Chemistry Paper 3 Questions and Answers - Asumbi Girls Pre Mock Examinations 2023

Instructions to Candidates

1. Answer ALL the questions
2. Mathematical tables and silent electronic calculators may be used.
3. All working MUST be clearly shown where necessary.

For Examiner’s Use Only

QUESTIONS

1. (20 marks) You are provided with:

• Solution P, hydrochloric acid
• Solution Q, containing 8.8g per litre of sodium hydroxide.
• 0.6g of an impure carbonate, solid B

You are required to determine the:

1. Concentration of solution P in moles per litre.
2. Percentage purity of the carbonate, solid B.

Procedure I
Fill the burette with sodium hydroxide, solution Q. Pipette 25.0cm3 of hydrochloric acid, solution P into a conical flask. Add 2 – 3 drops of methyl orange indicator and titrate. (The colour of the indicator changes from pink to yellow). Record your results in table 1 below. Repeat the titration two more times and complete the table.

Table I

(4 marks)

1. What is the average volume of solution Q used? (1 mark)
2. Determine the:
   1. Concentration of solution Q in moles per litre. (Na = 23.0; O = 16.0, H = 1.0) (1 mark)
   2. Concentration of solution P in mole per litre (2 marks) 

Procedure II
Using a 100ml measuring cylinder, measure out 100cm3 of solution P into a 250cm3 beaker.
Add all of solid B into the beaker containing solution P. Swirl the mixture and allow the reaction to proceed for about 4 minutes.
Label the solution obtained here as solution K.
Fill the burette with sodium hydroxide, solution Q. Pipette 25.0cm3 of solution K into a conical flask. Add 2- 3 drops of methyl orange indicator and titrate. Record your results in table II below. Repeat the titration two more times and complete the table.

Table II

(4 marks)
c) What is the average volume of solution Q used? (1 mark)

d) Calculate the :

1. Moles of hydrochloric acid in 25.0cm₃ of solution K. (1 mark)
2. Moles of hydrochloric acid in 100cm₃ of solution K. (1 mark)
3. Moles of hydrochloric acid in 100cm₃ of the original hydrochloric acid solution P. (1 mark)
4. Moles of hydrochloric acid that were used up in the reaction with solid B. (1 mark)
5. Moles of the carbonate that reacted with hydrochloric acid. (1 mark)

e) Given that the relative formula mass of the carbonate is 72, calculate the:

1. Mass of the carbonate that reacted. (1 mark)
2. Percentage purity of the carbonate, solid B. (1 mark) 

2. (10marks) You are provided with solid M. Carry out the tests below and record your observations and inferences in the spaces provided.

1. Place all of solid M in a boiling tube and add about 10cm3 distilled water and shake.
   

   Observations	Inferences

2. Divide the solution into five portions of about 2cm₃ each.
   1. To the first portion, add aqueous sodium hydroxide dropwise until in excess.
      

      Observations	Inferences

   2. To the second portion, add aqueous ammonia dropwise until in excess.
      

      Observations	Inferences

   3. Dip a glass rod in the third portion and heat it on a burnsen burner flame, identify the colour of the flame produced.
      

      Observations	Inferences

   4. To the fouth portion,add about 1cm3 of lead (II) nitrate solution.
      

      Observations	Inferences

   5. To the fifth portion, add about 1cm3 of dilute sodium hydroxide followed by a small piece of aluminium foil. Warm the mixture gently and carefully. Test any gas produced using blue and red litmus papers.
      

      Observations	Inferences

3. (10marks)You are provided with solid W. Carry out the tests below and record your observations and inferences in the spaces provided.

1. Place about one third of solid W on a metallic spatula and burn it using a Bunsen burner
   

   Observations	Inferences

2. Place the remaining solid W into a boiling tube. Add 10cm3 of distilled water and shake well. Use 2cm₃ portions of the mixture for each of the following reactions.
   1. To the first portion, add 2 drops of acidified potassium manganate (VII) and shake well.
      

      Observations	Inferences

   2. To the second portion, add three drops of bromine water
      

      Observations	Inferences

   3. To the third portion put universal indicator paper.
      

      Observations	Inferences

   4. To the fourth portion add a little solid Y, sodium hydrogen carbonate.
      

      Observations	Inferences

   5. To the fifth portion add about 2cm3 of ethanol and warm the mixture.
      

      Observations	Inferences

MARKING SCHEME 

1. (20 marks) You are provided with:

• Solution P, hydrochloric acid
• Solution Q, containing 8.8g per litre of sodium hydroxide.
• 0.6g of an impure carbonate, solid B

You are required to determine the:

1. Concentration of solution P in moles per litre.
2. Percentage purity of the carbonate, solid B.

Procedure I
Fill the burette with sodium hydroxide, solution Q. Pipette 25.0cm3 of hydrochloric acid, solution P into a conical flask. Add 2 – 3 drops of methyl orange indicator and titrate. (The colour of the indicator changes from pink to yellow). Record your results in table 1 below. Repeat the titration two more times and complete the table.

Table I

(4 marks)

1. What is the average volume of solution Q used? (1 mark)
   • 23.9 + 24.0 + 24.1 = 24.0cm³
                  3
2. Determine the:
   1. Concentration of solution Q in moles per litre. (Na = 23.0; O = 16.0, H = 1.0) (1 mark)
      • 8.8 / 40 = 0.22M
   2. Concentration of solution P in mole per litre (2 marks) 
      • M1V1 / M2V2 = 1/1
        M1 x 25 = 1
        0.22 x 24  1
        M1 = 0.22 x 24 
                       25
        = 0.2112 M

Procedure II
Using a 100ml measuring cylinder, measure out 100cm3 of solution P into a 250cm3 beaker.
Add all of solid B into the beaker containing solution P. Swirl the mixture and allow the reaction to proceed for about 4 minutes.
Label the solution obtained here as solution K.
Fill the burette with sodium hydroxide, solution Q. Pipette 25.0cm3 of solution K into a conical flask. Add 2- 3 drops of methyl orange indicator and titrate. Record your results in table II below. Repeat the titration two more times and complete the table.

Table II

(4 marks)
c) What is the average volume of solution Q used? (1 mark)

12.4 + 12.5 + 12.6 = 12.5cm³
              3

d) Calculate the :

1. Moles of hydrochloric acid in 25.0cm₃ of solution K. (1 mark)
   • Moles of HCL in 25cm₃ of A = moles of NaOH in 12.5 cm₃ of J
     0.22 x 12.5 
        1000
     = 0.00275 moles
2. Moles of hydrochloric acid in 100cm₃ of solution K. (1 mark)
   • 0.00275 x 100
             25
     = 0.011 moles
3. Moles of hydrochloric acid in 100cm₃ of the original hydrochloric acid solution P. (1 mark)
   • 0.2112 x 100
           1000
      = 0.02112 moles
4. Moles of hydrochloric acid that were used up in the reaction with solid B. (1 mark)
   • Ans a(iii) - a(ii)
     = 0.02112 - 0.011
     = 0.01012 moles
5. Moles of the carbonate that reacted with hydrochloric acid. (1 mark)

   • CO₃^2-_(aq) + 2H⁺_(aq)   →  CO_2(g) + H₂O_(l)

   • 0.01012 x 1 = 0.00506 moles
               2

e) Given that the relative formula mass of the carbonate is 72, calculate the:

1. Mass of the carbonate that reacted. (1 mark)
   • 0.00506 x 72= 0.36432g
             1
2. Percentage purity of the carbonate, solid B. (1 mark) 
   • percentage purity  = 0.36432 x 100
                                          0.6

2. (10marks) You are provided with solid M. Carry out the tests below and record your observations and inferences in the spaces provided.

1. Place all of solid M in a boiling tube and add about 10cm3 distilled water and shake.
   

   Observations	Inferences
   Colourless solution	Cu2+, Fe2+, Fe3+ absent

2. Divide the solution into five portions of about 2cm₃ each.
   1. To the first portion, add aqueous sodium hydroxide dropwise until in excess.
      

      Observations	Inferences
      White ppt (½mk) insoluble in excess½mk	Ca2+, Mg2+, Ba2+present - All three/any two correct ions given 1 mk. - Only one correct ion given ½ mk.

   2. To the second portion, add aqueous ammonia dropwise until in excess.
      

      Observations	Inferences
      White ppt (½mk) insoluble in excess½mk	Mg2+, Ba2+present - All two correct ions given 1 mk. - Only one correct ion given ½ mk.

   3. Dip a glass rod in the third portion and heat it on a burnsen burner flame, identify the colour of the flame produced.
      

      Observations	Inferences
      Green flame	Ba2+present

   4. To the fouth portion,add about 1cm3 of lead (II) nitrate solution.
      

      Observations	Inferences
      No white ppt 1mk NOTE - Accept “No ppt”/”colourless solution retained”/ “No observable change” for½mk but accept CORRECT inference and credit accordingly  - REJECT Colourless solution formed/No colour change/ No white substance/ No reaction/ No observation	SO42-, CO32-, SO32-absent - All 3 ions given 2mks - 2 ions given 1mk - 1 ion given ½ mk NOTE: - Where there is a contradictory ion mark out of 1½ mks and penalise ½ mk  for EACH contradictory ion given to a maximum of 1½ mks.

   5. To the fifth portion, add about 1cm3 of dilute sodium hydroxide followed by a small piece of aluminium foil. Warm the mixture gently and carefully. Test any gas produced using blue and red litmus papers.
      

      Observations	Inferences
      Colourless gas (½mk) the turns red litmus blue and blue litmus remains blue (½mk)	NO3- present

3. (10marks)You are provided with solid W. Carry out the tests below and record your observations and inferences in the spaces provided.

1. Place about one third of solid W on a metallic spatula and burn it using a Bunsen burner
   

   Observations

   Inferences

   Solid melts and burns with a yellow(½mk)/ luminous and sooty(½mk)/ smoky flame

   C = C  /   ̶ C ≡ C  ̶   present (1mk)   NOTE Accept either of the following given in words in place of the above structures for FULL credit. Carbon to carbon double/ triple bond present /Unsaturated organic compound /Long chain organic compound /Aromatic compound /Organic compound with high Carbon-hydrogen ratio. (Reject C = C / C ≡ C) Ignore Alkene / Alkyne present/Long chain hydrocarbon present. Penalize FULLY for any contradicting functional group given.

2. Place the remaining solid W into a boiling tube. Add 10cm3 of distilled water and shake well. Use 2cm₃ portions of the mixture for each of the following reactions.
   1. To the first portion, add 2 drops of acidified potassium manganate (VII) and shake well.
      

      Observations	Inferences
      Acidified potassium manganate (VII) solution is decolourised OR Purple colour of acidified potassium manganate (VII) changes to colourless OR purple colour of acidified KMnO4 solution turns colourless.	C = C  /   ̶ C ≡ C  ̶  (½mk), R – OH (½mk)  present   NOTE In absence of the above structures, accept the CORRECT inference given in words for FULL credit as “unsaturated organic compound and alcohol / alkanol present”.

   2. To the second portion, add three drops of bromine water
      

      Observations	Inferences
      Yellow/ orange bromine is decolourised. (1mk)	C = C  /   ̶ C ≡ C  ̶    present

   3. To the third portion put universal indicator paper.
      

      Observations	Inferences
      pH=4 (1mk) Accept pH = 5 or 6 Reject pH given as a range	Weakly acidic (1mk) Reject Weak acid/weak  alone

   4. To the fourth portion add a little solid Y, sodium hydrogen carbonate.
      

      Observations	Inferences
      Effervescence / Bubbles of a colourless gas / fizzing (½mk) NOTE Reject: hissing / fizzling / sizzling / “colourless gas” on its own	R – COOH present (½mk) NOTE Accept either of the following if given in place of the above structures for carboxylic / alkanoic acid present solution is acidic H3O+ / H+ present

   5. To the fifth portion add about 2cm3 of ethanol and warm the mixture.
      

      Observations	Inferences
      Fruity / Pleasant smell (½mk) Reject : Sweet smell	R-COOH, -COOH present (½mk) (Accept for above in words for ½mk) Ester formed if R-COOH is not mentioned). N/B: Penalize fully for any contradictory functional group.

CONFIDENTIAL

1. 200cm³ of hydrochloric acid, solution P.
2. 150cm³ of sodium hydroxide, solution Q.
3. 50g of solid B weighed accurately.
4. About 0.2g of solid Y.
5. About 0.5g of solid M.
6. About 0.5g of solid W.
7. Burette 0-50 cm³.
8. Pipette 25 cm³.
9. One pipette filler
10. Means of labeling (one label).
11. 100 cm³ measuring cylinder.
12. 10ml measuring cylinder
13. 250 cm³ beaker
14. Two conical flasks.
15. Six dry test-tubes.
16. About 1cm by 3cm aluminium foil
17. 1 red and 1 blue litmus papers.
18. Universal indicator paper
19. About 2cm³ ethanol placed in a test tube
20. 2 boiling tubes.
21. Metallic spatula
22.  Funnel
23. A test-tube holder.
24. Stop clock (wall clock)

BENCH REAGENTS/ ACCESS REAGENTS

1. Methyl orange indicator – supplied with dropper
2. Distilled water in a 500ml wash bottle
3. 2M sodium hydroxide – supplied with dropper
4. 2M aqueous ammonia – supplied with dropper
5. 5M lead (II) nitrate solution – supplied with dropper
6. Source of heat
7. Universal indicator solution supplied with a dropper
8. pH chart
9. Acidified potassium manganate (VII) – supplied with a dropper
10. Bromine water – supplied with a dropper

NOTES

1. Solution P is prepared by adding 18.0cm³ (density=1.18g/cm³) of concentrated hydrochloric acid into 600cm³ of distilled water contained in a one litre volumetric flask and diluting to one litre of solution.
2. Solution Q is prepared by dissolving 8.80g of sodium hydroxide in 600cm³ of distilled water contained in a one litre volumetric flask and diluting to one litre of solution. (22M)
3. Acidified potassium manganate (VII) is prepared by dissolving 2.0g of potassium manganate (VII) provided in about 100cm³ of 2M sulphuric (VI) acid, adding 800cm³ of distilled water and diluting to one litre of solution. Label this as acidified potassium manganate (VII).
4. Bromine water is prepared by adding 2ml of liquid bromine to 100cm³ of distilled water and the mixture stirred well in a fume cupboard.
5. Solids :
   • B is Na₂CO₃
   • M is Ba(NO₃)₂
   • W is Maleic acid
   • Y is NaHCO₃