KCSE Past Papers 2021 Chemistry Paper 3





Kenya certificate of primary Education
Practical

2021 Chemistry paper 3

1. You are provided with:

  • Solution A: 0.10 M solution of a monobasic acid A;
  • Solution B: Sodium hydroxide solution:
  • Solution C: containing 10.0g of acid C per litre of solution.
  • You are required to:
  • Standardise solution B using solution A;
  • Determine the number of moles of sodium hydroxide that react with one mole of acid C.
PROCEDURE I

Fill the burette with solution A. Using a pipette and pipette filler, place 25.0cm of solution B into 250 ml conical flask.

Titrate solution B with solution A using phenolphthalein indicator and record your results in Table 1. Repeat the titration and complete Table 1.

a. Table 1


I


II


III

   
Final   burette reading   

   
   

   
   

   
   

   
nitial burette   reading   

   
   

   
   

   
   

   
Volume of solution A
   
used, cm³   

   
   

   
   

   
   



(3 marks)
b. Calculate the:

i. average volume of solution A used. (1 mark)

ii. number of moles of solution A in the average volume used. (1 mark)

iii. number of moles of sodium hydroxide (N) in 25.0cm of solution B. (1 mark)

iv. concentration of sodium hydroxide in moles per litre. (1 mark)

PROCEDURE II

Clean


I


II


III

   
Final   burette reading   

   
   

   
   

   
   

   
nitial burette   reading   

   
   

   
   

   
   

   
Volume of solution A
   
used, cm³   

   
   

   
   

   
   


(3 marks)
d. Calculate the:

i. average volume of solution Cused. (1 mark)

ii. concentration in moles per litre, of solution C, given that the relative formula mass of acid C is 210.0. (1 mark)

iii. number of moles of acid C in the average volume used. (1 mark)

e.i. Write the ratio of moles of acid C to moles of sodium hydroxide (N) in the 25.0cm of solution B. (1 mark)

ii. Determine the number of moles of sodium hydroxide that react with one mole of acid C. (1 mark)

2. You are provided with solid D.

You are required to determine the freezing point of solid D.

PROCEDURE

i. Fill a 250 ml beaker with about 200 cm³of tap water and heat the water until it boils.

ii. Place all solid D provided in a dry test tube and insert a thermometer into the solid.

iii. Place the test tube in the boiling water and allow the solid to heat until it all melts.

iv. When the temperature of the melted solid is approximately 90°C, remove the test tube, wipe the sides with tissue paper and then place the test tube into an empty 250 ml beaker v. Start the stop watch or clock when the temperature of the melted solid is 85.0°C.

vi. As the solid cools, measure and record its temperature every 30 seconds and complete Table 3.

a. Table 3

Time
S


0

   
30   

   
60   

   
90   

   
120   

   
150   

   
180   

   
210   

   
240   

   
270   

   
300   

Temperature°C

   
   

   
   

   
   

   
   

   
   

   
   

   
   

   
   

   
   

   
   

   
   


(4 marks)
b. On the grid provided, plot a graph of temperature (vertical axis) against time. (3 marks)

c. Using the graph in (b), determine the freezing point of solid D. (1 mark)

3. You are provided with solid E. Carry out the following tests and record your observations and inferences in the spaces provided.

a. Place all the solid E in a boiling tube.

Add about 10 cm of dilute nitric(V) acid, warm the mixture and then allow to stand until all the solid dissolves. Add about 10 cm of distilled water to the solution and shake. Retain the solution for tests (b) and (c)


                                                Observations


Inferences

   
(1 marks)   

   
(1 mark)   


b. Use about 2 cm³ portions of the solution obtained in 3(a) for each of the following tests.
i. To the first portion add 2 or 3 drops of aqueous barium nitrate,


                                                Observations


Inferences

   
(1 marks)   

   
(1 mark)   


ii. To the second portion add 2 or 3 drops of aqueous lead(II) nitrate.


                                                Observations


Inferences

   
(1 marks)   

   
(1 mark)   


iii. To the third portion add aqueous sodium hydroxide dropwise until in excess.


                                                Observations


Inferences

   
(1 marks)   

   
(1 mark)   


iv. Place about 3 cm of aqueous ammonia in a test tube. To the fourth portion, add all the aqueous ammonia from the test tube dropwise.


                                                Observations


Inferences

   
(1 marks)   

   
(1 mark)   


c. To the remaining solution of solid in the boiling tube, add all the solid providedn Shake the mixture for about 2 minutes.
Filter the mixture into a boiling tube. Retain the filtrate for tests (i) and (ii) below


                                                Observations


Inferences

   
(1 marks)   

   
(1 mark)   


i. To about 2 cm portion of the filtrate, add aqueous ammonia dropwise until in excess


                                                Observations


Inferences

   
(1 marks)   

   
(1 mark)   


ii. To about 2 cm portion of the filtrate add 2 or 3 drops of dilute hydrogen peroxide solution.


                                                Observations


Inferences

   
(1 marks)   

   
(1 mark)   


Practical Answers

2021 K.C.S.E Chemistry paper 3
1.a

24.0

CT 1

I

II

III

D1

Final burette
reading

A1

Initial burette
reading

PA1

Volume of
solution A used,
cm3

FA1



b. i. Correct working

correct ans (b)(i)

ii. =correct ans (b)(i) x 0.1 (over) 1000 = correct ans (b)(ii)

iii. Ratio A:N b1:1

ans (b) (iii)

iv. =1000 x ans (b)(iii) (over) 25 = ans (b)(iv)

PROCEDURE II

16.5

CT1

I

II

III

D1

Final burette
reading

A1

Initial burette
reading

PA1

Volume of solution

FA1

A used, cm3


d. i. Correct working
correct ans (d)(i)

ii. =10/210 = 0.0476

iii. =correct ans (d)(i) x 0.0476 (Over) = correct ans (d)(iii)

e. i. ans (d)(iii) : ans(b)(iii)

ii. =ans(b)(iii)/ ans(d)(iii) = ans (e)(ii)

2.Table 3.a

Time, s

0

30

60

90

120

150

180

210

240

270

300

Tempe
rature,
°C

85.0

82.0

80.5

78.0

75.0

72.0

69.0

69.0

69.0

67.5

65.5


b.

c. showing
correct reading

3 a.

Observations

Inferences

No effervescence

CO32-/ SO3
2- absent

No effervescence

Cu2+ present
(1 mark)


b i


Observations

   
Inferences   

   
No white ppt formed   


SO²4- absent

ii


Observations


Inferences

   
No yellow ppt formed
   
No white ppt formed   

   
absent
   
CI-/Br- absent   

iii


Observations

   
Inferences   

   
Blue ppt formed insoluble in excess   


Cu2+ present

iv


Observations


Inferences

   
Blue ppt formed that dissolves in excess
   
to form a deep blue solution   

   
Cu2+present   

C.


Observations

   
Inferences   

   
Blue solution changes to green
   
Brown residue Green filtrate
   
Boiling tube becomes warm   


Cu2+ displaced by G/G is more reactive than

Cu/G is oxidized by Cu2+ /Cu2+ are reduced b\ G/ Cu2+ are displaced by Fe

(i)


Observations

   
Inferences   

   
Green ppt formed insoluble in excess   


Fe2+

(ii)


Observations

   
Inferences   

   
Green solution changes to brown/Yellow
   
Effervescence   


Fe2+ oxidized to Fe3+

Fe3+ formed