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KCSE CLUSTER TESTS 19

Chemistry Paper 3

1.

You are provided with:-

i) Magnesium ribbon, solid A.

ii) 0.7M hydrochloric acid, solution B.

iii) 0.3M Sodium hydroxide, solution C.

iv) Distilled water.

You are required to determine the;

a) Temperature change when magnesium reacts with excess hydrochloric acid.

b) Number of moles of hydrochloric acid that remain unreacted.

c) Number of moles of magnesium that reacted.

d) Molar heat of reaction between magnesium and hydrochloric acid.

Procedure I

Using a burette, measure 50cm3 of solution B and place it in a 100mls beaker. Measure the temperature of solution B in the 100mls beaker and record the value in table 1. Put the magnesium ribbon in the 50cm3 of solution B in the 100ml beaker and immediately start the stop watch. Stir the mixture with thermometer making sure that the magnesium ribbon remains inside the solution as it reacts. Measure the temperature after every 30 seconds and record the values in table 1. Complete table 1 below keep the resulting solution for procedure II below.

i) Plot a graph of temperature (y-axis) against time on the grid provided.

ii) On the graph, show the maximum change in temp.∆T and determine its value.

Value of ∆T...............................................................................

Procedure II

Transfer the contents of 100ml beaker into the 250ml volumetric flask. Wash the beaker with 100cm3 of distilled water and transfer the contents into the volumetric flask. Top up to the mark with distilled water, shaking the contents after each addition of water. Label this as solution D. Pipette 25cm3 of solution D into a conical flask. Add 3 drops of phenolphthalein indicator. Fill the burette with sodium hydroxide solution C and titrate against solution D in the conical flask until a pink colour just appears. Repeat two more times and complete the table below.

a) Calculate the average volume of solution C used.

............................................................................................................

b) Calculate the number of moles of solution C that reacted.

............................................................................................................

c) Calculate the number of moles HCl present in 25cm3 of solution D.

................................................................................................................................

d) Calculate the number of Moles of HCl in 250cm3 of solution D.

............................................................................................................

e) Calculate the number of moles of hydrochloric acid in 50cm3 of solution B.

............................................................................................................

f) Calculate the number of moles of hydrochloric acid that reacted with magnesium ribbon.

............................................................................................................

g) Calculate the number of moles of magnesium that reacted.

............................................................................................................

h) Determine the molar heat of reaction between magnesium and hydrochloric acid (S.H.C of solution =4.2J/g/K; density is 1.0g/cm3).

............................................................................................................

21 marks

2.

You are provided with solid K, a mixture of two salts. Carry out the tests below to confirm identity of the salts.

a) Place half a spatula endful of solid K in a dry test-tube and heat.

b) I. Place the remaining solid K in a boiling tube. Add 5cm3 of distilled water and shake.

Filter the mixture. Keep the residue and the filtrate for tests below.

Divide the filtrate into 3 portions.

i) To the 1st portion, add 1cm3 of lead (II) nitrate. Heat and allow to cool.

ii) Add 1cm3 of sodium hydroxide to the 2nd portion. Heat and test the gases produced using blue and red litmus papers.

iii) Heat the 3rd portion until no further change

II. Using a spatula, transfer the residue into a clean test-tube. Add 2cm3 of 2M nitric acid . Add 5cm3 of distilled water and shake. Divide the resulting solution into 4 portions.

i) To the 1st portion, add NaOH dropwise till in excess.

ii) To the 2nd portion, add 3 drops of potassium iodide solution.

iii) To the 3rd portion, add 1cm3 of dilute sulphuric acid.

iv) To the 4th portion, add ammonia solution dropwise till in excess

14 marks

3.

You are provided with liquid L carry out the tests below to confirm the functional groups in the liquid.

i) Put three drops of liquid L in a watch glass and ignite it using a burning splint.

ii) Put 1cm3 of liquid L in a test-tube. Add 1cm3 of water and shake. Allow to settle.

iii) Put 1cm3 of liquid L in a test-tube. Add 3drops of acidified KMnO4(aq)

iv) Put 1cm3 of liquid L in a test-tube. Add spatula of NaHCO3.

5 marks

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