Chemistry Paper 3 Questions and Answers - Form 3 End Term 3 2022 Exams

Share via Whatsapp

QUESTIONS

  1. (24 Marks)
    • You are provided with:-
    • Aqueous hydrochloric acid solution P in a beaker.
    • A solution Q containing 6.3 g of dibasic acid H2C2O4.2H2O per litre.
    • Aqueous sodium hydroxide solution R.
    • You are required to:
      1. Standardize the sodium hydroxide solution R.
      2. Use the standardized solution R to determine the concentration of solution P in moles per litre.

PROCEDURE I
Fill the burette with solution Q. Pipette 25.0 cm3 of solution R into a conical flask and 3 drops of phenolphthalein indicator. Titrate this solution with solution Q until the pink colour just disappears .Record your results in table I below .Repeat this procedure two more times and complete table 1.
TABLE 1

 

1

2

3

Final burette reading(cm3)

     

Initial burette reading(cm3)

     

Volume of solution Q used(cm3)

     

(4 marks)

  1. Calculate the average volume of solution Q used.   (1 mark)
  2. Calculate the concentration of the dibasic acid solution Q in moles per litre. (2 marks)
    (C=12,O=16,H=1)
  3. Calculate the number of moles of the dibasic acid solution Q used.(1 mark)
  4. Calculate the number of moles of sodium hydroxide in 25cm3 of solution R.(2 marks)
  5. Calculate the concentration of sodium hydroxide solution R in moles per litre (2 marks)

PROCEDURE II
Using a 100cm3measuring cylinder, measure 90cm3of distilled water and transfer it into a 250cm3 beaker. Dry the measuring cylinder and use it to measure 10cm3 of solution P and add it to the 90 cm3 of distilled in the beaker. Mix the solution well and label it solution T.
Fill the burette with solution T and pipette 25cm3 of solution R into a clean conical flask. Add 3 drops of phenolphthalein indicator. Titrate this solution with solution T until the pink colour just disappears .Record your results in table II below .Repeat this procedure two more times to complete table II.
Table II

 

1

2

3

Final burette reading(cm3)

     

Initial burette readings(cm3)

     

Volume of solution T used (cm3)

     
  1. Calculate the average volume of solution T used. (1 mark)
  2. Calculate the number of moles of solution R used.(1 mark)
  3. Calculate the number of moles of hydrochloric acid in solution T that reacted completely with 25cm3 of Sodium hydroxide solution R. (2 marks)
  4. Calculate the number of moles of hydrochloric acid in 100cm3 of solution T(2 marks)
  5. Determine the concentration of the original hydrochloric acid solution P in moles per litre.(2 marks)

2. (16 MARKS)
You are provided with solid A, which is a mixture of two compounds .Carry out the experiment s below .Record your observations and inferences in the table and identify any gas (es) evolved.

(a)Transfer a spatula end-full of solid A in a boiling tube and add 20cm3of distilled water. Shake thoroughly and filter .Rinse the residue with distilled water and keep both the filtrate and the residue.

Observations

Inferences

   

1 mark             1mark
(b)  Divide the filtrate into three portions, each of 2cm3.
(i)   To portion one, add ammonia solution dropwise until in excess.

Observations

Inferences

   

1 mark             1mark
(ii)  To portion two, add 4 drops of 0.5M Lead (II) nitrate solution.

Observations

Inferences

   

1 mark             1mark
(iii)To portion three, add 4 drops of Barium chloride solutions, followed by 5 cm3 of nitric (v) acid.

Observations

Inferences

   

1 mark             1mark
c) Scrap the residue from the filter paper (a) above using a spatula and transfer it into a boiling tube. Add  to it  1M Nitric acid until it dissolves .Keep the resulting solution for use in part(d) below.

Observations

Inferences

   

1 mark             1mark
(d)  Divide the solution into 3 parts.
(i)   To part two, add ammonia solution dropwise.

Observations

Inferences

   

1 mark             1mark
(ii) To part two, add 4 drops of 1M hydrochloric acid solution.

Observations

Inferences

   

1 mark             1mark
(iii) To part three, add 4 drops of potassium Iodide solution.

Observations

Inferences

   

1 mark             1mark

MARKING SCHEME

PROCEDURE I
TABLE I ……………. 5 Mks distributed as follows.

  1.                  
    • Complete table (1 Mark)
      1. Complete table with 3 titrations (1 Mark)
      2. Incomplete table with 2 titrations ( ½ Mark)
      3. Incomplete table with 1 titration (0 Mark)
    • Penalties
      1. Wrong arithmetic
      2. Inverted table
      3. Burette reading beyond 50cm3 unless where explained.
      4. Unrealistic titre values below 1cm3 or above 50cm3.
        NB: Penalise ½ mark each to a maximum of ½ mark.
  2. Use of decimals (tied to 1st and 2nd rows only) (1 Mark)
    1. Accept 1 or 2 d.p used consistently otherwise penalise fully.
    2. If 2 d.p is used, the 2nd d.p must be either ‘0’ or ‘5’ otherwise penalise fully.
  3. Accuracy (Tied to correct titre value) (1 Mark)
    Compare the candidate titre values with the teacher titre values and mark as follows;
    1. If at least one is within ± 0.1 of the teacher’s value. (1 Mark)
    2. If none is within ± 0.1, but atleast one is within ± 0.2 of teacher’s value ( ½ Mark)
      NB: If there are 2 possible values from the teacher’s result, use one that is closer to candidate’s value
      and mark accordingly.
  4. Principles of averaging (1 Mark)
    Conditions
    1. If 3 consistent values are averaged (1 Mark)
    2. If 3 titrations are done but only two are consistent and averaged (1 Mark)
    3. If 2 titrations done and are consistent and averaged. (1 Mark)
    4. If 3 titrations are done but are inconsistent and averaged. (0 Mark)
      NB: Values averaged must differ by a maximum of ± 0.2cm3 of each other.
  5. Final answer (1 Mark)
    (Tied to correct average titre)
    Compare the candidate’s average titre with the teacher’s average titre.
    1. If within ± 0.1 of teacher’s value. (1 Mark)
      (ii) If not within ± 0.1 but within ± 0.2 of the teacher’s value ( ½ Mark)
      (iii) If none is within ± 0.2 (0 Mark)
      NB:
      Accept final answer expressed to 2 d.p unless the answer works out to an exact figure otherwise penalise ½ Mark

PROCEDURE II

  1. Table II – 5 Marks
    NB: Marks as per table I
    Calculations
    PROCEDURE I
  2.  Rmm of H2C2O4.2H2O = 126
    . :Molarity of solution Q in moles per litre = 6.3/126 1 = 0.05 1
  3. Moles of dibasic acid solution Q used = (Average volume x 0.05 (ans i)) ½
                                                                                       1000
    Correct answer ½
  4. 2NaOH(aq) + H2C2O4(aq) → Na2C2O4(aq) + 2H2O(l)
    Acid : Base = 1:2 1
    . :Moles of sodium hydroxide used = 2 x ans iii ½
    Correct answer ½
    NB:
    If the equation is missing award zero marks for the working.
  5. Concentration of sodium hydroxide solution R = (1000 x Ans iv)
                                                                                           25
    Correct ans. 1
    NB: Ignore if units missing but penalise ½ mark for wrong units used.

PROCEDURE II

  1.                            
  2. Moles of solution R used = (25 x Ans (v) )  ½ = Correct ans ½
                                                   1000
  3. NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) 1
    Acid : Base = 1:1 1
    . : Moles of HCl in solution T that reacted with 25cm3 of solution R = Ans ii.
    NB: If Ans ii) is transferred intact to iii) but the equation is missing award zero marks.
  4. Moles of HCl in 100cm3 of solution T =                   (100 x Ans iii)                  1 = Corr. Ans 1
                                                                       (Average volume of solution T)
  5. Original concentration of HCl in solution P = (1000 × Ans iv) = Corr. Ans 1
                                                                                  10
    NB: If wrong units are used penalise ½ mark but ignore if units are missing.

2. 16 Marks

 

OBSERVATIONS

INFERENCES

(a)

  • Solid A is slightly soluble in water  ½
  • Colourless filtrate/white residue  ½ 

Soluble and insoluble salts present in solid A 1

(b)

(i) White precipitate  ½ soluble in excess

Zn2+ confirmed present
SO2-4, Cl-, CO2-3 or SO2-3 present

 

(ii) White precipitate formed 1

  1. 3 or 4 mentioned. 1
  2. 2 mentioned  ½
  3. One mentioned 0
 

(iii) White precipitate formed  insoluble in nitric acid  ½

SO2-4 present 1

(c)

Effervescence/bubbles produced

CO2-3 present

(d)

(i) White precipitate  ½ insoluble in excess  ½

Ca2+, Ba2+, Mg2+, Pb2+ or Al3+ present

  • 5 or 4 present 1
  • 3 or 2 mentioned ½
 

(ii) White precipitate formed 1

Pb2+ present 1
NB: For Pb2+ to score it must have been mentioned in d (i)

 

(iii) Yellow precipitate formed

Pb2+ confirmed present


Download Chemistry Paper 3 Questions and Answers - Form 3 End Term 3 2022 Exams.


Tap Here to Download for 50/-




Why download?

  • ✔ To read offline at any time.
  • ✔ To Print at your convenience
  • ✔ Share Easily with Friends / Students


Join our whatsapp group for latest updates
.
Subscribe now

access all the content at an affordable rate
or
Buy any individual paper or notes as a pdf via MPESA
and get it sent to you via WhatsApp

 

What does our community say about us?

Join our community on:

  • easyelimu app
  • Telegram
  • facebook page
  • twitter page
  • Pinterest