Chemistry Paper 3 Questions and Answers - Form 4 End Term 1 Exams 2021

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CHEMISTRY
FORM 4
END TERM EXAMS
TERM 1 2021
PAPER 3
(PRACTICAL)
TIME 2 ¼hrs

INSTRUCTIONS

  • Answer ALL the questions in the spaces provided.
  • All working must be clearly shown where necessary.
  • Mathematical tables and silent electronic calculators may be used.
  1. Question 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
       
      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 100cmmeasuring 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
       
      Final burette reading(cm3      
      Initial burette reading(cm3)      
      Volume of solution Q 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. Question 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.
    1. 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.
      Observation Inferences
         
      (1 mark)          (1mark)
    2. Divide the filtrate into three portions, each of 2cm3.
      1. To portion one, add ammonia solution drop wise until in excess.
        Observation Inferences
           
        (1 mark)          (1mark)
      2. To portion two, add 4 drops of 0.5M Lead (II) nitrate solution.
        Observation Inferences
           
        (1 mark)          (1mark)
      3. To portion three, add 4 drops of Barium chloride solutions, followed by 5 cm3 of nitric (v) acid.
        Observation Inferences
           
        (1 mark)          (1mark)
    3. 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.
      Observation Inferences
         
      (1 mark)          (1mark)
    4. Divide the solution into 3 parts.
      1. To part two, add ammonia solution dropwise until in excess
        Observation Inferences
           
        (1 mark)          (1mark)
      2. To part two, add 4 drops of 1M hydrochloric acid solution.
        Observation Inferences
           
        (1 mark)          (1mark)
      3. To part three, add 4 drops of potassium Iodide solution.
        Observation Inferences
           
        (1 mark)          (1mark)

CONFIDENTIAL

The information contained in this paper is to enable the head of the school and the teacher in charge of Chemistry to make adequate preparations for this year’s Chemistry practical examination .NO ONE ELSE should have access to this paper or acquire knowledge of its contents. Great care must be taken to ensure that the information herein does not reach the candidates either directly or indirectly.

The teacher in charge of Chemistry should NOT Perform any of the experiments in the same room as the candidates not make the results of the experiments available to the candidates nor make the results of the experiments available to the experiments to the candidates.

REQUIREMENT FOR CANDIDATES
In addition to the fittings and apparatus found in a Chemistry laboratory, each candidate will require the following:-

  • 30cm3 of solution P
  • 100cm3 of solution Q
  • 200cm3 of solution R
  • A burette
  • A 25cm3 pipette
  • A Pipette filler
  • 2 conical flasks
  • A stand and a clamp
  • 10 cm3 of measuring cylinder
  • 100cm3 of measuring cylinder.
  • One 250 cm3 beaker (plastic or glass)
  • A label.
  • Filter funnel.
  • 500cm3 of distilled water in a wash bottle
  • 2 filter papers
  • A spatula
  • About 1g of solid A.
  • A boiling tube.
  • 6 test-tubes in a rack.
  • Phenolphthalein indicator supplied with a dropper.

ACCESS TO:

  • 2M Ammonia solution
  • 2M Hydrochloric acid.
  • 0.5 M Lead (II) nitrate solution
  • 0.5M Barium Chloride.
  • 1M Nitric acid.
  • 0.5M Potassium Iodide solution
    (Each supplied with a dropper) 

NOTES

  1. Solid A is a mixture of Zinc sulphate and Lead (II) Carbonate in the ratio 1:2.
  2. Solution R is prepared by dissolving 4g of Sodium hydroxide in 600cm3 of distilled water and diluting it upto one litre of solution.
  3. Solution Q is prepared by dissolving 6.3g of oxalic acid (H2C2O4.2H2O) in 500cm3 of distilled water and diluting it upto one litre of solution .
  4. Solution P is prepared by dissolving 86 cm3 of concentrated hydrochloric acid(specific gravity 1.18) to 500cm3 of distilled water and diluting it upto one litre of solution.


MARKING SCHEME

  1. Question 1   
    1. 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: (i) 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)
        2. If not within ± 0.1 but within ± 0.2 of the teacher’s value ( ½ Mark)
        3. 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
    2. PROCEDURE II
      1. Table II – 5 Marks
        NB: Marks as per table I
        Calculations
    3. PROCEDURE I
      1.      
      2. Rmm of H2C2O4.2H2O = 126
        Molarity of solution Q in moles per litre = 6.3/126  = 0.05 
      3. Moles of dibasic acid solution Q used = (Average volume x 0.05 (ans i))/1000 
      4. 2NaOH(aq) + H2C2O4(aq)→ Na2C2O4(aq) + 2H2O(l)
        Acid : Base = 1:2 
        Moles of sodium hydroxide used = 2 x ans iii 
        NB: If the equation is missing award zero marks for the working.
      5. Concentration of sodium hydroxide solution R = (1000 x Ans iv)/25 
        NB: Ignore if units missing but penalise ½ mark for wrong units used.
    4. PROCEDURE II
      1.      
      2. Moles of solution R used = (25 x Ans (v) )/1000 ½ = Correct ans  ½
      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)/(Average volume of solution T) 1 = Corr. Ans 1
      5. Original concentration of HCl in solution P = (1000 X Ans iv)/10 = Corr. Ans 1
        NB: If wrong units are used penalise ½ mark but ignore if units are missing.
  2. Question 2
       OBSERVATION INFERENCES
    -Solid A is slightly soluble in water 
    -Colourless filtrate/white residue
    -Soluble and insoluble salts present in solid A
    b    -White precipitate soluble in excess Zn2+ confirmed present
    SO2-4, Cl-, CO2-3 or SO2-3 present
     ii -White precipitate formed  3 or 4 mentioned. 
    2 mentioned 
    One mentioned 
     iii White precipitate formed insoluble in nitric acid SO2-4 present 
     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 
    - 3 or 2 mentioned
     ii White precipitate formed 1 Pb2+ present 
    NB: For Pb2+ to score it must have been mentioned in d (i) 
    ii Yellow precipitate formed Pb2+ confirmed present

 

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