Chemistry Paper 3 Questions, Answers and Confidential - Mincks Group of Schools Mock Examinations 2022

INSTRUCTIONS TO CANDIDATES

• Write your name and index number in the spaces provided.
• Sign and write the date of examination in the spaces provided.
• Answer all the questions in the spaces provided in the question paper.
• You are not allowed to start working with the apparatus for the first 15 minutes of the 2 ¼ hours allowed for this paper. This time is to enable you to read the question paper and make sure you have all the chemicals and apparatus you need.
• All working must be clearly shown where necessary.
• Mathematical tables and electronic calculators may be used. 

For examiner’s use only

QUESTIONS

1. You are provided with:
   5.0g of solid R which is a hydrated acid with formula H₂C₂O₄.nH₂O
   Solution Q which is 0.25M NaOH
   You are required to determine the
   1. Solubility of R
   2. The value of n in the formula H2C2O4.nH2O

Procedure I
Fill the burette with distilled water
Place solid R in a clean boing tube
Transfer 4cm³ of distilled water from the burette into the boiling tube containing solid R. Heat the mixture while stirring carefully with thermometer to a temperature of 80ºC. Allow the solution to cool while stirring with thermometer. Record the temperature at which crystals start to form in the table 1 below.
Add a further 2cm³ of distilled water from the burette to the mixture. Repeat the procedure above by adding 2cm³ of distilled water and record the crystallization temperature after every experiment. Complete table 1 below by adding volumes of distilled water as indicated from table 1. PRESERVE THE CONTENTS OF THE BOILING TUBE TO BE USED IN PROCEDURE II

Table 1

(6mks)

1. On the grid provided, plot a graph of solubility of solid R (vertical axis against temperature. (3mks)
2. From the graph determine
   1. The solubility of solid R at 55ºC. (1mk)
   2. The temperature at which 40g of R dissolves in 50g of water. (1mk)

Procedure II
Transfer the contents of the boiling tube in procedure I to clean 250ml volumetric flask.
Add distilled water to the mark.
Label this as solution P.
Fill the burette with solution P
Pipette 25cm³ of solution Q into a clean conical flask. Add two drops of phenolphthalein indicator. Titrate solution P against solution Q until pink colour just changes to colourless. Record your results to the table II below. Repeat the procedure, two more times to complete that table II.
Table II

(4mks)
Calculate:

1. Average volume of solution P used. (1mk)
2. Moles of solution Q used. (1mk)
3. Moles of P used given that 2 moles of solution Q reacts with 1 mole of solution P. (1mk)
4. Concentration of solution P in moles per litre. (1mk)
5. The value of n in the formula C₂H₂O₄.nH₂O (C = 12, O = 16, H = 1) (1mk)

2.

1. You are provided with solid T. Put all solid T in a clean boiling tube. Add about 10cm³ of distilled water and shake. Filter the resultant mixture in a clean boiling tube. PRESERVE THE CONTENTS OF FILTER PAPER FOR USE IN PROCEDURE 2 (B) BELOW. Divide the filtrate into 3 portions of about 2cm³.
   

   Observations	Inferences
   (1mk)	(1mk)

   1. To the first portion add two drops of Lead (II) nitrate solution.
      

      Observations	Inferences
      (1mk)	(1mk)

   2. To the second portion add three drops of acidified Barium nitrate solution.
      

      Observations	Inferences
      (½ mk)	(½ mk)

   3. To the third portion add aqueous ammonia dropwise until in excess
2. Using spatula scoop the contents of the filter paper and transfer it in a clean boiling tube. Add about 6cm³ of 2M HNO₃ provided. Divide the resultant mixture into two portions of about 2cm³.
   

   Observations	Inferences
   (1mk)	(1mk)

   1. To about 2cm³ portion add sodium hydroxide dropwise until in excess.
      

      Observations	Inferences
      (1mk)	(1mk)

   2. To the second portion add two drops of potassium iodide solution
      

      Observations	Inferences
      (½ mk)	(½ mk)

3 You are provided with solid S. Use it to answer the questions that follow.

1. Describe the appearance of solid S. (1mk)
   
   
2. Place all solid S into a clean boiling tube and add about 8cm3 of distilled water and shake. Divide the resulting mixture into three portions of about 2cm3.
   

   Observations	Inferences
   (1mk)	(½mk)

   1. To the first portion add 2 drops of acidified potassium manganate (VII) and warm.
      

      Observations	Inferences
      (1mk)	(1mk)

   2. To the second portion, add magnesium ribbon provided
      

      Observations	Inferences
      (1mk)	(1mk)

3.                    
   1. Describe how the PH of the third portion can be determined. (1mk)
   2. Carry out the actual test in (c(i) above and record the observation and inference.
      

      Observations	Inferences
      (½mk)	(½mk)

MARKING SCHEME

Table 1

Table 1 attracts 6marks distributed as follows.

1. Complete table 1mrk
   • Award 1mrk for 5 – 4 correctly filled.
   • Award ½Mrk For 3 Correct Readings.
   • Award 0Mrk for less 3 readings.
2. Accuracy ½mrk
   • Tied to the 1st temperature.
   • Accept if within +12.0º C of school value
3. Decimal 1mrk
   • Accept whole number for temperature readings.
   • Accept one decimal places of .5 or .0 for temperature readings.
   • Penalize fully for decimal if not consistent.
4. Trend 1mrk
   • Accent temperature readings drops continuously.
   • Award ½ mrk if temperature reading all remain constant or rises
   • Penalize fully if temperature readings all remain constant 2 ½ mk
5. Solubility calculations 1mrk
   • Award 2 ½ marks for calculations of solubility.
   • Award ½ mark for each correct calculation done.

Graph
Graph attracts three marks awarded as follows:
7

1.                    
   1. Labeling of axes ½ mrk
      • Award ½mk for all axes correctly labeled with correct units.
      • Units may be given on both or not.
      • If units given, they must be correct.
   2. Scale ½mrk
      • Graph should cover at least ½ of the space provided (area covered by plots)
      • Scale should accommodate all plots whether plotted or not.
   3. Plotting 1mrk
      • Award 1mrk for 4 – 5 plots are correctly plotted.
      • Award ½mrk for 3 plots.
      • Award 0mrk for less than 3 plots.
   4. Curve (1mrk)
      • Award 1mrk for smooth curve drawn passing through correctly plotted plots.
        
2.                        
   1. Should be shown from the graph ½ correct reading from correctly drawn graph ½ (both should apply)
      E.g. 55ºC = 60g/100g water
   2. 40g → 50g
      ? → 100g
      40 x 100
          50
      = 80g/100g of water
      Reading correctly at 80g/100g water = 66ºC

Table II

Total of 5mks awarded as follows:

1. Complete table 1mk
   Awarded as;
   • Complete table with three titration done 1mk
   • Complete table with two titration done ½mk
   • Complete table with one titration done 0mk
   • Penalize 1/2mk for wrong arithmetic’s on complete table incomplete table or unrealistic burette readings
2. Decimals (1mk)
   • Award 1mk if decimals are used consistently tied to 1st row and second row readings.
   • Accept use of 0, 0.0, 0.00 on the initial burette reading. Incase of 2 decimals place the second decimal should be 0 or 5
   • Penalize fully if decimals are used inconsistently.
3. Accuracy (1mk)
   • Award 1mk if at least one of the readings is within -+0.2 of school value.
4. Principle of averaging (1mk)
   • Award 1mk if two or three consistent values are correctly chosen averaging that is within -+0.1 or -+0.2
   • If all three values are with the range and only two are averaged then Penalize fully and award (0mk)
   • If none of the values are within the range penalize fully, if averaged.
5. Final Accuracy (1mk)
   • Award 1mk if within -+0.1 0f the school value
   • Award ½mk if within -+0.2 of the school value

Calculations

1. Average volume of P
   19.8 + 19.9 +19.7 =19.8cm³
            3
2. Moles of Q
   0.25 x 25 ½ = 0.00625Moles (½mk)
      1000
3. Moles of P
   Ans in (b) above divided by 2 = ans ½
   E.g. 0.00625 ½ = 0.003125Moles
               2
   NB answer in (b) above Mn of be transferred wholly otherwise penalize fully.
4. Answer in (c) above x 1000 ½ = ans ½
           Average volume
   e.g. 0.003125 x 1000 = 0.157Moles
                             19.8
   NB answer should should be to 4 decimals otherwise penalize ½mk
5. Value of n
   5g → 250cm³                  5 x 1000 = 20
       →  1000cm³                    250
   20             
   ans in d above = Molar mass
   n= Molar mass – 90
                   18
   = (ans in whole Number)
   e.g. 127 – 90
                18
   =2.055
   =2.0

2.

1. You are provided with solid T. Put all solid T in a clean boiling tube. Add about 10cm³ of distilled water and shake. Filter the resultant mixture in a clean boiling tube. PRESERVE THE CONTENTS OF FILTER PAPER FOR USE IN PROCEDURE 2 (B) BELOW. Divide the filtrate into 3 portions of about 2cm³.
   

   Observation	Inference
   White residue ½ Colorless filtrate	Mixture of soluble and insoluble compounds 1mk

   1. To the first portion add two drops of Lead (II) nitrate solution.
      

      Observation	Inference
      White precipitate	CO32- SO42- SO32-, Cl- Present

   2. To the second portion add three drops of acidified Barium nitrate solution.
      

      Observations	Inferences
      White precipitate  Ignore – No effervescence Penalize – No white if mentioned	SO42- present SO42- should be mentioned in (i) above

   3. To the third portion add aqueous ammonia dropwise until in excess
      

      Observation	Inference
      White precipitate ½     Dissolve in excess ½	Zn2+ Present    1mk Penalize fully for any contradictory ion

2. Using spatula scoop the contents of the filter paper and transfer it in a clean boiling tube. Add about 6cm³ of 2M HNO₃ provided. Divide the resultant mixture into two portions of about 2cm³.
   

   Observations	Inferences
   Effervescence / bubbles of colorless gas 1mk	CO32- , SO32-     1mk 2 – ions mentioned – 1mk Present 1 – ion mentioned – 1/2mk

   1. To about 2cm³ portion add sodium hydroxide dropwise until in excess.
      

      Observations	Inferences
      White Precipitate   Dissolve in excess	Zn2+, Pb2+, Al3+ Present

   2. To the second portion add two drops of potassium iodide solution
      

      Observations	Inferences
      Yellow precipitate  Penalize – yellow only  Yellow solution	Pb2+  present Penalize fully for any contradicting ion mentioned

3 You are provided with solid S. Use it to answer the questions that follow.

1. Describe the appearance of solid S. (1mk) White ½ crystals/ solid ½
2. Place all solid S into a clean boiling tube and add about 8cm3 of distilled water and shake. Divide the resulting mixture into three portions of about 2cm3.
   

   Observations	Inferences
   Purple color of acidified potassium 1mk Manganate changes to colorless  Accept for 1mk KMnO4 is delocalized	C=C , or C=C , ½ ROH ½ Present

   1. To the first portion add 2 drops of acidified potassium manganate (VII) and warm.
      

      Observations	Inferences
      Effervescence of colorless gas            1mk	H, 3O+ R – COOH Present            1mk H+, H30+, R - COOH

   2. To the second portion, add magnesium ribbon provided
      

      Observations	Inferences
      (1mk)	(1mk)

3.                    
   1. Describe how the PH of the third portion can be determined. (1mk)
      

      Add/ put universal indicator to portion of e ½ mk
      Match the color formed with the PH chart scale. ½

   2. Carry out the actual test in (c(i) above and record the observation and inference.
      

      Observations	Inferences
      PH 1.0 ½ mk   Award ½ mk for PH=1 or PH=2 or PH=3 Penalize fully for given in range	Strongly Acidic ½ penalize fully for strong acid

CONFIDENTIAL

In addition to the equipment and fittings in the Chemistry laboratory, each candidate will require:

1. Solid R (5.0g of oxalic acid in a boiling tube. (Accurately weighed)
2. 100cm³ solution Q (0.25M NaOH)
3. Thermometer (-10^oC – 110^oC)
4. One 50ml burette
5. Filter funnel
6. Pipette and pipette filler
7. One label
8. 250ml volumetric flask
9. 2 pieces 250ml conical flask
10. About 500ml of distilled water
11. Bunsen burner
12. 5 of solid S (maleic acid)
13. 6 test-tubes in a rack
14. Two boiling tubes
15. Test tube holder
16. 0g solid T (PbCO₃ and ZnSO₄ mixture ratio 1:1)
17. Filter paper (Whatman)
18. 1cm length polished magnesium ribbon
19. PH chart scale

Access to the following

1. Phenolphthalein indicator supplied with a dropper
2. 0M NaOH supplied with a dropper.
3. 0M NH₃(aq) supplied with a dropper.
4. 0M dilute nitric acid supplied with a dropper
5. Lead (II) nitrate solution supplied with a dropper
6. Acidified Barium nitrate solution supplied with a dropper.
7. Potassium iodide solution supplied with a dropper.
8. Acidified potassium manganate (VII) supplied with a dropper
9. Universal indicator

NOTES: PREPARATIONS

6. Acidified Barium nitrate is prepared by weighing dissolving 26.0g of Ba(NO₃) in 600m³ of distilled water. Add 250cm³ of 2MHNO₃ and topping up to 1 litre with distilled water.
7. Acidified potassium manganate (VII) is prepared by dissolving 3.16g of solid KMnO₄ in 400cm³ of 1m H₂SO₄ and making it to one litre with distilled water.
8. Lead nitrate dissolve 33.1g in 800cm³ distilled water and make it to one litre.
9. Solution Q is prepared by accurately weighing 10.0g of sodium hydroxide pellets and dissolving and topping up to 1 litre.
10. 2M NaOH prepared by dissolving 80g of NaOH pellets in 800cm³ of distilled water and top up to 1 litre.
11. 2M NH₃(aq) measuring 298cm³ of ammonia solution top up to 1 litre. Label the solution aqueous ammonia (2MNH₃(aq)