Chemistry Paper 3 Questions and Answers - Mathioya Mock 2021 Exams

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

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	1½	2	2½	3	3½	4	4½	5	5½	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)

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

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

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

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

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

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)

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)

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

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

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