Chemistry Paper 3 Questions and Answers With Confidential - Cekana Mock Exams 2023

INSTRUCTIONS

• All working must be clearly shown where necessary.
• You are not allowed to start working with the apparatus for the first 15 minutes. This time is to enable you read the question paper and make sure you have all the requirements.

QUESTIONS

1. You are provided with:
   • Magnesium ribbon solid E
   • 2M hydrochloric acid, solution F
   • 0.375M sodium hydroxide, solution H
     You are required to determine
     1. The rate of reaction between magnesium and hydrochloric acid
     2. The mass of 1cm length of magnesium ribbon

PROCEDURE 1

Step 1: Cut out five pieces of exactly 1cm length of magnesium ribbon solid E. Carefully fill the burette with solution F.
Step 2: Drain from the burette 10cm3 solution F into a test tube. Put one piece of magnesium ribbon into the tube and                                immediately start a stopwatch. Record the time taken for the magnesium ribbon to completely react.
Step 3: Transfer all the contents of the test tube into a 250ml volumetric flask. Rinse the test tube with distilled water and put in into the volumetric flask.
Step 4: Repeat step 2 and 3 by placing 9cm³ of solution F and 1cm³ of distilled water shake the contents before use. After each experiment transfer the mixture into the volumetric flask.

Repeat the procedure using contents in test tube 3, 4 and 5 as shown in the table below.
RETAIN THE SOLUTION IN THE VOLUMETRIC FLASK FOR USE IN PROCEDURE II
Table I

(5mks)

1.  
   1. Plot a graph of rate(1⁄7) , against volume of solution F. (3mks)
      
   2. Use the graph to determine the time taken for 1cm length of magnesium to dissolve if volume of water added is 1.5cm³ . (2mks)
   3.  
      1. In terms of rate of reaction, explain the shape of the graph (2mks)

PROCEDURE II
Add distilled water into the solution in the 250ml volumetric flask up to the mark. Label it as solution G.
Clean the burette and fill it with Sodium hydroxide, solution H.
Pipette 25cm³ of solution G into 250ml conical flask. Add 2 drop of phenolphthalein indicator and titrate it with solution H from the burette.
Record your results in table below
Table II

(4mks)

2. 2.  Determine the
      1. Average volume of solution H used. (1mk)
      2. Number of moles of hydrochloric acid in 250cm³ of solution G. (2mks)
      3. Number of moles of hydrochloric acid solution F that reacted with solid E. (2mks)
      4. The mass of magnesium ribbon that reacted. (Mg=24). (2mks)
      5. Mass of 1cm length magnesium ribbon. (1mk)

2 . You are provide with solid K. Carry out the following tests and record your observations and inferences in the spaces provided.

1. Place all solid K into a boiling tube. Add about 5cm3 of distilled water and shake the mixture.
   Observation Inferences
   

   Observation	Inferences
   (1mk)	(1mk)

2. To about 2cm³ of the solution K , add half a spatula of sodium hydrogen carbonate.
   

   Observation	Inferences
   (½mk)	(½mk)

3. To the remaining solution K, add about 10cm3 of dilute hydrochloric acid. Shake thoroughly. Filter the mixture then wash the residue with distilled water. Dry the residue with filter paper.
   1. Place one third of the residue into a test tube. Add about 10cm3 of distilled water and shake the mixture. Add half a spatula sodium hydrogen carbonate.
      

      Observation	Inferences
      (½mk)	(½mk)

   2. To the remaining residue add 5cm³ of distilled water then 5cm³ of sulphuric (VI) acid, followed by 5cm3 of ethanol and warm the mixture.
      

      Observation	Inferences
      (1mk)	(1mk)

3. You are provided with solid L carry out the following test and record your observations and inferences in the space provided.

1. Place one third of the solid L in a dry test tube. Heat it and test any gas produced with blue and red litmus paper.
   Observation Inferences
   

   Observation	Inferences
   (2mks)	(1mk)

2. Place the remaining solid L in a boiling tube. Add about 10cm³ of distilled water and shake. Divide the solution in to four portions.
   1. To about 2cm³ of solution L add 2 drops of barium chloride solution
      

      Observation	Inferences
      (1mk)	(1mk)

   2. To about 2cm³of solution L add 5 drops of dilute hydrochloric acid
      

      Observation	Inferences
      (1mk)	(1mk)

   3. To 2cm³ of solution L add ammonia solution drop wise until in excess
      

      Observation	Inferences
      (1mk)	(1⁄2mk)

   4. To 2cm³ of solution L add sodium hydroxide drop wise until in excess
      

      Observation	Inferences
      (1mk)	(½mk)

CONFIDENTIAL

1.  Solid E
2. 50cm³of solution F
3. 80cm³of solution H
4. 0.5g of solid K
5. 0.5g of solid L
6. Distilled water
7. Burette
8. Pipette
9. Pipette filter
10. Label
11. 250ml volumetric flask
12. Two 250ml conical flasks
13. Test tube holder
14. 2 boiling tube
15. 6 test tubes
16. Stopwatch
17.  10ml measuring cylinder
18.  Spatula
19.  0.5g sodium hydrogen carbonate
20.  5cm³of ethanol
21.  Filter paper – 3 pieces
22.  Filter funnel
23.  Blue and red litmus paper
    Access to
    1. 0.1M BaCl₂
    2. 2M HCl
    3.  2M Ammonia solution
    4. 2M NaOH
    5. 2M H₂SO₄ acid
    6. Source of heat
    7. Phenolphthalein indicator
       Preparations
       1. Solid E – 5cm magnesium ribbon
       2. Solution F – 2M HCl
       3. Solution H – 0.375M NaOH
       4. Solid K – sodium benzoate
       5.  Solid L-Ammonium aluminium sulphate

MARKING SCHEME

Procedure I

Table 1…………………………….5 mks

1. Complete table (time)……..1mk
   complete table 1
   incomplete table (3-4) ½
2. Decimal (Time) ……..½mk
   Either whole number, 1d.p or 2d.p
   Consistently written ½
   Otherwise ward 0mk
3. Accuracy ….(experiment 1 ) ------½mk
   - compare candidates value with S.V of experiment 1. It should be within ±2 units otherwise award 0mk
4. Trend (time)……½mk
   Continuous increase
5. ¹/ _time (sec^-1) ……..2½mk
   . Each correct conversion is ½mk
   Condition
   To be at least 3d.p

Q 

1.  
   1. Graph
      • Labelling Axes…………..½mk
      • Scale………………………½mk
        Uniform scale, otherwise 0mk
        Plots to occupy at least half of grid in both axis
      • Plots…………………………………..1mk
        5 correctly plotted points
        3-4 correctly plotted points 
        Less than 3 points plotted 0mk
      • Line ………………………1mk
        Straight line passing through (0,0)
        Otherwise 0mk
   2. Show 
      recording 
      reciprocal 
      correct ans
   3. Rate is directly proportional to concentration of acid increase in the concentration of the acid increases the number of effective collisions per unit time
      Procedure II
      Table 2…………………5mks
      • Complete table …………………….1mk
        3 titration 
        2 titration 
        Penalties
        - Inverted table
        - Burette reading beyond 50cm3 unless explained
        - Volume of 1cm3 and below
      • Decimal………..1mk
        1d.p written consistently otherwise 0mk
      • Accuracy …………….1mk
        Any within ± 0.1 of s.v
        - Outside ±0.1 but within ±0.2 of s.v
        - Outside ±0.2………….0mk
      • Principle of averaging……………..1 mk
        Values within ±0.2 of each other 
        i.e working 
        Answer 
      • Final answer ………..1mk
        Average value within
        ±0.1of s.v
        ±0.2 of s.v
2.  
   2.  
      AV × ^0.375/ ₁₀₀₀ =ans(x)
      1:1 or equation
      ^{ans(x) × 250}/₂₅
   3. 2 × 40/ 1000 =0.08
      0.08-ans(y)
   4. mg:HCL=1:2
      mole of mg = ^ansb(iii)/ ₂
   5. ^ans(B)/ 5=ans(C)

2. 

1.  
   

   Observation	Inferences
   Dissolves ü1 to form a colourless solution	polar

2.  
   

   Observation	Inferences
   No effervescence	absent

3.  
   1.   
      

      Observation	Inferences
      Effervescence	R-COOH/H+

   2. 
      

      Observation	Inferences
      Pleasant smelling compound  Reject sweet	R-COOH

3 

1.   
   

   Observation	Inferences
   Colourless liquid formed on cooler part of boiling tube Colourless gas with pungent/ chocking smell Red litmus changes to blue Blue litmus paper persists White residue	Hydrated salt/ solid NH+4

2.  
   1.    
      

      Observation	Inferences
      White ppt	SO2-4CO2-3,SO2-3

   2.   
      

      Observation	Inferences
      No effervescenceü	SO2-4

   3.   
      

      Observation	Inferences
      White ppt Insoluble in excess	Mg2+ü½ Al3+

   4.  
      

      Observation	Inferences
      White ppt  Soluble in excess	Al3+ü½