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

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
   6
   8
   10
   12

   (6marks)
   
   1. On the grid provided plot a graph of the solubility of solid P against temperature (3marks)
      
   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

(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)

2. 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)

3. 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

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. 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

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.

QUESTION 3 – 9 MARKS