233/3 · CHEMISTRY (Practical) · Paper 3 · Mar. 2022 (KCSE 2021) · 2¼ 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 of the 2¼ hours allowed for this paper; use this time to read the question paper and make sure you have all the chemicals and apparatus you may need. All working must be clearly shown where necessary. Non-programmable silent electronic calculators and KNEC mathematical tables may be used.
Question 1 (15 marks)
You are provided with: solution A — 0.10 M solution of a monobasic acid A; solution B — sodium hydroxide solution; solution C — containing 10.0 g 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 25.0 cm³ of solution B into a 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 (3 marks)
| I | II | III | |
|---|---|---|---|
| Final burette reading | |||
| Initial burette reading | |||
| Volume of solution A used (cm³) |
(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 25.0 cm³ of solution B; (1 mark) (iv) concentration of sodium hydroxide in moles per litre. (1 mark)
Procedure II. Clean the burette and fill it with solution C. Using a pipette and pipette filler, place 25.0 cm³ of solution B into a 250 ml conical flask. Titrate solution B with solution C using phenolphthalein indicator and record your results in Table 2. Repeat the titration and complete Table 2.
(c) Table 2 (3 marks)
| I | II | III | |
|---|---|---|---|
| Final burette reading | |||
| Initial burette reading | |||
| Volume of solution C used (cm³) |
(d) Calculate the: (i) average volume of solution C used; (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 25.0 cm³ of solution B. (1 mark) (ii) Determine the number of moles of sodium hydroxide that react with one mole of acid C. (1 mark)
Question 2 (8 marks)
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 cm³ of tap water and heat the water until it boils.
(ii) Place all of 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 (4 marks)
| Time (s) | 0 | 30 | 60 | 90 | 120 | 150 | 180 | 210 | 240 | 270 | 300 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Temperature (°C) |
(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)
Question 3 (17 marks)
You are provided with solid E. Carry out the following tests and record your observations and inferences in the spaces provided.
(a) Place all of solid E in a boiling tube. Add about 10 cm³ of dilute nitric(V) acid, warm the mixture and then allow to stand until all the solid dissolves. Add about 10 cm³ of distilled water to the solution and shake. Retain the solution for tests (b) and (c). (2 marks, 1 mark)
| Observations | Inferences |
|---|---|
(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. (1 mark, 1 mark)
| Observations | Inferences |
|---|---|
(ii) To the second portion add 2 or 3 drops of aqueous lead(II) nitrate. (1 mark, 1 mark)
| Observations | Inferences |
|---|---|
(iii) To the third portion add aqueous sodium hydroxide dropwise until in excess. (1 mark, 1 mark)
| Observations | Inferences |
|---|---|
(iv) Place about 3 cm³ of aqueous ammonia in a test tube. To the fourth portion, add all the aqueous ammonia from the test tube dropwise. (1 mark, 1 mark)
| Observations | Inferences |
|---|---|
(c) To the remaining solution of solid E in the boiling tube, add all of the solid G provided. Shake the mixture for about 2 minutes. Filter the mixture into a boiling tube. Retain the filtrate for tests (i) and (ii) below. (1 mark, 1 mark)
| Observations | Inferences |
|---|---|
(i) To about 2 cm³ portion of the filtrate, add aqueous ammonia dropwise until in excess. (1 mark, 1 mark)
| Observations | Inferences |
|---|---|
(ii) To about 2 cm³ portion of the filtrate add 2 or 3 drops of dilute hydrogen peroxide solution. (1 mark, 1 mark)
| Observations | Inferences |
|---|---|