Chemistry Paper 3 Questions and Answers with Confidential - Mokasa II Joint Mock Exams 2023

INSTRUCTIONS TO CANDIDATES

• Answer all questions
• Mathematical tables and silent electronic calcualtors may be used and all working must be clearly shown where necessary

Question 1

You are provided with

1. Solution B containing NaOH 
2. Solution C containing 2M HCI
3. Solid D containing 2g NaCI 
4. 5cm piece of Magnesium ribbon
   You are required to
   1. Determine effect of concentration of HCl on the rate of reaction.
   2. Percentage of impure Sodium chloride in Hydrochloric acid solution.

PROCEDURE I

1. Label six test tubes 1,2,3, and 4, place them in a test tube rack.
2. Using a 10ml measuring cylinder, Measure the volumes of solution C as indicated in table 1 and pour them into the test tubes.
3. Wash the measuring cylinder and use it to measure volumes of water as indicated in table 1 and pour into the test tubes shake all the test tubes to ensure the solutions mix uniformly. 
4. Cut out five pieces each of exactly 1cm length of Magnesium. Transfer all the solution in the test tube 1 into a clean 100ml beaker.
5. Place one piece of magnesium into the beaker containing contents of test tube 1, and immediately start a stop watch. Swirl the beaker continuously ensuring that the Magnesium is always inside the solution.
6. Record in the table 1 the time taken for Magnesium to disappear. Wash the beaker each time.
7. Repeat procedure (e) above using the remaining test tubes 2.3.4 and 5 and complete table 1 below.
   1.  Table 1       (3mks)
      

      Experiment	Volume of C (cm3)	Volume of water (cm3)	Time (seconds)
      1	12	0
      2	10	2
      3	8	4
      4	6	6

      
      Table 2
      

      Experiment	Concentration of C  (moles/litre)	Rate 1/Time (sec−1)
      1
      2
      3
      4

   2. Complete table 2 above by determining for ¹/_Time (Rate) and concentrations of solution   (4mks)
   3. Plot a graph of ¹/_Time (Vertical axis) against concentration of solution B₁    (3mks)
   4. From the graph determine the time taken mfor Magnesium to disappear when 7.5cm³ of solution C is used

PROCEDURE II

1. Measure 75cm³ of solution C and transfer into 250ml volumetric flask, Add all the solid D and shake the mixture thoroughly.
2. Add 100cm³ of distilled water to the volumetric flask, shake the mixture and add more water up to the mark. Label the resulting solution as Solution C₁
3. Fill the burette with solution C₁ up to 0.0 mark
4. Pipette 25cm³ of solution B into a clean conical flask, Add 2-3 drops of phenolphthalein indicator.
5. Titrate the two solutions until pink colour changes to colourless. Record your results in table 3 below. Repeat this titration two more times and complete table 3 below.
   1. Table 3       (4 mks)
      

      	I	II	III
      Final burette Reading (cm3)
      Initial burette Reading (cm3)
      Initial burette Reading (cm3)

   2. Calculate the average volume of solution C₁ used     (1mk)
   3. Determine the number of moles of the following solutions used in titration.(2mks)
      1. Hydrochloric acid, Solution C₁
      2. Sodium Hydroxide, Solution B
   4. Calculate the Molarity of sodium Hydroxide in Moles per Litre     (1mk)
   5. Calculate the following present in 250cm³ of solution of C₁ 
      1. The mass of Hydrochloric acid in the solution H=1, Cl= 35.5
      2. The percentage purity of Sodium Chloride, Solid D in the solution  (1mk)

Question 2

You are provided with solution G and Soiid H, use them to answer the questions that follow.

Place about 2cm³ of solution G into five clean test tubes in a test tube rack

1. To the 1^st test tube add 3 drops of Sulphuric (VI) acid
   

   Observation (1mk)	Inference (2 mks)

2. To the 2^nd portion madd 3 drops of Barium Nitrate

   Observation (½mk)	Inference (½mk)

3. To the 3rd portion madd 3 drops of Lead (II) Nitrate

   Observation (1mk)	Inference (1mk)

4. To the 4^th portion add Sodium Hydroxide solution dropwise until in excess

   Observation (1mk)	Inference (1mk)

5. Place all the solid H into a clean test tube, add 10cm³ of water, shake the mixture thoroughly until all soild dissolves. Label the resulting solution as solution H 
   1. To the 5^th portion of solution G add 2cm³ of solution H

      Observation (1mk)	Inference (1mk)

   2. Place 2cm³ of solution H into a clean test tube, add 3 drops of Nitric (V) acid followed by 3 drops of Barium Nitrate

      Observation (1mk)	Inference (1mk)

Question 3

You are provided with Organic substance L in a boiling tube. Use it to answer questions that follow. Put 2cm³ of substance L into five test tubes.

1. To the 1^st portion add 2cm³ of distilled water

   Observation (½mk)	Inference (½mk)

2. Transfer the 2^nd portion into a clean watch glass mignite it using a burning wooden splint.

   Observation (½mk)	Inference (½mk)

3. To the 3^rd portion add all the Sodium hydrogen carbonate provided.

   Observation (½mk)	Inference (½mk)

4. To the 4^th portion add 3 drops of acidified potassium dichromate (VI) and warm

   Observation (½mk)	Inference (½mk)

5. To the 5^th portion add all the solid K and test any gases produced using a burning splint.

   Observation (½mk)	Inference (½mk)

CONFIDENTIAL

1. 1 Burette
2. 1 Pipette
3. 1250ml Volumetric flask
4. 100ml beaker
5. Complete stand
6. 7 test tubes in a rack (five labelled 1,2,3,4 and 5)
7. 2 boiling tubes
8. 2 labels
9. Stop watch
10. Distilled water in a wash bottle
11. 10ml measuring cylinder
12. 100ml measuring cylinder
13. 2 pieces of wooden splint 
14. Watch glass
15. 100cm³ of 0.5M NaOH labelled as solution B 
16. 150cm³ of 2M HCl labelled as solution C
17. 2g NaCl labelled as Solid D
18. 0.5g of Sodium Sulphate labelled as H
19. 15cm³ 0.3M Lead(II) Nitrate labelled as solution G
20. 15cm³ of Absolute ethanol labelled as Substance L
21. 6cm piece of Polished Magnesium ribbon 
22. 0.1g of Calcium turnings labelled as Solid K
23. 0.5g of Sodium Hydrogen Carbonate.

Access to:

1. Phenolphthalein indicator
2. Sodium Hydroxide
3. Barium Nitrate
4. Lead(II)Nitrate
5. Sulphuric(VI)acid
6. Acidified Potassium dichromate (VI)
7. Source of heat 
8. Nitric(V)acid

MARKING SCHEME

Question 1

Procedure I

a)Table 1

• Time increases
• uniform decimals

b)Table 2

• Accept values at least 3dp unless exact otherwise penalize FULLY
• Correct entry of ¹/_Time to at least 3d.p otherwise penalize fully to less than 3d.p

c) 

• Corect value of 1/T from correct graph
• Correct value of Time with correct units (otherwise penalize for wrong units used)

N/B graph must be correct. pass through the origin.

Procedure II

1. Acc. 24 ± 2
2. Correct average titre values
3.  
   1. Moles in 250cm³
      2 × 75 = 0.15 moles
       1000
      Moles used in titration
      0.15moles → 250
                        → correct average litre
   2. NaOH : HCl 
           1    :  1
      Ans in c(i) above
      At least 4d.p both in (i) and (ii)
4. Moles in C(ii) × 1000
                 25
   = correct ans
   (At least 3d.p unless exact)
5.  
   1. 0.15 moles in 250 × 36.5
           = Correct to atleast 2d.p in g
      Penalize ½ for missing units
   2. Ans in C(i) + 2g = Total mass
              2        × 100% = correct %
      Total mass

Question 2

1.  
   

   Observation (1mk)	Inference (2mks)
   - No effervescence  - White ppt is formed	-  SO32− , CO32−,  HCO3− Absent - Ca2+ ,Pb2+ ,Ba2+ present

2.  
   

   Observation (½mk)	Inference (½mk)
   No white ppt	SO42 Absent  Ignore SO32−, CO32− Absent

3.  
   

   Observation (1mk)	Inference (1mk)
   -No white ppt - No yellow ppt	- Cl−//Br− Absent   - I− Absent

4.  
   

   Observation (½mk)	Inference (½mk)
   - White ppt is formed soluble in excess	- Pb2+ present  Penalize fully for contradiction

5.  
   1.  
      

      Observation (½mk)	Inference (1mk)
      - White ppt is formed	H contains  (must mention)  SO42− , SO32−, CO32− , CI− , Br− ,

   2.  
      

      Observation (1mk)	Inference (1mk)
      - No efferevescence  - White ppt	SO42− Present  Must be mentioned in (i) above otherwise penalize fully.

Question 3

1.  
   

   Observation (½mk)	Inference (½mk)
   Miscible with H2O// forms uniform layer	Polar compound

2.  
   

   Observation (½mk)	Inference (½mk)
   Burns with a blue non sooty flame	l     l  C = C    or −C≡C−   Absent   l      l

3.   
   

   Observation (½mk)	Inference (½mk)
   No effervescence produced	R − COOH Absent

4.  
   

   Observation (½mk)	Inference (½mk)
   Orange colour of H+/ K2Cr2O7 changes to green	R−OH Present

5.  
   

   Observation (½mk)	Inference (½mk)
   Effervescence of colourless gas that extinguishes the burning splint with a pop sound	K is a metallic/ a metal (Tied to the pop sound)