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

• Solution A which is dilute hydrochloric acid.
• Solution B which is made of dissolving 5.3g of Na₂Co₃ in 500cm³ of dilute water.
  You are required to standardize solution A.

Procedure 1

1. Mark your conical flasks as X, Y, Z
2. Put solution A into the burette.
3. Place 25cm³ of solution B into conical flask X and titrate with solution A using 3 drops of phenolphthalein indicator. RETAIN THE CONTENT OF THE FLASK X.
4. Record your results in table1 below and repeat the procedure using other flask y and z respectively.
   Table 1 
   

   	I	II	III
   Find burette readings
   Initial burette readings
   Volume v1 A used (cm3)

   Calculate the average volume v1 of solution A used. (5mks)

Procedure 2  

1. Refill the burette with solution A.
2. Add 3 drops of methyl orange indicator to the content of conical flask X and continue titrating with solution A.
3. Record your result in table 2 below.
   Repeat procedure 2 using the other conical flask Y and Z respectively.  Fill the table below.
   

   Find burette readings
   Initial burette readings
   Volume v1 A used

4. Calculate the average volume of solution v₂ of solution A used. (5mks)
   1. Calculate the total volume of v₁ + v₂ used in table 1 and table 2. (1mk)
   2. Write the chemical equation between solution A and solution B. (1mk)
   3. Determine the concentration of solution B in moles per litre (Na=23 O=16 C=16) (1mk)
   4. Calculate the number of moles of solution A used in titration. (1mk)   
   5. Calculate the concentration of solution A in moles per litre. (2mks)

You are provided with solid E 

1. Place little amount of solid E on a watch glass.
   Describe its appearance. (2 mks)
2. Place all amount of solid E in a boiling tube.  Add about 10cm³ of distilled water.  Shake the mixture.
   

   Observations	Inferences

                           (1mk)                                                  (1mk)
3. Divide the resulting solution into 4 portions. To the first portion, add drops of Barium Nitrate provide.
   

   Observations	Inferences

                           (1mk)                                                  (1mk)
4. To the mixture attained in (iii) acidity using about 5 drops of nitric provided in the access.
   

   Observations	Inferences

                           (1mk)                                                  (1mk)
5. To the second portion add 3 drops of acidified potassium chromate (vi) provided.
   

   Observations	Inferences

                           (1mk)                                                  (1mk)
6. To the third portion add ammonia solution provided.
   

   Observations	Inferences

                           (1mk)                                                  (1mk)
7. To the last portion, dip a glass rod and place it on non luminous flame
   

   Observations	Inferences

                           (1mk)                                                  (1mk)

• Solution A 100cm³
• Solution B 100cm³
• Solid E 0.5g
• Burette
• Pipette
• Watch glass
• Glass rod
• Complete stand
• 3 conical flask
• White tiles
• Wash bottle with distilled/ rain water access to the following:
  • Phenolphthalein  indicator and dropper
  • Methyl orange indicator and dropper.
  • 0.5MB(NO₃)₂ and dropper
  • 2MNH⁴OH and dropper
  • Source of heat
  • Acidified K₂Cr₂O₇and dropper
  • 2MHNO₃ and dropper

Preparation 

• Solution A is 0.2 MHCl
• Solution B is made by dissolving 10.6g in 1litre of solution with distilled water.
• Acidified K₂Cr₂O₇ is made by distilling 5g of K₂Cr₂O₇ is 200cm³ of 2MH₂SO₄ then topping with distilled water to 1litre.
• Solid E is 0.5g sodium sulphite.

MARKING SCHEME

Q1. You are provided with;

• Solution A which is dilute hydrochloric acid.
• Solution B which is made of dissolving 5.3g of Na₂Co₃ in 500cm³ of dilute water.

You are required to standardize solution A. 

Procedure 1

1. Mark your conical flasks as X, Y, Z
2. Put solution A into the burette.
3. Place25cm³ of solution B into conical flask X and titrate with solution A using 3 drops of phenolphthalein indicator. RETAIN THE CONTENT OF THE FLASK X.
4. Record your results in table1 below and repeat the procedure using other flask y and z respectively.
   Table 1
   

   	I	II	III
   Find burette readings
   Initial burette readings
   Volume v1 A used (cm3)

   
   Calculate the average volume v1 of solution A used. (5mks)
   A=12.5 ± 0.1 √ ± 0.2 √
   CT√
   D√
   PA√
   FA√

Procedure 2  

1. Refill the burette with solution A.
2. Add 3 drops of methyl orange indicator to the content of conical flask X and continue titrating with solution A.
3. Record your result in table 2 below.
   Repeat procedure 2 using the other conical flask Y and Z respectively.  Fill the table below.
   
   

   	I	II	III
   Find burette readings
   Initial burette readings
   Volume v1 A used (cm3)

   
   A=12.5 ± 0.1 √ ± 0.2 √
   CT√
   D√
   PA√
   FA√
   Calculate the average volume of solution v₂ of solution A used. (5mks)
   1. Calculate the total volume of v₁ + v₂ used in table 1 and table 2. (1mk)
      12.5 + 12.5
      = 25.0cm³ √
   2. Write the chemical equation between solution A and solution B. (1mk)
      Na₂CO_{3 (aq)} + 2HCl _(aq) 2NaCl _(aq) + CO _(g) + H2O _(l)
   3. Determine the concentration of solution B in moles per litre (Na=23 O=16 C=16) (1mk)
      Molarity in moles/l =  g/l
                                      RFM √
                                   = 5.3 x 2 = 0.1m
                                          (23 x 2 + 12 + 16 3)√
   4. Calculate the number of moles of solution A used in titration. (1mk)   
      Moles  of B = 0.1 x 25                        Moles A moles ratio 1:2
                              1000                                                    = 0.0025 x 2                  
                           = 0.0025√                                                = 0.005 moles√   
   5. Calculate the concentration of solution A in moles per litre. (2mks)
                0.005 X 1000√
                         25                    = 0.2M√

Q2. 

You are provided with solid E 

1. Place little amount of solid E on a watch glass.
   Describe its appearance. (2 mks)
   • Write crystalline solid.√
2. Place all amount of solid E in a boiling tube.  Add about 10cm³ of distilled water.  Shake the mixture.
   

   Observations	Inferences
   Solid dissolved forming colorless solution√	Soluble salt/ compound   Absence of Fe2+/ Fe3+/cu2+      or colored ions.

                           (1mk)                                                  (1mk)
3. Divide the resulting solution into 4 portions.  To the first portion, add drops of Barium Nitrate provide.
   

   Observations	Inferences
   White precipitate formed	SO32- SO42-  CO32- present

                           (1mk)                                                  (1mk)
   All 3 = √
   2 = √
   1 =
4. To the mixture attained in (iii) acidity using about 5 drops of nitric provided in the access.
   

   Observations	Inferences
   White precipitate dissolved forming colorless solution	SO32-    CO32- present

                           (1mk)                                                  (1mk)
     2 =√
     1 =√
5. To the second portion add 3 drops of acidified potassium chromate (vi) provided.
   

   Observations	Inferences
   Orange potassium Turns green	SO32-  confirmed.

                           (1mk)                                                  (1mk)
6. To the third portion add ammonia solution provided.
   

   Observations	Inferences
   No white precipitate	AP+                        K+  Zn2+    Absent√      Lit           present +  Pb2                        Na+   √

                           (1mk)                                                  (1mk)
7. To the last portion, dip a glass rod and place it on non luminous flame
   

   Observations	Inferences
   Non luminous flame   Turns to golden yellow	Na+ confirmed.

                           (1mk)                                                  (1mk)