Chemistry Paper 3 Questions and Answers - Lugari Constituency Joint Pre Mock Exams 2023

1.  
   1. You are provided with:
      • Solution A containing an oxidizing agent A;
      • Solution B, 0.1 M aqueous sodium thiosulphate;
      • Solution C containing a reducing agent C;
      • Aqueous potassium iodide solution D;
      • Solution E, starch solution.
        You are required to determine theRate of reaction between the oxidizing agent A and the reducingagent C.
        Procedure I
        
        Step 1
        Using a 10 ml measuring cylinder, place 2 cm³of solution B into a 100 ml beaker. Add inside 2 cm³ of solution C followed by 2 cm³ of solution D. Add 5 drops of solution E and shake the mixture.
        
        Step 2
        Using a burette, place 12 cm³ of solution A into a test tube. Pour the measured 12 cm³ of solution A into the beaker in Step 1 above and immediately start a stopwatch/clock. Swirl the mixture and place on a white paper. Observe the mixture and immediately stop the stopwatch/clock when a blue colour appears.
        Record the time taken in table 1.
        
        Step 3
        Repeat steps 1 and 2 using the volumes of solutions given in table 1 for experiments 2, 3, 4, 5 and 6 and record in table 1.
        Complete table 1 by calculating the rate of reaction using the expression
                                               Rate =    1   x 1000 S^-1
                                                            time   
        Table 1
        
        

        Test-tube number	1	2	3	4	5	6
        Volume of distilled water (cm3)	0	2	3	5	6	7
        Volume of solution A (cm3)	12	10	8	6	4	2
        Time (seconds)
        Rate = 1   x 1000 S-1              time

                                                                                                                                                                     (8 mark) 
                                                  
        1. Plot a graph of rate (y-axis) against volume of solution A. (3 marks)
           
        2. Using the graph, determine the time that would be taken for the blue colour to appear if the volume of A used was 7cm³. (1 mark)
   2. You are provided with:
      • Solution J, 0.1M hydrochloric acid
      • 1g of Calcium hydroxide labelled solid K.
        You are required to determine the solubility of calcium hydroxide, SOLID K at room temperature.
        
        Procedure II
        
        Transfer all solid K into a 250ml beaker. Using a 100ml measuring cylinder, add inside 100cm³ of distilled water and stir gently for about 2 minutes. Allow the mixture to stand undisturbed for about 3 minutes. Decant the solution obtained into a clean 100ml beaker and label it as solution M.
        
        Place solution J into the burette. Pipette 25.0 cm³ of solution M into a 250 cm³ conical flask and titrate using methyl orange indicator. Record your results in table II below. Repeat the titration two more times and complete table II below.
        
        Table II
        
        

        	1	2	3
        Final burette reading (cm3)
        Initial burette reading (cm3)
        Volume of J used (cm3)

                                                                                                                                                               (4marks)
        Calculate the:
        1. Average volume of solution J used. (1mark)
        2. Number of moles of the solution J reacted. ( 1mark)
        3. Number of moles of solution K in the 25cm³ (1mark) Number of moles of solution K in the 100 cm³ of the solution. (1mark)
        4. Solubility of calcium hydroxide at room temperature. (1mark)
2. You are provided with solid Y. Carry out the tests below. Write your observations and inferences in the spaces provided.
   1. Using a clean metallic spatula, ignite about one third of solid H in a Bunsen burner flame
      

      OBSERVATION	INFERENCE
      1 mark	1 mark

   2. Add about 10cm³ of distilled water to the remaining solid Y and shake the mixture thoroughly. Divide the solution into three portions.
      1. To the first portion, add two drops of acidified potassium manganite (VII) solution.
         

         OBSERVATION	INFERENCE
         1 mark	1 mark

      2. To second portion, add two drops of bromine water
         

         OBSERVATION	INFERENCE
         1 mark	1 mark

      3. Determine the PH of the third portion using universal indicator paper
         

         OBSERVATION	INFERENCE
         1 mark	1 mark

3. You are provided with solid Z.Carry out the tests below and record your observations and references in the space provided.
   1. Place about one third of solid Z in a dry test-tube.Heat the solid gently and then strongly. Test any gases produced with blue litmus paper.
      

      OBSERVATION	INFERENCE
      1 mark	1 mark

   2. Dissolve the remaining portion of solid Z in 8cm³ of distilled water. Divide the solution into four portions.
      1. To the first portion add aqueous sodium hydroxide dropwise until in excess.
         

         OBSERVATION	INFERENCE
         1 mark	1 mark

      2. To second portion, add aqueous ammonia drop wise until in excess.
         

         OBSERVATION	INFERENCE
         1 mark	1 mark

      3. To the third portion, add 1cm³ of barium chloride solution.
         

         OBSERVATION	INFERENCE
         1 mark	1 mark

      4. To the fourth portion, add 3 drops of acidified potassium manganite (VII).
         

         OBSERVATION	INFERENCE
         1 mark	1 mark

      5. Identify the cation and anion present in solid Z.
         Cation....(½ mark)
         Anion...... (½ mark)

                                                                                       MARKING SCHEME 

1.  
   1. Table 1
      

      Test-tube number	1	2	3	4	5	6
      Volume of distilled water (cm3)	0	2	3	5	6	7
      Volume of solution A (cm3)	12	10	8	6	4	2
      Time (seconds)
      Rate = 1   x 1000 S-1              time

                  C.T -5
                  D-1
                  A-1
                  T-1
                                                                                                                                                                   (8 mark) 
      Marking points
      1. Complete table - (5 mks)
         • 6 experiments done, time correctly written and rate correctly calculated – 5 mks.
         • Penalise ½ mk for any unfilled space upto a max of 5 mks.
         • Accept rate given to at least 1 decimal place, otherwise penalise fully for rate given as whole numbers unless it works out exactly.
         • Penalise ½ mk once for time readings above 300 seconds.
      2. Decimal - (1 mk)
         • Accept time readings given as whole numbers or to two decimal places used consistently.
      3. Accuracy - (1 mk)
         • Compare the candidate’s time reading for experiment 1 with the school value . If within +/ - 5 seconds award (1mark), otherwise award 0.
      4. Trend – (1mk)
         • Time MUST increase continuously with increase in amount of water from left to right. Penalize fully for any contradictory trend.
           
           
           GRAPH 
         • Labelling of axes correctly with or without units- ½ mark
         • Scale- consistent intervals and plots occupying more than ½ of the grid provided- ½ mark Penalise fully for inconsistent intervals and plot area that is less than the one defined above
         • Plots
           • place a tick on every correct plot 
           • 5 -6 correct plots-1 mark
           • 3-4 correct plots – ½ mark
           • Only 1 or 2 correct plots -0
         • Shape of graph 
           • a straight line graph with positive gradient through the origin. The line MUST pass through at least two correct plots ( drawn using a ruler) – 1 mk
           • Award 0 for the line if it does not pass through the origin, is drawn using free hand sketch or is not passing through correct plots.
           • Show marking points on the graph as below:
             Lab- ½
             S- ½
             P- 1
             Lines- 1
             Total = 3
   2. MARKING POINTS
      Complete table-(1 mark) 
      • complete table with all three titrations done-(1 mark)
      • incomplete table with two titrations done- ½ mark
      • incomplete table with only one titration done -0
        Penalties
      • Penalise ½ mark once for:
      • inverted table
      • wrong arithmetic
      • titre values below 1cm³ and above 50cm³
        Use of decimals -(1 mark) 
      • accept one decimal place used consistently for (1 mark) 
      • Award 1 mark for two dp used consistently where the the 2^nd dp is a zero or a 5, otherwise penalise fully (award zero) for inconsistence in the number of decimal places.
        Accuracy -1 mark 
      • compare the teachers’s value with the candidate’s tire values.
      • If within +/- 0.1 award 1 mark
      • If outside +/- 0.1 but within +/- 0.2 award ½ mark, otherwise award zero
      • Put a tick on the candidate’s titre value that meets the criteria and award accordingly.
        Principles of averaging- (1mark) 
      • If all the three titrations are done, are consistent and correctly averaged, award (1 mark)
      • If only two titrations are done, are consistent and averaged – (1mark)
      • If three titrations are done but only two are consistent and are averaged-(1mark)
      • If three titrations are done, are inconsistent and averaged- 0
      • If three titrations are done, all values are consistent but only two are averaged -0
        Final Accuracy 
      • compare the candidate’s average titre with the teacher’s value.
        If within +/- 0.1 award 1 mark
        If outside +/- 0.1 but within +/- 0.2 award ½ mark, otherwise award zero
        Penalties 
      • Penalise fully for wrong arithmetic in calculating average volume.
        
        CALCULATIONS
        1. Average volume of solution J used. (1mark)
             T1+T2+T3 = correct answer
                    3                     
           • Award for correct answer even if units are missing.
           • Accept rounding off to 2 dp or whole number incase it works out exactly.
        2. Number of moles of the solution J reacted. ( 1mark)
                 av.titre x 0.1  = correct answer
                          1000               
        3. Number of moles of solution K in the 25cm³ (1mark)
                answer in (b) above = correct answer
                             2
        4. Number of moles of solution K in the 100 cm³ of the solution. (1mark)
                answer in c above x 100 = correct answer
                             25
        5. Solubility of calcium hydroxide at room temperature. (Ca = 40, O = 16, H =1) (2marks)
           • Correct Calculation of Molar mass of Calcium hydroxide = correct answer 1
           • Solubility = correct answer 1 x answer in (d) above = Correct answer 2 (g/100g of water)
           • Penalise ½ mk if the answer is greater than 1
             Common Penalties for parts (b) to (e) 
           • Correct answer MUST be given to at least 4dp.
           • Accept answer given to less than 4dp if it works out exactly.
           • Penalise ½ mk for use of wrong units but Penalise fully for use of wrong expression.
2. You are provided with solid Y. Carry out the tests below. Write your observations and inferences in the spaces provided.
   1. Using a clean metallic spatula, ignite about one third of solid H in a Bunsen burner flame
      
   2. Add about 10cm³ of distilled water to the remaining solid Y and shake the mixture thoroughly. Divide the solution into three portions.
      1. To the first portion, add two drops of acidified potassium manganite (VII) solution.
          INFERENCE
      2. To second portion, add two drops of bromine water
          
      3. Determine the PH of the third portion using universal indicator paper
         
         

         OBSERVATION	INFERENCE
         pH 5.0✔1	Weaky acidic.✔1

3. You are provided with solid Z.Carry out the tests below and record your observations and references in the space provided.
   1. Place about one third of solid Z in a dry test-tube. Heat the solid gently and then strongly. Test any gases produced with blue litmus paper.
      -acidic gas produced✔½​-acidic gas produced✔½​

      OBSERVATION	INFERENCE
      -Moist blue litmus paper changes to /turns red.   ✔½  -colourless liquid forms on cooler parts of the   test tube. ✔½	-acidic gas produced✔½  -solid contains water of crystalisation/   hydrated salt. ✔½  (inferences will only score if the corresponding observations are correctly written)

   2.  Dissolve the remaining portion of solid Z in 8cm³ of distilled water .Divide the solution into four portions
      1. To the first portion add aqueous sodium hydroxide drop wise until in excess.
         

         OBSERVATION	INFERENCE
         -White precipitate✔½, soluble✔½ in excess	Zn2+, Al3+, Pb2+ (✔1 for all the 3 ions, ✔½ for 2 ions and 0 for only 1). Penalise ½ mk for any contradictory ion to a maximum of 1 mk

      2. To second portion, add aqueous ammonia drop wise until in excess.
         

         OBSERVATION	INFERENCE
         -White precipitate ✔½ soluble ✔½ in excess	-Zn2+✔1 (penalise fully for any contradiction)

      3. To the third portion, add 3 drops of barium chloride solution.
         

         OBSERVATION	INFERENCE
         -White precipitate ✔ 1	SO42- , SO32- (✔1 for the 2 ions correctly inferred. ✔½ for only 1 ion correctly inferred). Penalise fully for any contradictory ions mentioned. Treat CO32- as a contradictory ion

      4. To the fourth portion, add 3 drops of acidified potassium manganate (VII).
         

         OBSERVATION	INFERENCE
         Purple colour of acidified potassium manganate (VII) is retained/remains✔1	SO42-✔1