Chemistry Paper 3 Questions and Answers - Mathioya Mock 2021 Exams

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Instructions to candidates

  1. Answer all the questions in the spaces provided
  2. Mathematical tables and Electronic calculators may be used
  3. All working must be clearly shown where necessary
  4. Students should check the question paper to ascertain that all the pages are printed as indicated and that no questions are missing

For Examiner’s use only

Questions

Max. score

Student’s score

1

20

 

2

10

 

3

10

 

Total score

40

 



QUESTIONS

  1. You are provided with:
    • 2.5g of anhydrous Sodium Carbonate labelled Solid A
    • Aqueous hydrochloric acid Solution B
    • Methyl Orange indicator.
      You are required to determine the following;
      1. Enthalpy change of solution for Sodium Carbonate
      2. Concentration of Hydrochloric Acid Solution B in moles per litre.

PROCEDURE 1
Using 100cm3 measuring cylinder, add 50cm3 of distilled water into 100cm3 beaker. Note the temperature and record it in Table 1. below. Measure the temperature of the water after every half minute and record the values in table 1. At exactly 1½ minute, add Solid A to the water. Stir the mixture gently with the thermometer and note the temperature of the mixture after every 1½ minute up to the 6th minute. Record the values in the table.
(Preserve the contents in the beaker for use in procedure II)

  1.                      
    1. TABLE 1 (5 mks)

      Time (minutes)

      0

      ½

      1

      2

      3

      4

      5

      6

      Temperature (0c)

           

      X

                       
    2. Plot a graph of temperature (y-axis) against time. (3 mks)
    3. From the graph, determine the highest change in temperature. (1 mk)
      (Attach graph page)
    4. Calculate the molar enthalpy of solution of sodium carbonate Solid A.
      (Assume that specific heat capacity of the mixture is 4.2 KJkg-1 k-1 and density of the mixture is 1g/cm3, Na = 23, C = 12, O = 16) (2 mks) 

PROCEDURE II

  1. Transfer the contents in the beaker into a 100ml measuring cylinder. Add about 20cm3 of distilled water and shake well. Add more distilled water to make up to the 100cm3 mark. Label this Solution A. Fill the burette with Solution B. Using a clean pipette and a pipette filler, place 25cm3 of Solution A into a 250 ml conical flask. Add 2-3 drops of Methyl Orange indicator and titrate with Solution B up to the end point. Repeat the titrations 2 times and complete Table II.
  2.                    
    1.                       
       

      1

      2

      3

      Final Burette reading (cm3)

           

      Initial Burette reading (cm3)

           

      Volume Solution B used (cm3)

           
    2. Determine the average solution B used. (1 mk)
    3. Calculate the molarity of solution A in moles per litre (Na = 23, O = 16, C = 12) (2 mks)
    4. iv) Calculate the number of moles of Sodium Carbonate Solution A in 25.0 cm3.(1 mk)
    5. Calculate the number of moles of Hydrochloric Acid Solution B in the volume used. ( 1 mk)
    6. Calculate the concentration of hydrochloric acid solution B in moles per litre.(1 mk)

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

  1. Place a spatula endful of Solid R into a clean dry test tube and heat it strongly on a Bunsen burner flame. Test it for any gas present. (2 mks)

    Observation

    Inferences

       
  2.                       
    1. Put the remaining Solid R into a boiling tube. Add 10cm3 of distilled water and shake. Label it R. (2 mks)

      Observation

      Inferences

         
    2. To about 1cm3 of solution R, add aqueous Sodium Hydroxide solution dropwise until in excess. (2 mks)
      Observation Inferences
    3. To about 1 cm3 of solution R add 5 drops of 20 volume Hydrogen Peroxide and shake. (1½mks)

      Observation

      Inferences

         
    4. To solution obtained in (iii) above add ammonia solution dropwise till in excess.(1½mks)

      Observation

      Inferences

         
    5. To about 1 cm3 of solutionR, add Barium Chloride solution followed by 2M HNO3 (AQ) (2 mks)

      Observation

      Inferences

         

3. You are provided with slid T. carry out the following tests and record your observations and inferences in the spaces provided

  1. Scoop half of Solid T with a metallic spatula and ignite it on a non-luminous flame of a Bunsen burner. Test for the gas produced using moist blue and red litmus papers. (2 mks)
  2.                  

    Observation

    Inferences

       
    1. Place the remaining half of solid T in a test tube. Add all the absolute ethanol provided to solid T in a test tube. Shake it well and divide it into 3 portions. (2 mks)

      Observation

      Inferences

         
    2. Determine the pH of the first portion using universal indicator solution and pH chart. (1mks)

      Observation

      Inferences

         
    3. To the second portion add one and a half of the solid Sodium Hydrogen Carbonate. (2mks)

      Observation

      Inferences

         
    4. To the third portion, add about 2cm3 of acidified Potassium Manganate (VII) solution. (2 mks)

      Observation

      Inferences

         


MARKING SCHEME

Question 1
Table 1.................................................(5mks)

  1.                                  
    1. Complete table ................................................. (2 mks)
      conditions
      1. Complete table with all readings ................................................. (2 mks)
        Incomplete table with 9-11 readings ................................................. (11/2 mk)
        Incomplete table with 6-8 readings ................................................. (1 mk)
        Incomplete table with less than 6 readings ................................................. (0 mks)
      2. Treat initial values of above 40º and below 10º as unrealistic and penalize 1 mk; tird to timw ie; t = 0
      3. Penalize 1/2 mk for each reading greater than 50º from t = 30 sec to a maximum of 1 mk
      4. Penalize 1 mk if all values given in the table are constant
    2. Use of decimals ................................................. (1 mk)
      Accept whole numbers value of reading to 0 or 5 used consistently. Otherwise penalize fully.
    3. Accuracy
      Compare the candidates value (initial temperature reading ie;  the time t  = 0 with the school value and if within ± 2ºc; award (1 mk)
      Otherwise if outside ±2ºc penalize fully,
    4. Trend.
      Award the first 1/2 mk for a continuous rise in temperature up to a maximum or constant values by a drop.
  2.                      
    1. Graph .................................................(3 mks)
      1. Labelling (Both axis) ................................................. (1/2 mk)
        Penalties
        Penalties fully for inversion of axis
        Penalize fully for wrong units given.
        Otherwise, ignore units if units are omitted
      2. Scale................................................. (1/2 mk)
        Conditions
        • Area covered by the plots should be at least 3/4 of the plotting area, otherwise penalize fully.
        • Intervals must be consistent, othersise penalize fully
      3. Plotting .................................................  (1 mk)
        Conditions
        • Award 1 mk if at least 9 points are correctly plotted.
        • Award 1/2 mk if 6-8 points are correctly plotted, otherwise 0.
        • Accept plots even if the axis are inverted
      4. Shape .................................................  (1 mk)
        • Award 1/2 mard for a straight line showing.
        • Progressive increase in temperature
        • Award the other 1/2 for an extra polated straight line showing a drop.
    2. ΔT ................................................. (1 mk)
      Showing T on the extrapolated graph award (1/2 mk)
      For correct value of T award (1/2 mk)
      NB:
      (Graph must be straight lines)
      ΔT to be accepted correct extrapolation is a must.
    3.                     
    4. Molar enthalpy of solution of sodium carbonate solid A 
      Solid A
      mass of water  = 50cm3 x 1 g/cm3 = 50g
      c = 4.2 kJ/kg/k
      ΔT = 
      Heat evolved  = McΔT = 50/1000 x 4.2 x ΔT
      = Ans y (1 mk)
      Moles of Na2CO3 = 2.5/106 = 0.0235 moles
      Molar enthalpy = Ans y/0.0235 = Ans Z   (1 mk)
      • Deny mark for final answer if wrong units 
        Accept fully if no units
      • Deny mark for the answer for unrealistic values of below 5 kgmol- or above 100 kgmol-1

Table II

  1. Complete table ................................................. (1 mk)
    Use of decimals ................................................. (1/2 mk)
    Accuracy ................................................. (1 mk)
    Principles of averaging ................................................. (1/2 mk)
    Final accuracy ................................................. (1 mk)
                                                                                  4    
  2. Determine the average of solutiion B used.
    Formula √ 1/2 mk   (already awarded)
    Ans √ 1/2 mk (above in table)

c). Calculate the molarity of solution A in moles per litre
Moles of Na2CO3 = 0.0235 moles
volume = 100cm3
molarity = 0.0235 ÷ 100/1000 = 0.235M

d) Calculate the number of moles of Na2CO3 solution A in 25 cm3
0.235 = 1000cm3
? = 25 cm3
0.235 x 25/1000 = 0.005875 moles

e) Calculate the number of moles of hydrochloric acid solution B in the volume used
Na2CO3 (aq) + 2HCl (aq) → 2NaCl (aq) + CO2 (aq) + H2O(l)
1 mole            2 mole
= 0.005875 x 2
= 0.01175 moles

f) Calculate the concentration of hydrochloric acid solution B in moles per litre
0.01175 moles = 28cm3
? = 1000cm3
0.01175 x 1000 = 0.42 m
            28
(Penalize answer for unrealistic high values of above 1 M or low values less than 0.1M)

Question 2
a). Place a spatula endful of Solid R into a clean dry test tube and heat it strongly on a Bunsen burner flame. Test it for any gas present.   (2 mks)

Observation

Inferences

Moist blue litmus paper remains blue (1/2) 
Moist red litmus paper turns blue  (1/2)
 NH4+ present

b)
i) Put the remaining Solid R into a boiling tube. Add 10cm3 of distilled water and shake. Label it R. (2 mks)

Observation

Inferences

 Solid dissolves (1/2) to form a light green solution (1/2)  Polar compound (1/2)
Coloured ions present (1/2)

ii) To about 1cm3 of solution R, add aqueous Sodium Hydroxide solution dropwise until in excess.(2 mks)

Observation

Inferences

Green precipitated (1/2) that do not dissolve in excess (1/2) Fe 2+ present (1/2)

iii) To about 1 cm3 of solution R add 5 drops of 20 volume Hydrogen Peroxide and shake. (1½mks)

Observation

Inferences

The green solution turns brown Fe2+ oxidised to Fe3+ (1/2)

iv). To solution obtained in (iii) above add ammonia solution dropwise till in excess.(1½mks)

Observation

Inferences

Brown precipitate (1/2) that do not dissolve in excess (1/2)  Fe3+ present (1/2)

v). To about 1 cm3 of solutionR, add Barium Chloride solution followed by 2M HNO3 (AQ) (2 mks)

Observation

Inferences

 A white precipitate that persists on adding drops of nitric acid (1/2)  SO42- present (1/2)

Question 3
a) You are provided with slid T. carry out the following tests and record your observations and inferences in the spaces provided
Scoop half of Solid T with a metallic spatula and ignite it on a non-luminous flame of a Bunsen burner. Test for the gas produced using moist blue and red litmus papers. (2 mks)

Observation

Inferences

 Moist red litmus paper remains red, moist blue litmus paper turns red  Acidic gas

b)
i). Place the remaining half of solid T in a test tube. Add all the absolute ethanol provided to solid T in a test tube. Shake it well and divide it into 3 portions.     (2 mks)

Observation

Inferences

Solid dissolves to form a colourless solution  Non-polar compound

ii) Determine the pH of the first portion using universal indicator solution and pH chart. (1mks)

Observation

Inferences

 PHH = 5.0  Weakly acidic

iii) To the second portion add one and a half of the solid Sodium Hydrogen Carbonate. (2mks)

Observation

Inferences

 Effervescence occur 
bubbles formed
Fizzling sound produced
H+, H3O+, -COOH 
present

iv). To the third portion, add about 2cm3 of acidified Potassium Manganate (VII) solution. (2 mks)

Observation

Inferences

Potassium manganate (vii) solution is decolourised or changes from purple to colourless sdss

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