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

Q1 = 11MKS
Q2 = 14MKS

Q1. You are provided with:

Solution A: Potassium Chromate VI
Solution B: 0.1mNa₂SO₃

You are required to determine the number of moles dichromate VI ions (Cr₂O^2-₇) in one litre of solution.

PROCEDURE

Fill the burette with solution B. Pipette 25cm³ of solution A and transfer it into conical flask. Titrate until a permanent green colour is obtained. Record your results in table below and repeat the procedure to fill the table.

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.
   

   Observation	inferences

    (1mk)                                                       (1mk)
2. Place one third of substance E in the test tube. Heat it strongly.
   

   Observation	inferences

    (1mk)                                                       (1mk)
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

    (1mk)                                                       (1mk)
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

       (1mk)                                                       (1mk)
   2. Add 2 to 3 drops of barium Nitrate to second portion of the mixture.
      

      Observation	inferences

       (1mk)                                                       (1mk)
   3. Add five drops of dilute nitric (v) acid to the mixture in (ii) above
      

      Observation	inferences

       (1mk)                                                       (1mk)
   4. To the last portion, add few then excess drops of sodium hydroxide.
      

      Observation	inferences

       (1mk)                                                       (1mk)
5. Give the formula of cat ion and anion present in substance E.
   Cat ion
   Anion

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. test tubes in rack
8. 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

Q1 = 11MKS
Q2 = 14MKS

Q1. You are provided with:

Solution A: Potassium Chromate Vi
Solution B : 0.1mNa₂SO₃

You are required to determine the number of moles dichromate VI ions (Cr₂O^2-₇) in one litre of solution.

PROCEDURE

Fill the burette with solution B. Pipette 25cm³ of solution A and transfer it into conical flask. Titrate until a permanent green colour is obtained. Record your results in table below and repeat the procedure to fill the table.

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	Hydrate salt// contains water   of crystallization

    (1mk)                                                       (1mk)
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 soluble salt   Absence of Fe2+,           Fe3+,Cu2+

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

       (1mk)                                                       (1mk)
   2. Add 2 to 3 drops of barium Nitrate to second portion of the mixture.
      

      Observation	inferences
      White precipitate	SO42-   SO32- All=1√   CO32- 2=√   1 = 0

       (1mk)                                                       (1mk)
   3. Add five drops of dilute nitric (v) acid to the mixture in (ii) above
      

      Observation	inferences
      White precipitate present	SO42- confirmed

       (1mk)                                                       (1mk)
   4. To the last portion, add few then excess drops of sodium hydroxide.
      

      Observation	inferences
      Observation inferences  White precipitate present   Insoluble in excess√	Mg2+ √ present

       (1mk)                                                       (1mk)
5. Give the formula of cation and anion present in substance E.
   Cat ion - Mg²⁺
   Anion - SO₄²