Chemistry Paper 3 Questions and Answers with Confidential - Chogoria Murugi Zone Pre Mock Exams 2023

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INSTRUCTIONS TO CANDIDATES

  • 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 2 ¼ hours allowed for this paper. This time is to enable you to read the Question paper and make sure you have all the chemicals and apparatus you need.
  • All working must be clearly shown where necessary.
  • Mathematical tables and electronic calculators may be used.

Question 1

You are provided with the following:

  • Solution A ; Hydrochloric acid
  • Solution B; 0.03M sodium hydroxide
  • Solution C, Containing 15.74g of Na2CO3. XH2O in 250ml of the solution.

You are required to determine: -

  1. The concentration of solution A
  2. The value of X in the carbonate Na2CO3. XH2O

PROCEDURE A

  • Fill the burette with solution A. Using clean pipette, place 25.0cm3 of solution B into a 250ml conical flask.
  • Add 2 drops of phenolphthalein indicator and titrate with solution A. Record your results in table I below. Repeat the experiment two more times and complete the table.
    Table I                                                                                                               (4mks)
       I  II  III
     Final burette reading (cm3)      
     Initial burette reading (cm3)      
     Volume of solution A (cm3) used      
  1. Determine the :-
    1. Average volume of solution A used.                                                                    (1mk)
    2. Number of moles of sodium hydroxide in 25 cm3 of solution B used.             (1mk)
    3. Number of moles of acid in volume of solution A used.                                   (1mk)
    4. Concentration of solution A in moles per litre.                                                  (1mk)

      PROCEDURE B
      • Using a clean pipette, place 25.0 cm3 of solution C into a 250ml volumetric flask. Add about 100cm3 of distilled water. Shake well and add more distilled water to make up to the mark. Label this solution D
      • Fill the burette with solution A. Using a clean pipette, place 25 cm3 of solution D into a conical flask. Add 2 drops of methyl orange indicator and titrate with solution A. Record your results in the table II.
      • Repeat the titration two more times and complete table II.
        Table II                                                                                                                   (4mks)
           I  II  III
         Final burette reading (cm3)      
         Initial burette reading (cm3)      
         Volume of solution A (cm3) used      
  2. Determine the: -
    1. Average volume of solution A used.                                                                (1mk)
    2. Moles of the acid of solution A that reacted with the carbonate solution D.   (1mk)
    3. Number of moles of the carbonate in 25 cm3 of solution D used.                  (1mk)
    4. Number of moles of carbonate in 250cm3 of solution D                                (1mk)
    5. Concentration of carbonate solution C in moles per litre.                              (1mk)
    6. Concentration of the carbonate solution C in grams per litre.                    (1mk)
    7. Value of X in Na2CO3. XH2O (H=1.0, C=12.0, O=16.0 Na=23.0)               (1mk)

Question 2

You are provided with solid M. Use it to carry out the tests below. Write the observations and inferences in the space provided.

  1. Place solid M in a boiling tube and add about 10cm3 of distilled water. Divide the resulting solution into five portions
     Observation  Inference 
     (1 mk)  1 (mk) 
  2. To the first portion add sodium hydroxide solution dropwise till excess.
     Observation  Inference 
     (1 mk)  1 (mk) 
  3. To the second portion add aqueous ammonia solution dropwise till excess.
     Observation  Inference 
     (1 mk)  1 (mk) 
  4.  To the third portion add 3 drops of sodium chloride solution.
     Observation  Inference 
     (1 mk)  1 (mk) 
  5. To the fourth portion add about 2cm3 of HNO3 acid.
     Observation  Inference 
     (1 mk)  1 (mk) 
  6. To the fifth portion add 3 drops of lead (II) nitrate solution and warm.
     Observation  Inference 
     (1 mk)  1 (mk) 

Question 3

You are provided with solid G. Use it to carry out the tests below. Write the observations and inferences in the space provided.

  1. Using a clean metallic spatula, take one third of solid G and place on Bunsen burner flame.
     Observation  Inference 
     (1 mk)  1 (mk) 
  2. Place the remaining solid G in a boiling tube. Add 10cm3 of distilled water and shake the mixture until all the solid dissolves. Divide the resulting solution into four portions.
    To the first portion add 3 drops of acidified potassium manganate (vii)
     Observation  Inference 
     (1 mk)  1 (mk) 
  3. To the second portion add 3drops of bromine water
     Observation  Inference 
     (1 mk)  1 (mk) 
  4. To the fourth portion dip universal indicator paper and determine the pH of the solution
     Observation  Inference 
     (1 mk)  1 (mk) 
  5. To the third portion add solid sodium hydrogen carbonate
     Observation  Inference 
     (½ mk)  ½ (mk) 

CONFIDENTIAL

INSTRUCTIONS

In additional to the apparatus and fittings common in a chemistry laboratory, each candidate will require the following.

  • About 150ml of solution labelled A.
  • About 100ml solution labelled B.
  • About 50ml of solution labelled C
  • About 0.2g of sodium hydrogen carbonate in a stoppered container.
  • About 0.5g of solid M in a stoppered container.
  • About 0.5g of solid G in a stoppered container
  • 0 – 50ml burette.
  • 25ml pipette.
  • Two 250ml conical flasks
  • 250ml volumetric flask
  • 10ml measuring cylinder.
  • Six test tubes on a test tube rack.
  • A boiling tube.
  • test tube holder.
  • Complete stand.
  • A white tile.
  • One metallic spatula.
  • Distilled water in a wash bottle.
  • One label

Access to:

  • Source of heat.
  • Universal indicator paper and its pH chart.
  • 2M aqueous ammonia supplied with a dropper.
  • 2M aqueous sodium hydroxide supplied with a dropper.
  • Pb(NO3)2(aq) supplied with a dropper
  • Acidified potassium manganate (VII) supplied with a dropper.
  • Bromine water supplied with a dropper.
  • 2M dilute nitric (V) acid.
  • Methyl orange indicator with a dropper
  • Phenolphthalein indicator with a dropper
  • Sodium chloride solution

NB:

  1. Solution A is prepared by dissolving 4.cm3 of concentrated HCl (1.18g/cm3 ) to 500cm3 of water and dilute to 1 litre.
  2. Solution B is prepared by dissolving 1.2g of NaOH pellets in about 600ml of distilled water and diluting to 1 litre.
  3. Solution C is prepared by dissolving 62.9g of Na2CO3 .10H2O in about 800ml of distilled water and then topping up to 1 litre.
  4. Acidified potassium permanganate is prepared by dissolving 3.5g of KMnO4(s) in 200cm3 of 2M H2SO4(aq)  and toping up to one litre solution.
  5. 2M H2SO4(aq) is prepared by diluting 110cm3 of concentrated Sulphuric (VI) acid to make one litre of solution.
  6. 2M NaOH (aq) is prepared by dissolving 80g of NaOH pellets in one litre of solution.
  7. 2M HNO3 is prepared by adding 128cm3 of Conc. HNO3 to about 500ml of water and dilute to 1 litre.
  8. Sodium chloride solution is prepared dissolving 5.85g of NaCl in 1 litre of water
  9. Lead (II) nitrate solution is prepared by dissolving 30g of Pb(NO3)2 in 1litre of water
  10. Solid M is aluminium sulphate
  11. Solid G is maleic acid.

MARKING SCHEME

Question 1

Procedure A

PROCEDURE II

Table I

Titration 

I

II

III

Final burette readings ( cm3)

15.0

15.0

15.0

Initial burette reading ( cm3)

0.0

0.0

0.0

Volume ]of solution R used ( cm3)

15.0

15.0

15.0

  • Complete table            C.T.  1mk
  • Decimal point             D.P    1mk
  • Accuracy                    AC      1mk
  • Principle averaging     P.A     1mk
  • Final Accuracy            FA      1mk
                                                5mks            
    Complete table………………………………………………….(1 Mark)

Conditions 

  • penalize ½ mk for inversion
  • penalize ½ mk for unrealistic reading of less than 0.1 and above 50
  • penalize ½ mk for wrong arithmetic

N/B penalize once only for ANY or ALL the above.

Decimal place …………………………………………………..(1 Mark)

  • All readings must be recorded to at least one decimal place used consistently or 2 decimal places used consistently.
  • If 2 decimal places are used then the second decimal place Must be a zero or a five otherwise penalize fully.

Accuracy ……………………………………………………….. (1 Mark)

  • If any of the titre values is within ± 0.1 of school value award 1mk
  • If any of the titre values is within ± 0.2 of school value award ½ mk or otherwise penalize fully

Principle of averaging ………………………………………….. (1 Mark)

N/B – Only consistent values should be averaged

  • The values averaged must be shown
  • The averaged value should be recorded to at least 2dp unless it works out to exactly a whole number or 1 dp.

Final answer

  • If the averaged value is within ± 0.1 of the school value – award 1mk
  • If the average value is outside ± 0.1 but within ± 0.2 – award ½ mk
  • If averaging is not done but there are consistent values average for the candidate credit accordingly.

( ii) Number of moles of NaOH, solution B used.        ( 1mk)

mols = 0.03 ×25  =0.00075mols
               1000

(iii) Moles of acid solution A used in the reaction  (1 mk)NaOH : HCl is 1:1 thus mols of HCl=0.00075mols

(iv) concentration of acid solution A in mols/litre.        ( 1mk)
molarity of soln A = 0.00075 ×1000  
                                 average titre

PROCEDURE B

Table II

Titration 

I

II

III

Final burette readings ( cm3)

22.0

22.0

22.0

Initial burette reading ( cm3)

0.0

0.0

0.0

Volume ]of solution R used ( cm3)

22.0

22.0

22.0

  • Complete table          C.T      1mk
  • Decimal point             D.P     1mk
  • Accuracy                    A C     1mk
  • Principle averaging     P.A      1mk
  • Final Accuracy             FA      1mk    
                                                    5mks            

Complete table………………………………………………….(1 Mark)

Conditions 

  • penalize ½ mk for inversion
  • penalize ½ mk for unrealistic reading of less than 0.1 and above 50
  • penalize ½ mk for wrong arithmetic

N/B penalize once only for ANY or ALL the above.

Decimal place …………………………………………………..(1 Mark)

  • All readings must be recorded to at least one decimal place used consistently or 2 decimal places used consistently.
  • If 2 decimal places are used then the second decimal place Must be a zero or a five otherwise penalize fully.

Accuracy ……………………………………………………….. (1 Mark)

  • If any of the titre values is within ± 0.1 of school value award 1mk
  • If any of the titre values is within ± 0.2 of school value award ½ mk or otherwise penalize fully

Principle of averaging ………………………………………….. (1 Mark)

N/B – Only consistent values should be averaged

  • The values averaged must be shown
  • The averaged value should be recorded to at least 2dp unless it works out to exactly a whole number or 1 dp.

Final answer

  • If the averaged value is within ± 0.1 of the school value – award 1mk
  • If the average value is outside ± 0.1 but within ± 0.2 – award ½ mk
  • If averaging is not done but there are consistent values average for the candidate credit accordingly.

(b)(ii) Moles of solution A that reacted with solution D  ( 1mks)
mols = Molarity of soln A × average titre
                             1000
(iii) Moles of solution D used in the reaction     ( 1mk)
Reacting ratio of Na2CO3 : HCl is 1:2 thus moles of D
mols = mols of HClsoln A used
                            2
(iv) Moles of solution D present in 250ml.      ( 1mk)
mols = mols in (iii) above × 250
                            25    
(v) Concentration of carbonate soln C in moles per litre                                             ( 1mks)
molarity = mols in (iv) above ×1000
                                 250  
(vi) Concentration of carbonate soln C in grams per litre                                            ( 1mks)
mols = 15.74×1000  = 62.96g/l
                  250
(vii) Value  of X in Na2CO3. XH2O.    (H=1.0, C=12.0, O=16.0 Na=23.0)         (1mk)

Question 2

Solid M is aluminium sulphate, Al2SO4 

   i)

 

Dissolves to form a colourless solution  ✓ 1                         

Soluble salt  ✓ 1      or 

Cu2+, Fe2+, Fe3+ absent ✔½

    ii

white ppt which dissolves in excess NaOH ✓ 1      

Pb 2+, Al 3+, Zn 2+  present ✓ 1      

   iii

Forms a white ppt insoluble in excess NaOH ✓ 1      

Pb 2+, Al 3+  present ✓ 1      

   iv

No white precipitate is formed ✓ 1      

Al 3+  present ✓ 1      

   v

No effervescence/no bubbles ✓ 1      

CO32-, HCO3-, SO32-    absent ✓ 1      

   vi

white precipitate insoluble on warming ✓ 1      

SO42- present ✓ 1      

 

Question 3

Solid G is maleic acid.

a

Melts and burns with a yellow sooty flame ✓ 1      

   \            /

    C  =   C   or –C ≡ C-  present  ✓ 1      

   /            \

b

Purple KMnO4 is decolourised✓ 1      

   \            /

    C  =   C   ,  –C ≡ C-  or R –OH    present ✓ 1      

   /            \   

c

Yellow bromine water is decolurised ✓ 1      

   \            /

    C  =   C   or –C ≡ C-  present ✓ 1      

   /            \                 

d

Accept pH= 2 to 3  ✓ 1      

Strongly acidic substance

e

Effervescence occurs / bubbles are produced ✔½

    H+ , - COOH  present ✔½

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