Chemistry Paper 3 Questions and Answers with Confidential - Form 3 Mid Term 3 Exams 2023

INSTRUCTIONS TO CANDIDATES

1. Answer all the questions in the spaces provided in the question paper.
2. 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 that you may need.
3. All working must be clearly shown.

Question 1

You are provided with:

• 0.2 M Sodium Hydroxide solution
• X M hydrochloric acid solution X
• Y M Na₂CO₃ solution Y
  You are required to standardize hydrochloric acid and hence determine the concentration of sodium carbonate.
  
  PROCEDURE I
  Using a pipette transfer 25cm³ of 0.2M Sodium hydroxide into a conical flask. Add 2-3 drops of phenolphthalein indicator and then titrate with hydrochloric acid provided in a beaker from a burette. Shake the conical flask after each addition and note the volume required to neutralize the sodium hydroxide solution. Record your results in the table below
  
  

  Titre	I	II	III
  Final burette reading (cm3)
  Initial burette reading (cm3)
  Volume of X used (cm3)

  (3 mks)
  1. What is the average volume of solution X used? (1 mk)
  2. Calculate the number of moles of solution X required to completely neutralize 25cm³ of sodium hydroxide solution.
     (2 mks)
  3. What is the morality of the hydrochloric acid, solution X? (2 mks)
     
     PROCEDURE II
     Rinse the pipette thoroughly then pipette 25cm³ of sodium carbonate into a conical flask. Add 2-3 drops of phenolphthalein indicator. Refill the burette with hydrochloric acid and use it to titrate the contents of the conical flask. Shake the flask after each addition of the acid and note the volume of the acid required to neutralize 25cm³ of the sodium carbonate. Record your results in the table below.     (3 mks)
     
     

     Titre	I	II	III
     Final burette reading (cm3)
     Initial burette reading (cm3)
     Volume of X used (cm3)

  4. What is the average volume of hydrochloric acid used in the second procedure? (1 mk)
  5. Calculate the number of moles of sodium carbonate in 25cm3 of solution. (2 mks)
  6. Calculate the concentration of sodium carbonate in moles per litre. (2 mks)

Question 2

You are provided with solid A. Carry out the tests below. Identify any gas or gases produced and record your observations and inferences.

1. Heat gently a spatula endful of solid A in a dry test tube. Test the gas with red and blue litmus paper.  
   

   Observations	Inferences
   (2mks)	(2mks)

2. Place a spatula endful of solid A in a boiling tube. Add 10cm³ of distilled water. Filter off the residue. Divide the filtrate into two portions. Retain the residue.
   1. To the first portion add 3 drops of ammonia solution and then excess. 
      

      Observations	Inferences
      (1mk)	(1mk)

   2. To the second portion add 3 drops of Lead (ii) nitrate solution.
      
      

      Observations	Inferences
      (1mk)	(1mk)

3. To the residue obtained in (b) above add 5cm³ of dilute nitric acid. Divide the solution into two portions.
   
   

   Observations	Inferences
   (1mk)	(1mk)

   1. To the first portion add 3 drops of Ammonia solution and then excess.
      
      

      Observations	Inferences
      (1mk)	(1mk)

   2. To the second portion add 3 drops of lead (ii) nitrate solution.
      

      Observations	Inferences
      (1mk)	(1mk)

Question 3

You are provided with solid M. carry out the tests below and record your observations and inferences in the spaces provided.

1.  Place a half spatula of solid M in a non-luminous flame of a Bunsen burner.
   
   

   Observations	Inferences
   (1mk)	(1mk)

2. Place the remaining solid M in a boiling tube and add 8cm³ of water. Divide the solution into 4 portions.
   1. Place the remaining solid M in a boiling tube and add 8cm³ of water.
      
      

      Observations	Inferences
      (½mk)	(½mk)

   2. To the first portion, add 3 drops of acidified potassium manganate(vii)
      
      

      Observations	Inferences
      (1mk)	(1mk)

   3. To the second portion, add 3 drops of acidified potassium dichromate (vi) and warm.
      
      

      Observations	Inferences
      (1mk)	(1mk)

   4. To the third portion, add a little sodium hydrogen carbonate.
      
      

      Observations	Inferences
      (½mk)	(½mk)

   5. To the fourth portion, add 3 drops of universal indicator and determine the PH of the solution.
      
      

      Observations	Inferences
      (1mk)	(1mk)

CONFIDENTIAL

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

1. About 100cm³ of 0. 2M Sodium hydroxide solution.
2. About 200cm³ of 0.2M hydrochloric acid solution labeled solution X.
3. About 100cm³ of 0.1M sodium carbonate solution labeled solution Y.
4. One burette (0 - 50ml)
5. One 25.0ml pipette
6. One filter funnel
7. Retort stand
8. Pipette filler
9. Phenolphthalein indicator
10. Two conical flasks (250 ml)
11. White tile
12. 6 dry test tubes
13. 1 boiling tube
14. Metallic spatula
15. 1.5g of solid A
16. 1g pf solid M
17. About 0.2g of sodium hydrogen carbonate
18. Distilled water
19. 1 wooden splint
20. 1 red and 1 blue litmus paper

Access to:

1. Means of heating
2. 2M ammonia solution with a dropper
3. 2M nitric(v) acid with a dropper.
4. Acidified potassium manganate(vii) with a dropper.
5. Acidified potassium dichromate(vi) with a dropper.
6. Universal indicator with a dropper.
7. Standard PH chart
8. 0.2M lead(ii)nitrate solution

NOTE

1. Solid A is a mixture of Zinc carbonate and anhydrous zinc sulphate in the ratio 1:1.
2. Solid M is oxalic acid
3. Solution X is 0.2M Hydrochloric acid prepared by dissolving 17.2cm³ of concentrated hydrochloric acid in 1 litre.
4. 0.2M sodium hydroxide is prepared by dissolving 8g of sodium hydroxide pellets in 1 litre.
5. Solution Y is prepared by dissolving 10.6g of sodium carbonate in 1 litre.
6. 0.2M lead(ii)nitrate solution is prepared by dissolving 66.2g of lead(ii)nitrate in 1litre of solution.
7. Acidified potassium manganate(vii) is prepared by dissolving 3.2g of potassium manganate(vii)in about 600cm³ of 2M sulphuric (vi) acid and diluting to one litre of solution.
8. Acidified potassium dichromate(vi) is prepared by dissolving 25g of solid potassium dichromate(vi) in about 600cm³ of 2M sulphuric (vi) acid and diluting to one litre of solution.
9. 2M ammonia solution is prepared by dissolving 112cm³ of concentrated ammonia solution in 1 litre.
10. 2M nitric(v) acid is prepared by dissolving 128cm³ of concentrated acid to water and make up to 1 litre.

MARKING SCHEME

1. CT – 1mk
   D.P – 1 mk
   Accuracy (1 mk)
   1. Average volume of solution X = 25.0cm3 (1 mk)
   2. Moles of solution X required
      NaOH_(aq) + HCl_(aq) → NaCl_(aq) + H₂O_(l)
      1:1 (1/2 mk)
      Moles of NaOH = 0.2×25 = 0.005 moles (1/2 mk)
                                    1000
      Moles of solution X = 0.005×1 = 0.005 moles (1 mk)
                                              1
   3. Molarity of solution X
      No. of moles = 0.005 moles
      0.005 moles   →        25cm³
          ?                →        1000 cm³
      = 0.005×1000  (1 mk) = 0.2m (1 mk)
                 25
      Procedure II
      CT – 1 mk
      D.P – 1 mk
      Accuracy – 1 mk 
   4. Average volume of solution X
      = 25.0cm³ (1 mk)
   5. Number of moles of Na₂CO₃ in 25cm³
      Na₂CO_3(aq) + 2HCl_(aq) → 2NaCl_(aq) + CO_2(g) + H₂O_(l)
      1:2
      Moles of HCl reacting = 0.2×25 
                                              1000
                                          = 0.005 moles (1/2 mk)
      Mole ratio – Na₂CO₃:HCl
                                     1:2 (1/2 mk)
      Moles of Na₂CO₃ = ½ x 0.005(1/2 mk) = 0.0025 moles (1/2 mk)
   6. Concentration of sodium carbonate in moles per litre.
      0.0025 → moles 25cm³
          ?     →   1000cm³
      0.0025×1000  (1mk)   =0.1 M (1 mk)
                 25
2.  
   1.  
      

      Observations	Inferences
      - Solid turns to yellow and then to white on cooling. (1 mk) - A gas that turns moist blue litmus paper to red.	Zn2+ present (1 mk) The gas is acidic

   2.   
      1. 
         

         Observations	Inferences
         A white precipitate soluble in excess. (1mk)	Zn2+ present (1mk)

      2.  
         

         Observations	Inferences
         A white ppt present (1mk)	SO42-, Cl−, CO32- present (1 mk for any two)(1mk)

   3.   
      

      Observations	Inferences
      Effervescence present  (1mk)	CO32−(1mk)

      1.  
         

         Observations	Inferences
         A white ppt which dissolves in excess   (1mk)	Zn2+ present  (1mk)

      2.  
         

         Observations	Inferences
         No white ppt    (1mk)	SO42-, Cl−, CO32-    (1mk)

3.  
   1.  
      

      Observations	Inferences
      Solid burns with a sooty flame     (1mk)	(½ mk)

   2.  
      1.  
         

         Observations	Inferences
         Solid dissolves forming a colourless solution (½mk)	The solid is polar  (½mk)

      2.  
         

         Observations	Inferences
         Purple acidified potassium manganate (vii) is decolourised   (1mk)	present(½mk)

      3.  
         

         Observations	Inferences
         Orange acidified Potassium dichromate (vi) turns to green (1mk)	(½mk)

      4.  
         

         Observations	Inferences
         Effervescence   (1mk)	H+ present (1mk)

      5.  
         

         Observations	Inferences
         pH  is 5 (1mk)	Solution is weakly acidic  (1mk)