KCSE Past Paper 3 Chemistry 2015

CHEMISTRY

Paper 3

(PRACTICAL)

1. You are provided with:

– 2.0 g of substance A, labelled solid A.

– Solution B, 0.05 M hydrochloric acid.

– Methyl orange indicator.

You are required to determine the:

– solubility of substance A in water.

– relative formula mass of substance A.

PROCEDURE I

(i)Place 200 cm’ of. tap water in a 250 ml beaker and keep it for use in step (vi).

(ii)Place all of substance A in a dry boiling tube.

(iii)Using a burette, measure 10.0 cm3 of distilled water and add it to the substance A in the boiling tube.

(iv)While stirring the mixture in the boiling tube with a thermometer, warm the mixture using a Bunsen burner, until the temperature rises to 65°C. Stop warming the mixture.

(v)Allow it to cool while stirring with the thermometer.

(vi)When the temperature drops to 60°C, start the stop watch/clock, place the boiling tube in the beaker with tap water prepared in step (i) above.

(vii)Continue stirring and record the temperature of the mixture after two minutes, then thereafter record the temperature of the mixture after every one minute interval and complete table 1. Retain the mixture with the thermometer inside for use in procedure II below.

(4 marks)

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

(a) Using the graph, determine the temperature (Ts) when 2.0 g of substance A dissolves completely in 10.0 cm3 of distilled water. (1 mark)

(b) Calculate the solubility of substance A in grams per 100 g water at temperature, Ts. (2 marks)

PROCEDURE II

Using a funnel, transfer all the mixture obtained from Procedure I into a 250 ml volumetric flask. Rinse the boiling tube and the thermometer with about 20 cm3 of distilled water and add the rinses into the volumetric flask. Repeat the rinsing two more times. Add about 100 cm3 of distilled water to the volumetric flask. Shake until all the solid dissolves. Add more distilled water to the mark. Label this as solution A. 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. Add three (3) drops of the indicator provided and titrate using solution A. Record your readings in table 2 below. Repeat the titration two more times and complete the table.

(3 marks)

(a) Calculate the:

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

(ii) number of moles of hydrochloric acid, solution B used. (l mark)

(b) Given that two moles of acid react with one mole of substance A, calculate:

(i) number of moles substance A used. (1 mark)

(ii) concentration of solution A in moles per litre; (1 mark)

(iii) concentration of solution A in g per litre; (l mark)

(iv) relative formula mass of substance A. (1 mark)

2. You are provided with solid C. Carry out the following tests and record your observations and inferences in the spaces provided. Place all the solid C in a boiling tube. Add about 15 cm3 of distilled Water and shake until all the solid dissolves. Use 2cm3 portions of the Sodium solution in a test tube, for each of the tests in (a), (b), (c), (d), (e) and (f).

3 You are provided with substance L. Carry out the following tests and record your observations and inferences in the spaces provided. Use about 2 cm3 portions of substance L in a test-tube for each of the tests, (a), (b), (c) and (d).

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