KCSE 2023 Chemistry Paper 3

233/3 · CHEMISTRY (Practical) · Paper 3 · Nov. 2023 · 2½ hours

Instructions: Answer all the questions in the spaces provided. You are not allowed to start working with the apparatus for the first 15 minutes; use this time to read the question paper and check that you have all the materials and apparatus. All working must be clearly shown where necessary. KNEC mathematical tables and silent electronic calculators may be used.

Question 1 (19 marks)

You are provided with:

• Solution A — a mixture containing sodium thiosulphate and starch;
• Solution B — aqueous potassium iodide;
• Solution C — aqueous hydrogen peroxide;
• Solution D — 0.01 M potassium manganate(VII).

You are required to determine the:

• concentration in moles per litre of hydrogen peroxide in solution C;
• rate of reaction between hydrogen peroxide and potassium iodide.

Procedure I

Using a pipette and pipette filler, pipette 25.0 cm³ of solution C into a 250 ml volumetric flask. Add distilled water to the mark and label this as solution E.

Fill the burette with solution D. Using a clean pipette and pipette filler, place 25.0 cm³ of solution E into a 250 ml conical flask. Titrate with solution D until a permanent pink colour persists. Record the results in Table 1. Repeat the titration two more times and complete Table 1.

(a) Table 1 (4 marks)

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

(b) Determine the:
 (i) average volume of solution D used; (1 mark)
 (ii) number of moles of potassium manganate(VII) that reacted; (1 mark)
 (iii) number of moles of hydrogen peroxide in 25.0 cm³ of solution E (5 moles of hydrogen peroxide react with 2 moles of potassium manganate(VII)); (1 mark)
 (iv) concentration in moles per litre of hydrogen peroxide in solution E; (1 mark)
 (v) concentration in moles per litre of hydrogen peroxide in solution C. (1 mark)

Procedure II

(i) Rinse the burette with solution E and then with distilled water. Fill the burette with solution C. Place 5 test tubes on a test tube rack and label them 1, 2, 3, 4 and 5. To each test tube place 10 cm³ of solution C from the burette.

(ii) Clean the burette and fill it with solution B. Place 25.0 cm³ of solution B into a 100 ml beaker from the burette.

(iii) Using a 100 ml measuring cylinder, add 20 cm³ of solution A to the beaker containing solution B.

(iv) Pour the contents of test tube 1 into the mixture in the beaker and immediately start the stop watch. Swirl the contents of the beaker and allow to stand. Record in Table 2 the time, in seconds, taken for a blue colour to just appear. Measure the temperature of the final mixture and record it in the space provided. Wash the beaker and proceed to step (v).

(c) Record the temperature of the final mixture: ……… (1 mark)

(v) Place 20.0 cm³ of solution B into the 100 ml beaker from the burette. Using the 100 ml measuring cylinder, add 20 cm³ of solution A to the beaker, followed by 5 cm³ of distilled water measured using a 10 ml measuring cylinder. Add solution C in test tube 2 to the mixture in the beaker and immediately start the stop watch. Swirl the contents of the beaker and allow to stand. Record in Table 2 the time taken for a blue colour to just appear. This is experiment 2 in Table 2. Wash the beaker.

(vi) Repeat step (v) with solution C in test tubes 3, 4 and 5 with the corresponding volumes of solution B, solution A and distilled water as shown in Table 2 for experiments 3, 4 and 5.

(vii) Complete Table 2 by calculating the rate for each experiment, given by $\text{rate} = \dfrac{1000}{\text{time}}\ \text{s}^{-1}$.

(d) Table 2

Experiment	Solution A (cm³)	Solution B (cm³)	Distilled water (cm³)	Solution C (cm³)	Time (s)	Rate $= \dfrac{1000}{\text{time}}$ (s⁻¹)
1	20	25	0	10
2	20	20	5	10
3	20	15	10	10
4	20	10	15	10
5	20	5	20	10

(e) Plot a graph of rate (y-axis) against volume of solution B. (3 marks)

(f) State why it was necessary to add distilled water to the mixture in experiments 2, 3, 4 and 5. (1 mark)

Question 2 (13 marks)

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

(a) Describe the appearance of solid F. (1 mark)

(b) Place about one-third of solid F in a dry test tube. Heat the solid gently at first and then strongly. Test any gases with red and blue litmus papers. (2 marks, 2 marks)

Observations	Inferences

(c) Place the remaining amount of solid F in a boiling tube. Add about 15 cm³ of distilled water and shake until it all dissolves. Use about 2 cm³ portions of the solution in a test tube for tests (i) to (iv).
 (i) Measure the pH of the first portion using universal indicator paper and chart. (1 mark, 1 mark)

Observations	Inferences

 (ii) To the second portion, add aqueous sodium hydroxide dropwise until in excess. (1 mark, 1 mark)

Observations	Inferences

 (iii) To the third portion, add three drops of aqueous barium chloride. Shake the mixture and then add about 1 cm³ of dilute hydrochloric acid. (1 mark, 1 mark)

Observations	Inferences

 (iv) To the fourth portion, add about 3 cm³ of aqueous hydrogen peroxide. Shake the mixture and then add aqueous ammonia dropwise until in excess. (1 mark, 1 mark)

Observations	Inferences

Question 3 (8 marks)

You are provided with an organic compound, solid G. Carry out the following tests and record the observations and inferences in the spaces provided.

(a) Place all of solid G in a boiling tube. Add about 15 cm³ of distilled water and shake the mixture for use in test (b). (1 mark, 1 mark)

Observations	Inferences

(b) Use about 2 cm³ portions of the mixture, in a test tube, for each of the following tests:
 (i) To the first portion, add all of the solid sodium hydrogen carbonate provided. (1 mark, 1 mark)

Observations	Inferences

 (ii) To the second portion, add three drops of acidified potassium manganate(VII). (1 mark, 1 mark)

Observations	Inferences

 (iii) To the third portion, add about 1 cm³ of acidified potassium dichromate(VI) and warm the mixture. (1 mark, 1 mark)

Observations	Inferences