KCSE 2010 Chemistry Paper 3 Questions and Answers

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You are provided with:
Acid A labeled solution A;
2.0 M sodium hydroxide solution labeled solution B.
Solution C containing 25.0 g per litre of alkanoic acid.

You are required to:
1. Prepare a dilute solution of sodium hydroxide, solution B
2. Determine the:
  1. Molar mass of the alkanoic acid.
  2. Reaction ratio between sodium hydroxide and acid A.
    
    Procedure 1
    Using a pipette and a pipette filler, place 25.0cm3 of solution B in a 250.0 ml volumetric flask. Add about 200 cm³ of distilled water. Shake well. Add more distilled water to make up to the mark. Label this solution D. Retain the remaining solution B for use in procedure II.
    
    Fill a burette with solution C. Using a clean pipette and a pipette filler, place 25.0 cm³ of solution D into a 250 ml conical flask. Add two drops of phenolphthalein indicator and titrate with solution C. Record your results in table 1. Repeat the titration two more times and complete the table.
    
    Determine the:
    (i) Average volume of solution C used; (1 mark)
    (ii) Concentration of solution D in moles per litre; (1 mark)
    (iii) Concentration of the alkanoic acid in solution C in moles per litre (1 mole of the acid reacts with 3 moles of the base); (1 mark)
    (iv) Molar mass of the alkanoic acid. (1 mark)
    
    Procedure II
    Fill a clean burette with solution A. Place 5 cm³ of solution A into a 100 ml beaker. Measure the initial temperature of solution A in the beaker and then record it in table II. Using 10 ml or 100 ml measuring cylinder, measuring 25 cm³ of solution B. Add it to solution A in the beaker and immediately stir the mixture with a thermometer. Record the maximum temperature reached in table II. Repeat the experiment with other sets of volumes of solution A and B and complete the table.
    
    (a) On the grid provided, plot graph of ΔT (Vertical axis) against the volume of solution A. (3 marks)
    
    (b) From the graph, determine the volume of solution A which gave the maximum change in temperature. (1 mark)
    (c) Determine the volume of solution B that reacted with the volume of solution A in (b) above. (1 mark)
    (d) Calculate the:
    (i) Ratio between the volumes of solutions A and B that neutralised one another. (1 mark)
    (ii) Concentration in moles per litre of the acid in solution A.
    (Assume that the volume ratio is the same as the mole ratio). (1 mark)
You are provided with solids E, F and G.
Carry out the tests below and write your observations and inferences in the spaces provided.
1. Place all of solid E in a boiling tube. Add 20 cm³ of distilled water and shake until all the solid dissolves. Label this as solution E.
  1. To about 2 cm³ of solution E in a test tube, add four drops of 2 M sulphuric (VI) acid.
  2. To about 2 cm³ of solution E in a at test-tube, add 2 M sodium hydroxide dropwise until in excess.
  3. Place one half of solid F in a test tube. Add 2 cm³ of distilled water and shake well. Add 4 drops of this solution to about 2 cm³ of solution E in a test-tube.
  4. To about 2 cm³ of solution E in a test tube, add two drops of aqueous potassium iodide.
2. 1. Using a metallic spatula, ignite about one half of solid G in a Bunsen burner flame.
  2. Place the other half of solid G into a boiling tube.
    Add 15 cm³ of distilled water and shake well. Label this solution G.
    Use this solution for the following tests.
    1. Place 2 cm³ of solution G in a test tube and determine its pH.
    2. To about 2 cm³ of the solution obtained in (ii) above, add 3 drops of acidified potassium manganate (VII).
    3. To about 2 cm³ of the solution obtained in (ii) above, add 2 drops of bromine water.
  3. To the remaining solution G in the boiling tube, add the other half of solution F.

Marking Scheme

	I	II	III
Final burette reading	13.80	27.80	40.70
Initial burette reading	0.00	13.80	27.30
Volume of solution used (cm³)	13.80	13.50	13.40

Average volume used
13.50 + 13.40 = 13.45 cm³
2
Mava = Mbvb
2 x 25 = 250 x Vb = 0.20M
Moles of NaOH used = 0.2 x 25/1000 = 0.005 moles
Moles of acid used = 1/3 x 0.0005
Concentration of acid = 0.005 x 100 = 0.12M
13.45 x 3
Molar mass of acid = 25/0.12 = 208.3

Table 2

Volume of solutin A (cm³)	5	9	13	17	21	25
Volume of soluton B (cm³)	25	21	17	13	9	5
Maximum temperature (°C)	30.5	34.0	36.5	36.5	34.0	30.5
Initial temperature (°C)	26.5	26.5	26.5	26.5	26.5	26.5
ΔT change in temperature	4.0	7.5	10.0	10.0	7.5	4.0

15cm3
30 - 15 = 15cm3
1. 15:15 = 1:1
2. MaVa = MbVb
  Ma x 15 = 1
  2 x 15 1
  Ma = 2 x 15 = 2
  15
  Ma = 2M

a
(i)	Observations	Inferences
	White ppt formed No effervesence	CO^2-₃ and SO^2-₃ ion absent Probably Pb²⁺, Ba²⁺ or Ca²⁺ may be present
(ii)	Observations	Inferences
	White ppt which Dissolves in excess	Pb²⁺ present
(iii)	Observations	Inferences
	White ppt formed	Insoluble cpd of Pb²⁺ is formed
(iv)	Observations	Inferences
	Yellow ppt	Pb²⁺ ions confrimed or Pbl₂ formed
b
(i)	Observations	Inferences
	Burns with a smoky flame	Unsaturated organic cpd or long chain hydrocarbon
(ii)	Observations	Inferences
	Colourless solution, turns red pH 1- 2	Carboxylic acid present
(iii)	Observations	Inferences
	Effrevesence colourless gas evolved Odourless gas	Confirm G was acid and F was carbonate
I	Observations	Inferences
	Decolourised KMnO4(l)	Unsaturated alkene or alcohol present
II	Observations	Inferences
	Bromine water decolourised	Unsaturated alkene present or alkyne