Chemistry Paper 3 Questions and Answers - Form 4 Term 1 Opener Exams 2023

1. Determine the average volume of solution B. (5mks)
2. Calculate the number of moles of solution B. (2mks)
3. Given that the ionic equation for the reaction between dichromate ion and sulphate is;
   Cr₂O^2-7(aq) + 3SO^2-3(aq) + 8H⁺(aq)  →   2Cr³⁺(aq) + SO^2-4(aq) + 4H₂O(l)
   Calculate;
   1. Number of moles of dichromate IV ion in 25cm3 (2mks)
   2. Moles of dichromate ion one litre of solution.

Q2: You are provided with substance E, carry out the tests below and write your observations and inferences in the space provided.

1. Describe the appearance of substance E.
2. Place remaining amount of E in boiling tube.  Add about 10cm³ of distilled water and shake well. Retain the mixture for tests in d) below.
   

   Observation	Inferences
   (1 mk)	(1 mk)

3. Use about 2cm³ potion of the mixture obtained in (c) for tests (i) to (iv)
   1. Add 2 to 3 drops of lead (ii) Nitrate to the  mixture.
      

      Observation	Inferences
      (1 mk)	(1 mk)

   2.  Add 2 to 3 drops of barium Nitrate to second portion of the mixture.
      

      Observation	Inferences
      (1 mk)	(1 mk)

   3. Add five drops of dilute nitric (v) acid to the mixture in (ii) above
      

      Observation	Inferences
      (1 mk)	(1 mk)

   4. To the last portion, add few then excess drops of sodium hydroxide.
      

      Observation	Inferences
      (1 mk)	(1 mk)

4. Give the formula of cation and anion present in substance E. 

CONFIDENTIAL
In addition of apparatus formed in lab, each student is expected to have;

1. 100cm³ of solution A
2. 100cm³ of solution B
3. 0.5g of solid E
4. Burette
5. Pipette
6. 2 conical flask
7. 6 test tubes in rack
8. 1 boiling tube holder
9. Test tube holder
10. Distilled water in wash bottle.

Access

• 0.5 mBa (NO₃)₂ solution and dropper.
• 2M NaOH solution and dropper.
• Source of heat.
• 0.1MPb (NO₃)₂ solution and dropper.
• 2MHNO₃ solution and dropper.
• Solution A is acidified 0.01MK₂CrO₇ .
• Solution B is 0.1MNa₂SO₃
• Solution E is about 0.5g of MgSO₄.H₂O.

NOTE:
Solution A is prepared by dissolving 2.94g of K₂CrO₇ in 200cm³ of 2M H₂SO₄ then adding distilled water to 1l of solution.

MARKING SCHEME

1. Determine the average volume of solution B. (5mks)
   7.5 + 7.5 = 7.5cm³
         2
   CT√
   D√
   PA√
   A± 0.1√
   ±0.2√
   FA√
2. Calculate the number of moles of solution B. (2mks)
   1.0 x 7.5
      1000
   = 0.00075moles
3. Given that the ionic equation for the reaction between dichromate ion and sulphate is;
   Cr₂O^2-7(aq) + 3SO^2-3(aq) + 8H⁺(aq)  →   2Cr³⁺(aq) + SO^2-4(aq) + 4H₂O(l)
   Calculate;
   1. Number of moles of dichromate IV ion in 25cm3 (2mks)
      0.00075
          3
      = 0.00025 moles
   2. Moles of dichromate ion one litre of solution.
      0.00025 x 1000
                25
      = 0.01moles/litre

Q2 : You are provided with substance E, carry out the tests below and write your observations and inferences in the space provided.

1. Describe the appearance of substance E.
   • White crystalline solid// colourless crystalline solid.
2. Place one third of substance E in the test tube.  Heat it strongly.
   

   Observation	Inferences
   colourless vapor on condenses on cover  plots  (1 mk)	Hydrate salt// contains water of crystallization  (1 mk)

3. Place remaining amount of E in boiling tube.  Add about 10cm³ of distilled water and shake well. Retain the mixture for tests in d) below.

   Observation	Inferences
   Solid dissolved forming colorless solution  (1 mk)	soluble salt          Absence of Fe2+, Fe3+,Cu2+  (1 mk)

4. Use about 2cm³ potion of the mixture obtained in (c) for tests (i) to (iv)
   1. Add 2 to 3 drops of lead (ii) Nitrate to the  mixture.
      

      Observation	Inferences
      White precipitate  (1 mk)	SO42- SO32-  Present All=1√ CO32-                     3=√ Cl-                           2 or 1 = 0(1 mk)

   2. Add 2 to 3 drops of barium Nitrate to second portion of the mixture.
      

      Observation	Inferences
      White precipitate  (1 mk)	SO42- SO32-    All=1√ CO32-        2=√                  1 = 0    (1 mk)

       
   3. Add five drops of dilute nitric (v) acid to the mixture in (ii) above
      

      Observation	Inferences
      White precipitate present  (1 mk)	SO42- confirmed (1 mk)

   4. To the last portion, add few then excess drops of sodium hydroxide.
      

      Observation	Inferences
      White precipitate present  Insoluble in excess√   (1 mk)	Mg2+ √ present   (1 mk)

5. Give the formula of cation and anion present in substance E.
   • Cation Mg²⁺
   • Anion SO₄²