CHEMISTRY PAPER 3 - KCSE 2019 MASENO MOCK EXAMINATION (WITH MARKING SCHEME)

INSTRUCTIONS TO CANDIDATES.

• Answer all the questions
• You are not allowed to start working with the apparatus for the first 15 minutes of the 2 ¼ hours allowed in this paper.
  This is to enable you read the question paper and make sure you have all the chemicals and apparatus you may need.
• Mathematical tables and Electronic calculators may be used.
• All working must be clearly shown where necessary.

1. You are provided with:
   • 5g of dibasic acid H₂X. 2H₂O, Solid A
   • Solution B, 0.4M sodium hydroxide.
     
     You are required to:
   • You are required to determine the solubility of Solid A in water at different temperatures.
   • Determine RAM of dibasic A in H₂2H₂O.

Procedure I:

1. Place all solid A into a clean boiling tube. Using a burette, add 4cm³ of distilled water to solid A in the boiling tube. Heat the mixture while stirring with a thermometer to about 70^o When all the solid has dissolved allow the solution to cool while stirring with the thermometer. Note the temperature (Ts) at which crystals of solid A first appear. Record this temperature in table I
2. Using the burette, add 2cm³ of distilled water to the contents of the boiling tube, warm the mixture while stirring with the thermometer until ALL the solid dissolves. Allow the mixture to cool while stirring. Note and record the temperature at which crystals of solid A first appear.
3. Repeat procedure (b) two more times and record the temperatures in table I.
   Retain the contents of the boiling tube for use in procedure (II)
4. Complete table I by calculating the solubility of solid A at different temperatures.
   NOTE:
   You may hasten cooling for the first two temperatures readings by pouring cold water from the tap on the sides of the boiling tube.
   1. TABLE I (6mks)
      

      Volume of water added (cm3)	4	6	8	10
      Crystallization temperature, Ts (°C)
      Solubility of solid C in g/l00g water

   2. Plot a graph of solubility of solid A with crystallization temperature Ts. [3mks]
   3. Use your graph to determine
      1. the solubility of solid A in water at 55°C. [1 mk]
      2. determine the temperature at which 100g of solid A would dissolve in 100 cm³ of water. [1 mk]

Procedure II
Transfer the content of the boiling tube into 100ml Measuring cylinder. Rinse both the boiling tube and thermometer with distilled water and add to the measuring cylinder and shake thoroughly. Add more water carefully to make up to 100 ml mark. Label this solution A. Fill the burette with solution A (H₂X. 2H₂O). Pipette 25cm³ of solution B into a conical flask. Add 2-3 of Phenolphthalein indicator and titrate with solution A. Record your readings in table II below. Repeat the procedure and complete table II.

Table II (4mks)

1. Calculate the average volume of solution A used.                                               (1 mk)
2. Calculate the ;
   1. Number of moles of sodium hydroxide in 25 cm³ solution B.   ( 1mk)
   2. number of moles dibasic acid solution A used, given the equation for the reaction as;  
      2NaOH +H₂X  →Na₂X + 2H₂O       (1 mark)
   3. number of moles of dibasic acid in 100cm³ of solution A           1 mark
3. Determine the ;
   1. Relative formula mass of dibasic acid, H₂X.2H₂O.         1 mrk
   2. RAM of X in dibasic acid, H₂X.2H₂O.     (H=1,O=16)       1 mark

2. You are provided with solid E containing two cations and one anion. Carry out the tests given and record your observations and deductions in the space provided.
   1. Place half of solid E in a clean dry test-tube and heat gently then strongly. Test any gases produced with both blue and red litmus papers.
      

      Observation	Deductions
      (2 mks)	( 1 mk)

   2. Place the remaining solid E into a boiling tube. Add about10cm³ of distilled water and shake thoroughly. Divide the resultant mixture into 4 portions.
      1. To the first portion add a few drops of sodium hydroxide solution till in excess.
         

         Observation	Deductions
         ( 1 mk)	( 1 mk)

      2. To the second portion, add a few drops of ammonium hydroxide solution till in excess.
         

         Observation	Deductions
         ( 1 mk)	( 1 mk)

      3. To the third portion, add 2-3 drops of dilute hydrochloric acid.
         

         Observation	Deductions
         ( 1 mk)	( 1 mk)

      4. To the third portion, add 2-3 drops of Lead (II) nitrate solution.
         

         Observation	Deductions
         ( 1 mk)	( 1 mk)

      5. To the third portion, add a few drops of Barium chloride solution.
         

         Observation	Deductions
         ( 1 mk)	( 1 mk)

3. You are provided with solid F. Carry out the tests below. Record your observations and inferences in the spaces provided.
   1. Place about half of solid F on a metallic spatula and burnt it using a non-luminous flame
      

      Observations	Inferences
      (1mk)	(1mk)

   2. Place the remaining solid F in a clean boiling tube and add about 5cm³ of water and shake thoroughly.
      1. To about 2cm³ of the solution F, put the universal indicator paper provided to determine its P^H.
         

         Observations	Inferences
         (1mk)	(1mk)

      2. To about 2cm³ of solution F, add three drops of acidified potassium manganate (VII) solution and warm.
         

         Observations	Inferences
         (1mk)	(1mk)

      3. To about 2cm³ of solution F, add solid sodium hydrogen carbonate.
         

         Observations	Inferences
         (1mk)	(1mk)

MARKING SCHEME

1.  
   1. TABLE I (6marks)
      

      Volume of water in boiling tube(cm3)	Temperature at which crystals of solid A first appear (0C)	Solubility of solid A (g/100g of water)
      4	66	112.5
      6	56	75.0
      8	49	56.0
      10	44	45.0

      
      
      1. Complete table.........1mk
         complete table 1 mark otherwise penalise fully
         Penalise ½ mk if all temperature readings given in the table are constant.
         For initial temp, treat temp. below 40^oC and those above 80^o C as unrealistic and penalise ½ mk once.
      2. Use of decimals......... (Tied temperature readings)................. 1mk
         Accept ONLY if all readings are recorded CONSISTENTLY as whole numbers or ONE decimal point of .0 or .5 other wise penalise fully.
      3. Accuracy (AC)........ 1mk
         Compare candidates first temperature reading with the SCHOOL VALUE / CENTRE VALUE. If   within + 2^oC of the S.V /C.V award 1 mk otherwise penalise fully.
         Award 1mk for continuous decrease in the temperature.
      4. solubility calculation............2mks
         Award ½ mk for each value of solubility correctly calculated
   2. Graph ..................                3mks  
      Marks distributed as follows
      1. L.A   ................................................                ½ mk
         Conditions / Penalties
         Penalise fully for inverted axis.
         Penalise fully for wrong units: if no units are given ignore and award fully .
         If only one axis is labelled / units given, condition (ii) above is applied.
      2. Scale.......................................................½ mk
         • Area occupied by the ACTUAL plots MUST be at least ¾ of the graph paper provided.
         • Scale intervals MUST be constant / consistent.
         • The scale chosen must be able to accommodate all the plots / points.
      3. Plotting          .................................................               1mk
         • Award 1mk if 4 points are correctly plotted.
         • Award ½ mk if only 3 points are correctly plotted: otherwise if less than 3points are plotted correctly: award 0 mk.
         • If scale intervals are inconsistent then accept plots if any within the FIRST interval only.
         • Accept plots even if the axis are inverted and award accordingly.
      4. shape/ curve...............................................................1mk
         Award 1mk for descending curve/ line passing through all the plotted points.
         Otherwise penalise fully.
   3.  
      1. Showing on the graph                 ½ mk
         Correct answer (63⁰C+1)                   ½ mk
      2. Showing on the graph                 ½ mk
         Correct answer (72+1g/100g H₂O)                  ½ mk
         
         TABLE II (5 marks )
         

         	I	II	III
         Final burette reading (cm3)	15.0	15.0	15.0
         Initial burette reading (cm3)	0.0	0.0	0.0
         Volume of solution A used (cm3)	15.0	15.0	15.0

         CT lmk
         DP lmk
         AC 1mk
         PA 1mk
         FA lmk
         1. Complete table: penalize to a maximum of   ½ mk for:
            • Inverted table
            • Wrong arithmetic
            • Burette readings beyond 50cm³ except where explained
            • Unrealistic titre values (below 1cm³) and above 50cm³
         2. Use of decimals
            • One decimal or 2 decimal places throughout otherwise penalize fully
            • For use of 2 decimal places, the 2^nd digit after the decimal is either ‘0’ or ‘5’ otherwise penalize fully.
            • Accuracy
         3. Compare any of the teachers’ titre value. If any
            • Within ±0.1 of T.V...lmk
            • Within ±0.2 of T.V. ½ mk
            • Non-within ±0.2 of T.V....Omk
         4. Averaging
            • If 3 Averaged and within ±0.2 of each... lmk
            • If 2 averaged and within ±0.2 of each....1 mk
            • Otherwise penalize fully for averaged values outside ±0.2 of s.v
         5. Final answer
            • Compare teacher’s averaged titre
            • If within±0.1 of T. average titre....lmk
            • If within ±0.2 of T. average titre.. ..½mk
            • If outside of ±0.2 of T. averaged titre 0mk
      3. Average volume= ^{V₁+V₂+V₃}  (transferred to table II as shown above)
                                        3                     
         Calculations
         1. No of moles of NaOH in 25 cm³ of solution = molarity x volume /1,000
            = 0. 3 x ans(a) /1000 ½ mk = ans  ½ mk
         2. No of moles of dibasic acid used
            Mole ratio = 2: 1
            = ½ x ans P ½ mk = Ans P ½ mk
         3.  
            

2. You are provided with solid E containing two cations and one anion. Carry out the tests given and record your observations and deductions in the space provided.
   1. Place half of solid E in a clean dry test-tube and heat gently then strongly. Test any gases produced with both blue and red litmus papers.
      

      Observation	Deductions
      -Blue litmus remains blue -Red litmus turns blue½   mk -Colourless liquid forms on cooler parts½   mk -White residue ½   mk	-NH4+ (tied to Red litmus turns blue)  ½   mk -Hydrated salt (tied to Colourless liquid forms on cooler parts) ½   mk

   2. Place the remaining solid E into a boiling tube. Add 10cm³ of distilled water and shake thoroughly. Divide the resultant mixture into 4 portions.
      1. To the first portion add a few drops of sodium hydroxide solution till in excess.
         

         Observation	Deductions
         White precipitate  ½   mk dissolves in excess½ mk	Zn2+, Al3+, Pb2+ 3 mentioned   1 mk 2 mentioned  ½   mk 1 mentioned 0 mk Penalize fully for any contradictory ion

      2. To the second portion, add a few drops of ammonium hydroxide solution till in excess.
         

         Observation	Deductions
         White precipitate ½   mk insoluble in excess ½   mk	Al3+, Pb2+ 2 mentioned P1 mk 1 mentioned  ½   mk Penalize fully for any contradictory ion

      3. To the third portion, add 2-3 drops of dilute hydrochloric acid.
         

         Observation	Deductions
         No White precipitate 1 mk No effervescence	Pb2+ absent SO 3 2- , CO 3 2- absent

      4. To the third portion, add 2-3 drops of Lead (II) nitrate solution.
         

         Observation	Deductions
         White precipitate  1 mk	SO4 2-   ,Cl- 2 mentioned 1 mk 1 mentioned  ½   mk Penalize fully for any contradictory ion

      5. To the third portion, add a few drops of Barium chloride solution.
         

         Observation	Deductions
         White precipitate  ½ mk	SO4 2-  ½   mk Penalize fully for any contradictory ion

3. You are provided with solid F. Carry out the tests below. Record your observations and inferences in the spaces provided.
   1. Place about half of solid F on a metallic spatula and burnt it using a non-luminous flame
      
   2. Place the remaining solid F in a clean boiling tube and add about 5cm³ of water and shake thoroughly.
      1. To about 2cm³ of the solution F, put the universal indicator paper provided to determine its P^H.
         
      2. To about 2cm³ of solution F, add three drops of acidified potassium manganate (VII) solution and warm.
         
      3. To about 2cm³ of solution F, add solid sodium hydrogen carbonate.