Chemistry Paper 3 Questions and Answers - Sukellemo Joint Pre Mock Exams 2022

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QUESTIONS

QUESTION 1
You are provided with:

  • 6.0g of an alkanoic acid labelled solid P in a boiling tube.
  • 2M sodium hydroxide solution labelled solution Q.

You are required to:

  1. Determine the solubility of solid P at different temperatures.
  2. Determine the number of moles of water of crystallization in solid P.
  3. Find the molar mass of the alkanoic acid.

Procedure I

  1. Using a burette add 10cm3 of distilled water to the solid P in the boiling tube. Heat the mixture while stirring with a thermometer to about 70ºC. When the entire solid has dissolved allow the solution to cool while stirring with the thermometer. Note the temperature at which crystals of solid P first appear. Record this temperature in the Table 1 below.
  2. Using the burette add 2cm3 of distilled water to the contents of the boiling tube. Warm the mixture, while stirring with the thermometer until all the solid dissolves. Allow the mixture to cool while stirring. Note the temperature at which the crystals of solid P first appear.
  3. Repeat the procedure (ii) two more times and record the temperatures in Table I. Retain the contents of the boiling tube for use in procedure II.

TABLE I

Volume of water in the boiling tube (cm3) Temperature at which crystals of solid P first appear (ºC) Solubility of Solid P (g/100gH2O)
     
  1.    
    1. Complete Table 1 by calculating the solubility of solid P at different temperatures (6 mks)
    2. On the grid provided, plot a graph of the solubility of solid P (vertical axis) against temperature. (3 mks)
    3. Using the graph determine the temperature at which 55g of solid P would dissolve in 100cm3 of water. (1 mk)

Procedure II

  1. Transfer the contents of the boiling tube in Procedure I into a 250 ml volumetric flask. Rinse both the boiling tube and the thermometer with distilled water and add to the volumetric flask. Add more distilled water to make up to the mark. Transfer the solution to a 250 ml beaker , Label this solution R. Wash the volumetric flask then rinse it with distilled water ready for use in Step(ii).
  2. Using a measuring cylinder place 25.0 cm3 of solution Q into a 250 ml volumetric flask. Add about 200cm3 of distilled water. Shake well. Add more distilled water to the make up to the mark. Label this solution T.
  3. Fill a burette with solution R. Using a pipette and pipette filler, place 25 cm3 of solution T into a 250 ml conical flask. Add 2-3 drops of phenolphthalein indicator and titrate with solution R. Record your results in table II. Repeat the titration two more times and complete the table.

TABLE II

Titre 1 2 3
Final burette reading (cm3)      
Initial burette reading (cm3)      
Volume of solution R used (cm3)      

(4 marks)

DETERMINE:

  1. The average volume of solution R used (1 mk)
  2. The concentration of solution T in moles per litre. (1 mk)
  3. The concentration of alkanoic acid solution R in moles per litre (1 mole of acid reacts with 2 moles of the base) (2 mks)
  4. The relative formula mass of the alkanoic acid, solid P (2 mks)
  5. The formula of P has the form M.xH2O. Determine the value of x in the formula. (The relative formula mass of M is 90; O=16;H = 1) ( 1mk)

QUESTION 2
You are provided with solid Y. Use it to carry out the tests indicated below and record your observations and inferences in the spaces provided.

  1. Put half a spatula of solid Y in a boiling tube. Add about 5 cm3 of distilled water and shake. Divide the resultant mixture into four portions of 1 cm3 each.
    Observations Inferences
       
  2. To the first portion add aqueous sodium hydroxide dropwise till in excess.
    Observations Inferences
       
  3. to the second portion add aqueous ammonia dropwise till in excess
    Observations Inferences
       
  4. To the third portion add lead (II) nitrate solution and warm.
    Observations Inferences
       
  5. To the fourth portion add dilute nitric (V) acid followed by Barium nitrate solution.
    Observations Inferences
       

QUESTION 3
You are provided with solid M and solid F.
Carry out the tests below and write the observations and inference in the spaces below:

  1. Using a metallic spatula, ignite about one half of solid M in a Bunsen burner flame.
    Observations Inferences
       
  2. Place the other half of solid M in a boiling tube. Add 15cm3 of distilled water and shake well. Label this solution M. Use this solution for the following tests.
    Observations Inferences
       
  3. Place 2cm3 of solution M in test tube and determine pH.
    Observations Inferences
       
  4. To about 2 cm3 of the solution M obtained in (b) above add 3 drops of acidified potassium manganate (VII)
    Observations Inferences
       
  5. To about 2 cm3 of the solution M obtained in (b) above add 2 drops of bromine water.
    Observations Inferences
       
  6. To the remaining solution M in the boiling tube, add half of solid N
    Observations Inferences
       


MARKING SCHEME

  1.    
    Volume of water in the boiling tube (cm3) Temperature at which crystals of solid P first appear (ºC) Solubility of Solid P (g/100gH2O)
    10 55 6x100 = 60
      10
    12 50 6x100 = 50
       12
    14 45 6x100 = 42.9
       14
    16 40 6x100 = 37.5
       16
    1.    
      1.    
        • complete table
        • decimal point(whole number throughout/1do)
        • calculation
        • school value
        • trend(decrease in temperature)
      2.    
        1
      3. 52.5ºC

TABLE II

Titre 1 2 3
Final burette reading (cm3) 13.2 27.2 13.2
Initial burette reading (cm3) 0.0 14.0 0.0
Volume of solution R used (cm3) 13.2 13.2 13.2

(4 marks)

  1.  
  2. 100cm3 = 2moles
    25cm3 = 25/1000 x 2 = 0.05moles
    0.05moles = 250cm3
    1000 x 0.05 = 0.2M
     250
  3. Moles of base in 25cm3
    0.2 x 25 = 0.005moles
      1000
    Moleratio of acid:base
    1:2
    0.005 x 1 = 0.0025moles acid
       2
    Acid 
    0.0025moles = av. volume
       100cm 3 x 0.0025 
    av. volume
  4. ans in (i) above 250 = correct ans
             1000
    6
    ans in (c) above x 250
            1000
    OR
           6       
    ans above
  5. RFM of P is M.H2O = Ans in (d) above
    x = ans in (d) above - 90
                  18
    = correct ans

QUESTION 2

Observations Inferences
  1. Solid dissolves to form a colourless solution      
    1. White precipitate dissolves to form a colourless solution
    2. White precipitate dissolves in excess to form a colourless solution
    3. White precipitate does not dissolve on warming 
    4. No effervescence, white precipitate
  1. Soluble salt
    1. Colored ions absent Cu2+, Fe2+, Fe3+
    2. Zn2+, Al3+, Pb2+ Present
    3. Zn2+ present
    4. SO42-, SO32-, CO32-, present
      Cl- absent
    5. SO42- present 

QUESTION 3

Observations Inferences
  1. Solid melts and burns with a yellow sooty flame
  2.    
    1. pH = 3
      Accept aby valve within (1-3) 
    2. Purple KMnOq(aq) to colourless
    3. Yellow bromine water changes colourless
    4. Effervescence
  1. Strongly acidic
  2.    
    1. 2
    2. 3
    3. 4
    4. 5

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