

Observation	Inference
a) Burns with a yellow sooty flame//smoky//luminous
b) i) PH of 1,2,3    ½	Strongly acidic      ½
ii) ) Effervescence//production of gas bubbles//fizzing	Acidic
iii)  Purple acidified potassium manganate (VI) decolourised. Penalise if the purple colour of acidified potassium manganate(VII) is not mentioned

CONFIDENTIAL

In addition to the fitting and apparatus found in the Chemistry laboratory, each student will require the following

1. Solution A; About 150ml of 0.02M acidified potassium manganate(VII)
2. Solution B; About 100ml of ammonium iron (II) sulphate.
3. Solution C; About 70ml of 0.25M oxalic acid
4. Solid P; Mixture of zinc carbonate(ZnCO3) and sodium sulphite (Na2SO3) in the ratio 1:1
5. Solid T; maleic acid
6. Burette
7. Pipette
8. 250ml Conical flask (2)
9. 10ml measuring cylinder
10. Thermometer
11. Boiling tube
12. Test tubes(6)
13. Filter funnel
14. Filter paper
15. Nichrome wire
16. Distilled water
17. Spatula
18. PH chart
19. Test tube holder

Access

• 2M sodium hydroxide
• 2M ammonia solution
• Barium nitrate solution
• Dilute nitric (V) acid
• Acidified potassium manganate (VII) solution
• Universal indicator
• Solid Sodium hydrogen carbonate
• Source of heat
• Warm water bath.

Note:

1. Solution A is prepared by dissolving 3.2g of KMnO4 in 400cm³ of 1M sulphuric (VI) and diluting to one litre of solution using distilled water.(0.02M)
2. Solution B is prepared by dissolving 23.52grams of hexahydrous ammonium iron (II) sulphate in 400cm³ of distilled water and diluting to one litre of solution.(0.06M)
3. Solution C is made by dissolving 31.5grams of oxalic in about 400cm³ of distilled water and diluting to one litre of solution.(0.25M)