Chemistry Paper 3 Questions and Answers - Mumias West Pre Mocks 2022

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QUESTIONS

  1. You are provided with:
    • 4.5g of solid P in a boiling tube
    • 0.2M sodium hydroxide solution Q
      You are required to determine:
      1. the solubility of solid P at different temperatures
      2. The value of ‘n’ in the formula of compound P, (HX) n. 2H2O.

PROCEDURE I

  1.        
    1. Fill the burette with distilled water. Using the burette, add 4.0cm3 of distilled water to solid P in a boiling tube. Heat the mixture in a water bath while stirring with a thermometer to about 70ºC until all the solid dissolves.
    2. Allow the solution to cool while stirring with the thermometer and note the temperature at which crystals of solid P start to appear. Record this temperature in table I.
    3. Using the burette, add 2.0cm3 of distilled water to the contents of the boiling tube. Heat the mixture while stirring with the thermometer until all the solid dissolves while in the water bath.
    4. Allow the mixture to cool while stirring and note the temperature at which crystals of solid P start to appear. (FOR FASTER COOLING PROCESS, USE COLD TAP WATER)
    5. Repeat the procedure (c) and (d) three more times, heating the solution in a water bath and record the temperature in the table. Retain the contents of the boiling tube for use in procedure II.
  2. Complete the table by calculating the solubility of solid P at the different temperatures. (The solubility of a substance is the mass of that substance that dissolves in 100cm3(100g) of water at a particular temperature.
    Table I
    Volume of water in the boiling tube (cm3) Temperature at which crystals of solid P first appear (ºC) Solubility of solid P (g/100g) of water
    4    
       
    8    
    10    
    12    
    (6marks)
    1. On the grid provided plot a graph of the solubility of solid P against temperature (3marks)
      1
    2. Using your graph determine the temperature at which 100g of solid P would dissolve in 100cm3 of water. (1mark)
    3. Determine the solubility of solid P at 55ºC (1mark)

PROCEDURE II
Transfer the contents of the boiling tube into a 250ml volumetric flask. Rinse the boiling tube and the thermometer with distilled water and add to the volumetric flask. Add more distilled water to make up to the mark. Label this solution P.
Fill the burette with solution P. Using a pipette place 25.0cm3 of solution Q into a conical flask. Titrate solution Q with solution P using phenolphthalein indicator.
Table II

  I II III
Final burette reading cm3      
Initial burette reading cm3      
Volume of solution P used cm3      

(4 marks)
Calculate:

  1. Average volume of solution P used in the experiment. (1mark)
  2. Number of moles of sodium hydroxide used in solution Q. (2marks)
  3. Number of moles of solution P in the average volume given that the relative formula mass of P, (HX) n. 2H2O is 126. (2marks)
  4. The number of moles of sodium hydroxide required to react with one mole of P. Hence find the value of n in the formula (HX) n .2H2O (2marks)
  1. You are provided with solid A. Use it to carry out the following tests. Write the inferences and observations in the spaces provided.
    1. Place all of solid A in a boiling tube. Add about 8cm3 of distilled water and shake. Divide the solution formed into 4 portions.
      Observations Inferences
      (1mark) (1mark)
    2. To the first portion, add sodium hydroxide drop wise until in excess
      Observations Inferences
      (1mark) (1mark)
    3. To the second portion, add ammonia solution drop wise until in excess.
      Observations Inferences
      (1mark) (1mark)
    4. To the third portion, add 3 drops of sodium chloride solution
      Observations Inferences
      (1mark) (1mark)
    5. To the fourth portion add about 3 drops of barium nitrate followed by 4 drops of dilute nitric (V) acid
      Observations Inferences
      (1mark) (1mark)
  2. You are provided with an organic compound, solid Q. Use it to carry out the following tests
    1. Scoop about a third of the solid B using a metallic spatula and ignite it on the non-luminous flame of the Bunsen burner
    2.        
      1. Place the remaining solid B in a clean boiling tube, add about 10cm3 of distilled water and shake. Divide the resulting solution into six (3) portions
        Observations Inferences
        (1mark) (1mark)
      2. To the first portion of the solution, add 2 drops acidified potassium manganite (vii) and warm
        Observations Inferences
        (1mark) (1mark)
      3. To the third portion of the solution, add 2 drops of acidified potassium dichromate (VI)
        Observations Inferences
        (1mark) (1mark)
      4. To the fourth portion, add in the whole of solid sodium hydrogen carbonate provided
        Observations Inferences
        (1mark) (1mark)

CONFIDENTIAL

  • The information contained in this paper is to enable the head of school and teacher in charge of Chemistry to make adequate preparations for this year’s chemistry JOINT 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 chemistry teacher is NOT expected to perform the experiments
  • The apparatus required by each candidate for the chemistry mock practical examination are set out on the next page. It is expected that the ordinary apparatus of a chemistry laboratory will be available.
  • The chemistry teacher should note that it is his/her responsibility to ensure that each apparatus acquired, for this examination  agrees with specifications on the next page.

In addition to the apparatus and fitting in a chemistry laboratory each candidate will require the following.

  1. 4.5 g solid P in a boiling tube (weighed accurately)
  2. About 100cm3 solution Q
  3. 1 label
  4. One burette
  5. One pipette
  6. Two conical flasks ( 250ml)
  7. Pipette filler
  8. Filter funnel
  9. 250ml volumetric flask
  10. 10ml measuring cylinder
  11. One boiling tube
  12. Thermometer (-10ºc to 110ºc)
  13. Distilled water in a wash bottle
  14. 1g solid A in a stoppered container
  15. 1g solid B in a stoppered container
  16. One metallic spatula
  17. Test tube holder
  18. 1g sodium hydrogen carbonate in a stoppered container
  19. Six empty clean test tubes

Access to

  • Phenolphthalein indicator supplied with a dropper
  • Bunsen burner
  • Acidified potassium manganate (VII) supplied with a dropper
  • Acidified potassium dichromate (VI) supplied with a dropper
  • 2M sodium hydroxide supplied with a dropper
  • 2M aqueous ammonia supplied with a dropper
  • Barium nitrate solution supplied with a dropper
  • 2M nitric (V) acid supplied with a dropper
  • 0.1M sodium chloride solution supplied with a dropper
  • Water bath

Notes.

  1. Solid P is oxalic acid (COOH)2.2H2O
  2. 0.2M Sodium hydroxide solution labeled Q
  3. Solid A – Aluminium sulphate
  4. Solid B - oxalic acid


MARKING SCHEME

Volume of water in the boiling tube (cm3) Temperature at which crystals of solid P first appear (ºC) Solubility of solid P (g/100g) of water
4 68.0 112.5
64.0 75.0
8 62.0 56.25
10 50.0 45.0
12 48.0 37.5

DISTRIBUTED AS FOLLOWS:
COLUMN 2 – 4MARKS

  1. COMPLETE TABLE – 2 MARKS (Applicable to column 2 on temperature readings only)
    Award ½ mark for each temperature reading to a maximum to 2marks
    Penalties
    Penalize ½ mark for the unrealistic values (above 100ºc or below 20ºc)
  2. DECIMAL – ½ Mark (Applicable only to column 2)
    Award ½ mark for temperatures consistently recorded to whole numbers or 1dp (.0 or .5) otherwise award 0 mark.
  3. ACCURACY – ½ Mark (Tied to the reading on column 2 row 1)
    Award ½ mark for the candidate’s within the range of 2 ± units of the school value otherwise award 0mark.
  4. TREND – 1mark (Tied to temperature readings only)
    Award 1 mark for continuous decrease in temperature otherwise, award 0mark.

COLUMN 3 – 2MARKS

  • Award ½ mark for each correct calculation of solubility to maximum of 2marks
    Conditions and Penalties
  • Penalize ½ mark ONCE for any value rounded off to whole number.
  • Penalize ½ mark for any missing or wrong value of solubility

GRAPH – 3MARKS

  1. Axes – ½ mark
    Conditions
    • Award ½ mark if all axes are correctly labeled
    • Units may be included or not. If units are included must be correct, otherwise award 0mark for axes
  2. SCALE - ½ mark
    • Correctly plotted points MUST cover ¾ of the grid provided for ½ mark
    • The scale MUST be linear in both axes otherwise award 0 mark for the wrong scale.
  3. PLOTTING – 1 MARK
    • All 5 points plotted correctly …….1mark
    • 3-4 points plotted correctly …………. ½ mark
    • 1-2 points plotted ………………… 0mark
      NOTE
      Award 0 mark for points plotted on a wrong scale.
  4. CURVE - 1MARK
  5.        
    1. Award 1 mark for a curve showing continuous increase in solubility with increasing temperature.
      Award 0 mark for use of a straight line.
      2
    2. Award 1mark for the correct reading from a correct graph
      The student to give the temperature when solubility is 100g/100g of water.
      Conditions
      • Award ½ mark for correct showing and ½ mark for correct reading
      • If the candidate has not shown on the graph, but correct reading is given, award 1mark
      • If the candidate shows but does not give the correct reading, award ½ mark.
    3. The candidate to give the solubility when the temperature is 55ºc for 1mark.
      • Award ½ mark for correct showing and ½ mark for correct reading
      • If the candidate has not shown on the graph, but correct reading is given, award 1mark
      • If the candidate shows but does not give the correct reading, award ½ mark.

TABLE II - 5MARKS

  I II III
Final burette reading cm3 17.5 17.5 17.5
Initial burette reading cm3 0 0 0
Volume of solution P used cm3 17.5 17.5 17.5

Marking Points

  1. Complete table (CT) ……………………………………………….1mark
    The table should be completed.
    Penalize ½ mark for the following errors if any occurs.
    • Arithmetic error in subtraction.
    • Values recorded beyond 50cm3
    • Inversion of table
    • Penalize ½ mk only on any one of these errors.
  2. Decimal point (d.p)………………………………………………..1mk
    • All values to be recorded to 1d.p or
    • All values to be recorded to 2dp second decimal value being 0 or 5 only
    • Award 0-mark if whole numbers used or more than 3dp are used or inconsistency in the number of d.p
  3. Accuracy mark (AC)…………………………………….. 1mark
    • Consider any one candidates’ titre if within ± 0.10cm3 of school value award 1mk.
    • If it is ± 0.11 to 0.20 award ½ mark. If beyond 0.20cm3 award 0mk
  4. Principle of Averaging (P.A)……………………………..1Mark
    • Three titres to be averaged if within ±0.2cm3 to one another.
    • Two titres can only be arranged if they are consistent.
    • N/B- If a student averages two titres when three are consistent award 0mk.
    • If a student averages three inconsistent values, award 0 mark
  5. Final answer (F. A)………………………………………………….1Mark
    • If averaged titre is within 0.0 to 0.10cm3 of S.V award 1mk
    • If within 0.11 to 0.2cm3 of s.v award ½ mk
    • If beyond 0.20cm3 award 0mk.
      Summary
      Complete table (CT) = 1mk Type equation here.
      Correct use of decimals (dp) = 1mk
      Accuracy (AC) = 1mk
      Averaging (PA) = 1mk
      Final answer (FA) = 1mk)
      5mks
      N/B – For school value (SV), teacher to perform practical to obtain school value.

CALCULATIONS

  1. Marked on the Table
  2. Moles of sodium hydroxide in 25cm3 = 0.2 x 25
                                                                    1000
    = 0.005 1mark
  3. Moles in 250cm3 = 4.5  ½ marks
                                  126
    = 0.0357 ½ mark
    Moles of P in 25cm3 = Average volume × 0.0357 ½ mark
                                                         250
    = correct answer (III) ½ mark
  4.  0.005 
    Ans(III)
    = Ans (IV) ½ Mark
    Value of ‘n’ = Ans (IV) (Given as a whole number) ½ Mark
    • The value of n MUST be written as a whole number to earn a mark.
    • The number of moles MUST be given to at least 4dp unless it divides completely. OTHERWISE penalize ½ mark for correct answer.

QUESTION 2 – 9 MARKS
Place all of solid A in a boiling tube. Add about 8cm3 of distilled water and shake. Divide the solution formed into 4 portions.

Observations Inferences
  1. Solid dissolves to form a colourless solution… ½ mark
    • Accept for FULL credit a colourless solution formed.
    • REJECT: Colourles liquid.
      (1/2 mark)
  2. White ppt ½ soluble ½ in excess
  3. White ppt ½ insoluble ½ in excess
  4. No white ppt.
    REJECT
    • No ppt
    • No change
    • Colourless solution formed
  5. White precipitate ½ that does not dissolves ½ in addition of nitric (V) acid.
    OR white ppt persists on addition of HNO3
  1. Soluble compound
    • Cu 2+, Fe 2+, Fe3+ absent
      N/B: Accept any of the inferences for FULL credit.
    • Penalize FULLY for any contradictory inference.
  2. Zn2+, Al3+, Pb2+ 1mark
    • Award 1mark for 3 ions mentioned
    • Award ½ mark for 2 ions mentioned
    • Award 0 mark for 1 ion mentioned
    • PENALIZE ½ mark for any contradictory ion to a maximum of 1mark.
  3. Al 3+, Pb 2+ present
    • Award ½ mark for each ion
    • PENALIZE ½ mark for any contradictory ion to a maximum of 1 mark.
    • Accept the two ions only if they are mentioned in (b) above.
  4. Al 3+ present
    • ACCEPT for ½ mark Pb 2+ absent
    • Penalize FULLY for any contradictory ion.
  5. SO42- present
    - PENALIZE FULLY for any contradictory ion.

QUESTION 3 – 9 MARKS

Observations Inferences
  1. Burns with a yellow sooty/smoky flame ---1mark
  2. Solid dissolves to form a colourless solution.
  3. Purple KMnO4 changes to colourles/ is decolourises
  4. Orange colour of acidified K2Cr2O7 persists/ does not change to green….1mark
  5. Effervescence/ bubbles of a colourless odourless gas …..1mark
  1.    
    • =C= C = OR - C≡≡ C – present
    • Accept unsaturated organic compound for FULL credit.
    • PENALIZE FULLY for any contradictory functional group.
  2. Polar compound
  3. =C= C = / - C≡≡ C – ½ mark, R-OH ½ mark present
    PENALIZE ½ mark for any contradictory ion to a maximum of 1mark.
  4. R-OH absent………1mark
    Note:
    PENALIZE FULLY for any contradictory ion
  5.      
    • R – COOH present…. ½ mark
      Note
    • Penalize FULLY for any contradictory ion.
    • REJECT: H+, H3O+

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