Chemistry Paper 3 Questions and Answers with Confidentials - Asumbi Girls High School KCSE Mock 2021

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Questions

Instructions to the candidates:-

  • Answer all the questions
  • Mathematical tables and electronic calculators may be used.
  • All working MUST be clearly shown where necessary.
  • Use the first 15minutes of the 2 ¼ hours to ascertain you have all the chemicals and apparatus tha you may need.

QUESTION 1

  • You are provided with solution K and L
  • Solution K is 1M H2SO4
  • Solution L contains 8.7g of the hydroxide of metal M {with formulae MOH] in 600cm3 of the solution
  • You are required to carry out the experiment to determine;
    1. Concentration of solution L
    2. R.A.M of metal M

Procedure

  1. Measure 75cm3 of solution K and put into a clean 250cm3 volumetric flask and add distilled water up to the mark
    -label this solution W
  2. Fill a clean burette with solution W
  3. Pipette 25cm3 of solution L into a clean conical flask and add 2 drops of phenolphthalein indicator
  4. Titrate the solution W in the burette against solution L in the conical flask and record the results in the table below
  5. Repeat {3} and {4} above as you fill the table below.

TABLE 1

   I  II  III
Final burette reading {cm3}       
Initial burette reading {cm3}       
Volume of solution W used {cm3}       

[4mks]

  1. Calculate the;
    1. Average volume of solution W used [1mk]
    2. Concentration of solution W [1mk]
    3. Number of moles of solution W that reacted with each 25cm3 portion of solution L {2mks]
  2. Calculate the;
    1. Number of moles of the metal hydroxide {MOH} in solution L that reacted with each portion of solution W [2mks]
    2. Concentration of solution L [1mk]
    3. Number of moles of the metal hydroxide [MOH]in 600cm3 of solution L [2mks]
    4. R.A.M of metal M [2mks]
      [0=16, H=1]

QUESTION 2
You are provided with solution N and P

  • Solution N is 2M HCl
  • Solution P is 0.16M sodium thiosulphate
  • You are required to carry out the experiment below to determine how concentration affects the rate of reaction between HCl and sodium thiosulphate solutions

PROCEDURE

  1. Fill a clean burette with solution P.
  2. Measure 25cm3 of the solution P from the burette into a clean 100cm3 glass beaker and place on a white piece of paper with a cross[x] marked on it
  3. Add 10cm3 of solution N into it and immediately start a stop watch and note the time taken for the cross beneath the mixture to become invisible
  4. Clean the 100cm3 beaker and measure into it 20cm3 of solution P form the burette, and add 5cm3 of distilled water into the solution solution
  5. Repeat step [2] above and note the time taken for the cross to become invisible
  6. Repeat the experiment using volumes indicated on the table below and as you record the results

TABLE 2

Expt   1  2  3  4  5
Volume of solution p(cm3)  25 20 15 10 5
Volume of water added to solution p{cm3}   0  5  10  15 20
Volume of solution N  10  10  10  10  10
Time taken for the cross to become invisible [in seconds]           
1/t S-1           

(4mks)

  1. plot a graph of 1/t against volume of solution P on the grid provided [3mk]
  2. From the graph
    1. Determine the time taken for the cross to become invisible when 12.5cm3 of solution P is used [2mks]
    2. Explain the effect of concentration on the rate of reaction between HCl and sodium thiosulphate solution [2mks]

QUESTION 3

  • You are provided with solids Q and R
  • You are required to carry out the tests below as you record your observations and inferences
    1. SOLID Q
      -Add about 5cm3 of distilled water to solid Q, shake the mixture thoroughly for a while and then filter it
      NOTE: Retain both the filtrate and the residue for the tests below
     TEST  OBSERVATION  INFERENCES
 A i  Divide the filtrate into 4 portion
To the first portion, add 4 drops of NaOH
   
  ii Scoop the 2nd portion on a metallic spatula
and ignite on a non-luminous flame
   
  iii  To the 3rd portion, add 2 drops of Pb{NO3}[aq]    
  iv To the 4th portion ,add acidified KmnO4    
 B i Put the residue in a test tube and add about 2cm3 of HNO3    
  ii To the mixture in b{i} above, add 2 drops of KI solution    
 ii SOLID R   
    TEST OBSERVATION INFERENCES
A i Scoop a portion of solid R on a
Metallic spatula and burn on a
Non-luminous flame
   
B i Put the remaining portion of solid R into a clean test tube and
add about 3cm3 of distilled water, shake and divide into 2 portions
   
  ii To the 1st portion, add 2 drops of acidified KMnO4 and warm    
  iii To the 2nd portion add NaCO3    

 

Confidential

  1. Each student should be supplied with the following
    1. Burette
    2. Pipette
    3. Pipette filler
    4. Filter funnel
    5. White tile
    6. Clamp and stand
    7. 2 conical flask
    8. 100cm3 glass Beaker (empty)
    9. Stop watch
    10. 100cm3 measuring cylinder
    11. 10cm3 measuring cylinder
    12. 250cm3 volumetric flask
    13. Metallic spatula
    14. 6 clean test tubes
    15. Test tube holder
    16. 500ml distilled water
    17. White piece of paper or filter paper
    18. 1 filter paper
    19. 1 labelling paper
    20.  Phenolphthalein indicator
    21. About 90cm3 solution K
    22.  About 100cm3 solution L
    23.  About 70cm3 solution N
    24. About 90cm3 solution P
    25. About 0.5gNaHCO3
    26. About 1.0g solid Q
    27. About 0.5g solid R
  2. Each student should have access to the following solutions:
    1. Mean of heating
    2. 2M NaOH
    3. 2M HNO3
    4. Pb(NO3)
    5. Acidified KMNO4
    6. Potassium iodide solution
      NB: the above solutions should be supplied with a dropper each.
  3. SOLUTIONS PREPARATION AND SOLID MEASUREMENTS
    1. SOLUTION K IS 1M H2SO4
    2. SOLUTION L IS 0.36 M NaOH CONTAINING 14.4g OF NAOH IN 1 LITRE OF THE SOLUTION
    3. SOLUTION N IS 2M HCl
    4. SOLUTION P IS 0.16 M Na2S2O3(SODIUM THIOSULPHATE)
    5. SOLID Q IS A MIXTURE OF SODIUM SULPHITE(Na2SO3) AND LEAD (II) CARBONATE (PbCO3) MIXED IN THE RATIO 1:1 (should be thoroughly mixed)
    6. SOLID R IS MALEIC ACID

Marking Scheme

QUESTION 1
TABLE 1

  • complete table CT 1 mk
  • decimal point D 1 mk
  • accuracy A (tied to school value)1 mk
  • principal of averaging 1 mk

 

  1.        
    1. Average volume / final accuracy 1(tied to school value)
      NB theoretically expected value = 15.0 cm3
    2. 1M x 75 cm3   Â½ mk
         250cm3
      = 0.3M ½ mk
  2. Ans in (b)above X ans in (a)above½ mk  = ans ½ mk
             1000
  3. 2MOH(aq) + H2SO4 (aq) → M2SO4(aq) + H2O(l) 1mk
  4. Mole ratio = 2:1 ½ mk
  5. Moles of solution W = 2 x ans in a(iii) ½ mk
  6. ans in b(i)x 1000½ mk = ans ½mk
                25
  7. ans in (i)x 600 = ans½ mk
       25½ mk
  8.       8.7/½ mk= RFM OF MOH½ mk
    ans in b (iii)
  9. RAM of metal M = RFM -(16+1) ½ mk= ans½ mk

QUESTION 2
TABLE 2

  • complete table CT1mk
  • decimal point D 1mk
  • trend (increasing time )1mk
  • 1/t row completed 1mk

 

  1. Graph
    Plotting 1mk
    Scale 1mk
    Straight line touching origin (0,0) 1mk
  2. Correctly read value from the graph 1mk
  3. Correct reciprocal of value read from the graph 1mk
  4. Rate at reaction increase with increase in concentration of the sodium thiosulphate because increase in concentration increases number of successfulcollisions

QUESTION 3

  1. Solid Q
    1.        
           Observation  inferences 
      A  No white ppt formed Â½ mk Ca 2+ Mg2+, Pb2+, Al3+ Zn2+ Absent
      ii   Burns with a golden yellow flame Na+ confirmed
      iii  White ppt formed ½ mk Cl-, SO42-, SO32-,CO32-1mk
      iv  KMNO4 decolorized½ mk SO32-, Confirmed1mk
      B  i  Effervescence occurs ½ mk   CO32-, SO32-1mk
       ii

       Yellow ppt formed½ mk

       Pb2+ confirmed1mk
  2.          
    1. Solid R
           Observation  inferences
       A Burns with a yellow sooty flame½ mk  =C = C = , = C = C
       B  i Dissolves½ mk forming a colorless solution½  Polar substance½ mk
       ii  KMNO4 decolorized½ mk   ROH, =C = C =, = C = C =
       ii  Effervescence½ mk  H+ or RCOOH1mk

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