Chemistry Paper 3 Questions and Answers with confidential - Nginda Girls Mock Examination 2023

INSTRUCTIONS

• Answer all the questions in the spaces provided.
• Mathematical tables and electronic calculators may be used.
• All working MUST be clearly shown where necessary.

QUESTION 1

You are provided with;

• solution P, potassium manganate (VII)
• solution Q, containing 3 g/litre of dibasic acid H₂X.nH₂O
• Solution R, containing 4.0 g/litre of Sodium hydroxide solution.
  You are required to determine:
  1. The value of n in H₂nH₂O
  2. How the rate of reaction of solution P with solution Q varies with change in temperature.
     

Procedure 1 

• Fill the burette with solution Q.
• Pipette 25cm³ of solution R into a conical flask and titrate solution Q with solution R using phenolphthalein indicator.
• Record your results in table I below and repeat the titration two more times and complete the table:

 Table I.                                                                                                                    (4mks)

1. Determine the average volume of solution Q used. (1mk)
2. Calculate the concentration of solution R in moles per litre (Na = 23.0, 16.0, H= 1.0 )   (1mk)
3. Given that the equation of the reaction taking place and R.F.M  of X = 88
   H₂X_(aq) + 2 NaOH_(aq) → Na₂X_(aq) + H₂O_(l)
   Calculate:
   1. The number of moles of the dibasic acid solution Q that reacted                             (1½mks)
   2. The number of moles of the dibasic acid solution Q in 1000cm³ of solution.              (1mk)
   3. The R.F.M of the dibasic acid, hydrated.                                                                    (1mk)
   4. The value of n in the formula of the hydrated acid given that the RFM of X= 88. (O =16.0, H = 1.0)                                                                                                          (1½mks)
      

Procedure II 

1. Using a measuring cylinder, place 10cm³ portion of solution P into 5’ test tubes placed in a test tube rack.
2. Clean the measuring cylinder and use it to place 10.0 cm³ of solution Q into a boiling tube.
3. Insert a thermometer in the solution Q in the boiling tube and place the boiling tube in the water bath to attain a temperature of 50^o
4. Remove the boiling tube from the water bath and place it in a test- tube rack and add the first portion of solution P. and at the same time start the stopwatch.
5. Record the time taken for the purple colour and the mixture to decolourise in table II
6. Repeat the experiment using 10cm³ of solution Q at 50^oC , 60^oC, 70° C and 80°
7. Record the time in table II below. Complete the table by computing sec^-1.
   Table II                                                                                                                      (5marks)
   

   Temperature of solution Q  (oC)	40	50	60	70	80
   Time of colour to decolourise (sec )
   1/t (sec-1)

2. 1. Plot a graph of (sec^-1) against temperature.(3mks)
      
   2. From the graph , determine the time taken for decolourisation of the mixture, if the temperature of solution B was 65°C (1mk)
   3. How does the rate of reaction of potassium manganate (VII) with oxalic acid vary with temperature? Explain                                                                                     (2mks)

Question 2

You are provided with solid V. Carry out the tests below and record your observations and inference in the spaces provided.

1. Strongly heat a spatula – end full of solid V in a dry test–tube test any gases produced using both blue and red litmus papers. 
   

   Observations	Inferences
   (1mk)	(1mk)

2.  
   1. Place the remaining solid V in a boiling tube. Add 10cm³of distilled water. Divide the solution into four portions. To the first portion, add 2-3 drops of aqueous lead (II) nitrate solution and warm.
      

      Observations	Inferences
      (1mk)	(1mk)

   2. To the second portion add 2-3 drops of barium nitrate solution followed by about 2cm3 dilute nitric (V) acid.
      
      

      Observations	Inferences
      (1mk)	(1mk)

   3. To the third portion add a few drops of sodium hydroxide until in excess.
      

      Observations	Inferences
      (1mk)	(1mk)

   4. To the fourth portion add few drops of aqueous ammonia until in excess’
       

      Observations	Inferences
      (1mk)	(1mk)

   5. To the five portion add few drops of hydrochloric acid and warm. 
      

      Observations	Inferences
      (½mk)	(½mk)

Question 3

You are provided with solid T. Carry out the tests and your observations and inferences.

1. Place about a half of solid T on a metallic spatula and burn it using Bunsen burner.
    

   Observations	Inferences
   (1mk)	(1mk)

2. Place the remaining solid T in a test tube. Add about 6cm³ of distilled water and shake well. Divide the solution into three portions.
   

   Observations	Inferences
   (1mk)	(1mk)

   1. To about 2cm³ of the mixture add a small amount of sodium hydrogen carbonate. 
      

      Observations	Inferences
      (1mk)	(1mk)

                   
   2.  To about 2cm³ of the mixture add two drops acidified potassium manganate (VII)
      

      Observations	Inferences
      (1mk)	(1mk)

CONFIDENTIAL

INSTRUCTIONS TO SCHOOLS
In addition to the fittings and apparatus found in a chemistry laboratory each candidate should have:

1. About 150 cm³ of Solution Q
2.  About 150 cm³of solution P.
3.  About 80cm³of solution R.
4. Burette 50ml
5.  Filter funnel
6.  25 cm3 pipette
7.  Clamp and stand
8. White tile
9.  2 conical flasks
10. 10ml Measuring cylinder
11. .6 test tubes
12. . Boiling tube
13. . Distilled water
14. . Thermometer -10-110°c
15. . A stop watch
16.  About 1g solid V
17.  About 1.5g of solid T
18.  Metallic spatula
19. About 1g of sodium hydrogen carbonate
20.  Water bath
21.  Pipette filter

Access to

1. Phenolphthalein indicator
2. Source of heat
3.  2M Pb(NO3)2(aq)
4. 2M HNO3 (aq)
5. .0.5M Ba(NO3)2 (aq)
6. .2M Ammonia solution
7. .2M NaOH
8.  Acidified KMnO4 solution
9.  2M HCl (aq)

NOTES

1. Solution Q is 0.05M oxalic acid
2. Solution P is 0.01M potassium manganate (vii) solution.
3. Solution R is 0.1M sodium hydroxide solution.
4.  A hot Water bath prepared by placing about 200 cm3of water in a 250ml beaker.
5. Solid V mixture (NH4)2SO4 and Al2(SO4)3 in the ratio 1: 1
6. Solid T is maleic acid crystals.
7.  Acidified KMnO4 is made by dissolving 3.169 of the solid KMnO4 in about 500 cm3of 2M
   H2S04 acid and diluting to one litre of solution.

MARKING SCHEME

1. Procedure Table 1 - (5mks)
   1. Complete table - 1mk
      • Complete table with 3 titrations done – 1mk
      • Incomplete table with 2 titrations done ½ mk
        Penalties
      • wrong subtraction
      • Inverted table
      • Burette readings beyond 50cm³ unless explained
      • Unrealistic titre values eg less 1cm3 or in hundrends Pennalise ½ mk to Maximum ½ mk
   2. Use of decimals – (Tied to first and second rows only) -1mk
      • Accept one or two decimals used consistency otherwise penalize fully ( A ward zero marks)
      • Accept inconsistency of use of zero’s on the initial burette readings eg 0, 0.0, 0.00.
      • If two decimals, use the second digit as 0. Or 5 otherwise penalize fully.
   3. Accuracy – 1mk
      Compare the candidate’s titre values with the school value and award as follows.
      1. If at least one is within ± 0.1 of S.V – 1mk
      2. If none but at least one within ± 0.2 of S.V – ½ mk
      3. If none within ±0.2 of S.V award 0 mk.
   4. Principles of averaging - 1mk
      1. 3 Titrations done, 2 are consistent – 1mk
      2. If only 2 titrations done, are consistent – 1mk
   5. Final Accuracy - 1mk
      • tied to correct average of titre - 1mk
      • Compare the candidate correct average titre with S.V. award as follows.
        1. If within ±0.10cm3 of S.V - 1mk
        2. If not ± 0.10 but within ±0.20 of S.V ½ mk
        3. If not within ± 0.20 cm3 of S.V 0omk
2. Concentration of R = 4/40 ✓½=0.1✓ ½moles/litre
3.  
   1. mole ratio of H2X : NaOH = 1: 2
      Therefore moles of dibasic acid that reacted = 1/(2 )×0.1✓½= 0.05 ✓½
   2. (answer in c(i))/(average violume ) ✓½= correct answer ✓½
   3. (6.3 )/(Answer C (ii) ) ✓½= correct answer ✓½
   4. m.m
      = 2 + 88 + 18n = answer C (iii) above ✓½
      18n = ans -90
      n = (answer-90 )/(18 ) ✓½ = final correct answer ✓½
      (N/B: accept final correct answer only if within the range 123 to 129)

Procedure II – table II

1.  
   1. Complete table with 10 readings ( 3mks)
   2. Incomplete table with 8 or 9 readings ( 2mks)
   3. Incomplete table with 5 or 6 readings ( 1mk
   4. Less than 2 readings ( 0 –mk)
      Penalise ½ mk for incorrect 1/t or value rounded to less than 3 d.p. unless exact (maximum penalty 1mk)
2. Use of decimal (½ mk)
   • should be whole member or 2 d.p consistent otherwise penalize fully.
3. Accuracy – 1mk
   • Compare the first record at 40o C of S.V to candidates readings.
     If ± 2oC award 1mk otherwise penalize fully
4. Trend - ½ mk
   • Time progression should be consistent or continuous drop from 40oC to 80oC
     1. Graph - 3mks as shown
        1. Labelling of axes ( ½ mk)
        2. Scale ( ½ mk)
           • Area covered by the graph ( plots) should be at least half of the grid provided.
           • scale interval must be consistent on each axis.
        3. Plotting – (1mk)
           • 3- 4 [points correctly plotted award - 1mk
           • 2 points are correctly plotted ½ mk
           • Mark all points plots with a tick or cross
        4. Line/ shape of graph- (1mk)
           • Accept a correct line passing through at least 2 correctly- plotted points and the origin for - 1mk
     2. Calculation for time at 65o C from graph time
        = Reciprocal of 1/t at 65o C ✓½ = correct answer ✓½
     3. The rate of reaction increases as the temperature increases ✓1
        due to increase in kinetic energy ✓½ of colliding particles hence the reacting particle collide more frequently ✓½

Question 2.

QUESTION 3