KCSE Past Papers 2021 Chemistry Paper 3

 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