Chemistry Paper 3 Questions and Answers - Kijiset Revision Mock Exams 2022

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Questions

Instructions to candidates

  • Answer all the questions in the spaces provided in the question paper.
  • You are not allowed working with the apparatus for the first 15 minutes of the 21/4 hours allowed for this paper. This time is to enable you the question paper and ensure you have all the chemicals and apparatus that you may need.
  • All workings must be clearly shown where necessary.
  • Mathematical tables and silent electronic calculators may be used.

Question 1.
You are provided with the following:

  • Solution A containing 95g of a mixture of sodium carbonate and sodium chloride per litre of solution.
  • 1M HCl solution B

You are required to determine the percentage of sodium chloride in the mixture

Procedure:
Pipette 25cm3 of A and place it in a conical flask. Titrate with B from the burette using 3 drops of methyl orange indicator until a permanent pink colour appears. Repeat the experiment and complete the table below.

Table I

   I II  III 
 Final burette reading (cm3)      
 Initial burette reading (cm3)      
 Volume of B used (cm3)      

(4mks)

  1. Calculate the average volume of B used. (1mk)
  2. Determine the number of moles of B used (1mk)
  3. Write the ionic equation for the substance that react. (1mk)
  4. Calculate:
    1. the number of moles of the base used (1mk)
    2. the concentration of sodium carbonate (2mks)
    3. the mass of sodium carbonate in 1 litre of the solution. (Na=23, C=12, O=16) (2mks)
    4. the percentage of sodium chloride in the mixture (1mk)

Question 2.
You are provided with:
-1M HCl solution B
-1M NaOH solution C

You are expected to determine the Molar heat of Neutralization hydrochloric acid

Procedure:
Measure 23cm3 of solution B and put it in a 100ml beaker. Measure its temperature and record in the table below under first column. By use of measuring cylinder, measure 5cm3 of solution C and add to solution B in the beaker. Stir with the thermometer and record the final steady temperature. Continue adding 5cm3 at a time and record the temperature till 35cm3 has been added.

  1. Complete the table below (4mks)
     Volume of C added(cm3) 10  15  20  25  30  35
     Temperature (°C)                
  2. Plot a graph of temperature (vertical axis) against volume of NaOH added. (3mks)
  3. From your graph, determine:
    1. Volume of 1M NaOH needed to neutralize 23cm3 of 1M HCl (1mk)
    2. Rise in temperature ∆T (1mk)
  4. Calculate the amount of heat evolved in the above reaction. (Take specific heat capacity of solution to be 4.2J/g/K, density of solution, 1g/cm3) (2mks)
  5. Calculate :
    1. the number of moles of HCl used (1mk)
    2. hence determine the Molar heat of neutralization of hydrochloric acid (2mks)

Question 3.
You are provided with mixture P. You are required to perform tests on the mixture in order to determine its composition. Record your observations and inferences in the spaces provided.

  1. Place a spatula of solid P on a white tile and observe its appearance
     Observation Inferences 
     





     
  2. Place the remaining portion of solid P in a boiling tube and add 10cm3 of distilled water. Shake vigorously, filter and retain both the residue and filtrate.
     Observation Inferences 
     





     

    1. Divide the filtrate into 3 portions. To the first portion, add sodium hydroxide drop-wise until excess.
       Observation Inferences 
       





       
    2. Dip a glass rod in the second portion and place it at the hottest part of the non-luminous flame.
       Observation Inferences 
       





       
    3. To the third portion, add 3 drops of dilute HNO3(aq), followed by 3 drops of BaCl2(aq).
       Observation Inferences 
       





       
  3. Scrap the residue from the filter paper and place a half of it in a clean dry test-tube. Add about 3cm3 of 2M HNO3(aq). Test foe any gas produced by use of calcium hydroxide solution on a glass rod. Preserve the solution for use in procedure (d) below.
     Observation Inferences 
     





     
  4. Add about 3cm3 of distilled water to the solution obtained in (c) above and shake to mix. Divide the solution into 3 portions.
    1. To the first portion, add sodium hydroxide drop-wise until excess.
       Observation Inferences 
       





       
    2. To the second portion, add ammonia solution drop-wise until in excess.
       Observation Inferences 
       





       
    3. To the third portion, add 2 drops of potassium iodide solution.
       Observation Inferences 
       





       

Confidential

REQUIREMENTS
In addition to the equipment, apparatus and chemicals found in the Chemistry laboratory, each candidate will require the following:

  1. 100cm3 of solution B
  2. 80cm3 of solution A
  3. 50cm3of solution C
  4. 25ml pipette
  5. 50ml Burette
  6. 2 conical flasks
  7. Filter funnel
  8. Filter paper
  9. 10ml Measuring cylinder
  10. 50ml Measuring cylinder
  11. Thermometer (-10 to 1100C)
  12. 100ml plastic beaker
  13. 6 test-tubes on a test-tube rack
  14. 1 Boiling tube
  15. White tile
  16. Stand and clamp
  17. Solid P in a stoppered container
  18. A glass rod
  19. Metallic spatula
  20. Distilled water in a wash bottle
  21. Calcium hydroxide in a stoppered container

Access to:

  1. Methyl orange indicator with a dropper
  2. 2M sodium hydroxide with a dropper
  3. 2M nitric (V) acid with a dropper
  4. 2M ammonia solution with a dropper
  5. 0.5M barium chloride with a dropper
  6. 0.05M potassium iodide with a dropper
  7. 2M hydrochloric acid with a dropper
  8. Source of heat

Preparation of solutions:

  1. Solid P is the mixture of Lead (II) carbonate and Sodium sulphate in the ratio of 1:1.
  2. Solution A is prepared by mixing 53g of sodium carbonate and 42g of sodium chloride solid and dissolving to make one litre of solution.
  3. Solution B is prepared by dissolving 86cm3 of concentrated hydrochloric acid in distilled water and dilute to one litre.
  4. Solution C is prepared by dissolving 40g of sodium hydroxide pellets in distilled water and top up to one littre.

 

Marking scheme

QUESTION 1

Table 1

   I II  III 
 Final burette reading (cm3)      
 Initial burette reading (cm3)      
 Volume of B used (cm3)      

A ward a total of 5 mks as follows

  1. Complete table 1 mk
    1. Complete table with 3 titrations 1mk
    2. Incomplete table with 2 titrations ½
    3. Incomplete table with only 1 titration done 0 mk

      Penalties
      • Wrong arithmetic/ subtraction
      • Inverted table
      • Burette reading beyond 50cm3
      • Unrealistic titre value ie 1.0cm3

        NOTE: Penalize ½mk for each for a maximum penalty of ½
  2. Use of decimals (tied to the 1st and 2nd row only) 1mk
    1. Accept either 1 or 2 decimals used CONSISTENTLY , otherwise penalize fully.
    2. If 2 decimal places are used the 2nd decimal must be 0 or 5 otherwise penalize fully.
  3. Accuracy 1mk
    1. Compare candidate titre value with the school value (SV)
    2. If at  least one value is within ±0.1 award 1mk
    3. If within ±0.2 of the SV award ½
    4. If no value is within ±0.2 of the SV award 0mk
  4. Principle of averaging
    Value average must be shown and must be within 0.2 of each other ½mk, followed by ½mk for the correct answer.

    NOTE
    1. If 3 values are possible but averaged, award 0mk
    2. If 3 titration are done, are inconsistent and averaged, award 0mk
    3. If only 2 titrations are done, are consistent and averaged, award 0mk
  5. Final answer (tied to correct average titre)  1mk
    Compare the candidate CORRECT AVERAGE TITRE with the SV
    1. If it is within 0.1 of S.V, award 1mk
    2. If it is within 0.2 of S.V, award ½mk
    3. If it is beyond 0.2 of S.V, award 0mk

Calculations

  1. Table 2
    • Complete table 1mk
    • Accuracy 1mk
    • Decimal place 1mk
      Accept 1d.p or whole number for temperature readings
      Award 0mk if the second d.p is not 0 or 5 
    • Reject 2d.p
    • trend 1mk
      Temperature readings to increase then decrease.
  2. Graph
    1. Labelled axes ½mk
      Award half a mark if both axes are correctly labelled (temeparture (°C) vertical axis) against colume (cm3) on horizontal axis.
    2. Scale ½mk
      • Area covered by the actual plots must be at least half.
      • Scale interval must be consistent
      • Scale chose must accomodate all plot
    3. Plot
      • If 7 to 8 are correctly plotted 1mk
      • If 4 to 6 plots are correctly plotted half a mark
      • Otherwise a ward 0mk
    4. Shape 1mk
      • Accept straight lines of best fit, one for increase in temperature and another for decrease joined with dotted lines at a vertex 1mk
  3.  
    • From the graph
    • Shown in graph
    • Correct reading
      ΔT - 
      • Shown in graph
      • correct answer from graph
  4. ΔH = MCΔT = (23 + C(i) x 4.2 x c(ii))
    correct ans
  5. Moles =   1 x 23  
                   1000
    = 0.023 moles

Question 3

   Observations  Inferences
a

White powder

Fe2+, Fe3+ , Cu2+ absent 
b  White residue
Colourless filtrate
Sold P is partiallt/ sparingly
soluble (pegged on colourless filtrate and white residue otherwise penalize fully)
 Fe2+, Fe3+ , Cu2+ absent  (pegged on colourless filtrate)
   i.  colourless solution retained/ persists// no white ppt  Zn2+, Al3+, Pb2+, Mg2+, Ca2+ present
Any 3 ions correct 1mk
Any 2 ions correct ½mk 
(otherwise penalize half a mark for any contradictory ion to a mazimum of half a mark)
   ii. Yellow flame  Na+ present
Penalize fully for any contradictory ion mentioned to a maximum of half a mark
   iii white ppt SO2-4 present
(penalize fully for any contracictory ion ,entioned to a maximum of half a mark) 
 Eggrevesence/ bubbles / hissing sound 
Colourless gas froms white ppt with Ca(OH)2aq
Solid dissolves to form colourless solution
 CO2-3(aq) present
Penalize half a mark for any contradictory ion menbtioned to a maximum of half a mark
 i. white ppt , soluble in excess Pb2+ , Zn2+, Al3+ present
Any 2 ions correct 1mk
Any 1 ion correct ½mk
Otherwise penalize fully 
  ii yellow ppt Pb2+ present
Penalize fully for any contradictory ion mentioned

 


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