Chemistry Paper 3 Questions and Answers with Confidential - Sunrise Pre Mock Exams 2023

INSTRUCTIONS TO CANDIDATES

• Answer all the questions in the spaces provided.
• You are not allowed to start working with the apparatus for the first 15 minutes of the 2 ¼ hours allowed for this paper.
• This time is to enable you to read the question paper and make sure you have all apparatus and chemicals that you may need.
• All working must be clearly shown, mathematical and electronic calculators may be used.

1. You are provided with:
   • Solid A 2.0g of dibasic acid, H₂X.
   • Solution B, 0.5M solution of dibasic acid, H₂X.
   • Solution C, Sodium hydroxide solution
   • Solution D, 0.02M acidified Potassium Manganate (VII) solution.
     You are required to determine:
     1. The heat of reaction of solid A, H₂X with sodium hydroxide.
     2. The number of moles of solution E that reacts with 2 moles of acidified potassium manganate (VII) solution.
        
        Procedure 1 (a)
        Place 40cm³ of distilled water into 100ml beaker. Measure the initial temperature of water and record in the table 1 below. Add all the solid A provided at once. Stir the mixture carefully with the thermometer until ALL the solid dissolves. Measure the final temperature and record in table 1.
        
        TABLE 1
        

        Final temperature (°C)
        Initial temperature(°C)

        (1 mk)
        1. Determine the change in temperature ΔT. (1mk)
        2. Calculate the
           1. Heat change when H₂X dissolves in water. (Assume the heat capacity of solution is 4.2J/g/ºC and density of the solution is 1 g/cm³ ).            (1mk)
           2. The molar heat of solution, ΔH 1 solution of the acid H₂X. (Molar mass of acid H₂X is 126g)               (2 mks)
              
              Procedure 1 (b)
              Place 40cm³ of solution B into 100ml beaker. Measure the initial temperature and record in table II below. Measure 40cm 3 of sodium hydroxide solution C. Add all the 40cm³ of solution at once to solution. Stir the mixture carefully with thermometer. Measure the final temperature reached and record in table II. (Keep remaining solution n B to use in procedure II)
              
              Table II
              

              Final temperature (°C)
              Initial temperature(°C)

              (1 mk)
              1. Determine the change in temperature, ΔT. (1 mk)
              2. Calculate the
                 1. Heat change for the reaction. (Assuming the heat capacity of the solution is 4.2J/g/°C and density of the solution is 1g/cm³ ) (1mk)
                 2. Heat for the reaction of one mole of the acid H₂X with sodium hydroxide, ΔH₂ . (2 mks)
              3. Given that H₂X(s) + 2OH⁻(aq) → 2H₂O(l) + X^2- (aq) Determine ΔH₃ using an energy circle diagram. (2 mks)
                 
                 Procedure II
                 
                 Measure exactly 15cm³ of solution B and put in 250ml volumetric flask. Add water as you shake up to the mark. Label as solution E. Using a pipette, pipette 25cm³ of solution E and place in a conical flask. Warm solution E to boiling. Fill the burette with solution D and titrate with hot solution E. Stop just when a permanent change in color appears. Record your results in table III below. Repeat the procedure to complete table III below.
                 

                 	I	II	III
                 Final burette reading (cm3 )
                 Initial burette reading (cm3 )
                 Volume of solution D used

                 (4 mks)
                 1. Calculate the average volume of solution D used. (1mk)
                 2. Calculate the number moles of solution D reacting. (1mk)
                 3. Calculate the number of moles of solution E used. (1mk)
                 4. Calculate the number of moles of E which react with 2 moles potassium Manganate (VII). (2 mks)
2.  
   1. You are provided with solution F in a conical flask. Carry out the following test and record your observations and inferences in the spaces provided.
      1. Add 20cm³ of 2M sodium hydroxide solution to solution F in the flask; shake well. filter the mixture in a clean boiling tube. Retain the filtrate and the residue.
         

         Observation	Inferences
         (1mk)	(1mk)

      2. Place about 2cm³ of the filtrate in a test tube. Add 2M nitric (v) acid drop wise until in excess. Retain the mixture.

         Observation	Inferences
         (1mk)	(1mk)

      3. Divide the mixture in (ii) above into two portions. To one portion add 2M sodium hydroxide solution drop wise until in excess.
         

         Observation	Inferences
         (1mk)	(1mk)

      4. To portion two, add 2M ammonia solution drop wise until in excess.

         Observation	Inferences
         (1mk)	(1mk)

      5. Place about 2cm³ of filtrate in a test tube. Add 3 drops of barium chloride solution.
         

         Observation	Inferences
         (½mk)	(½mk)

      6. To the residue add about 5cm³ of 2M nitric (v) acid a and filter into a test tube. Place about 2cm³ of this filtrate in a test tube. Add 2M ammonia solution drop wise until in excess.
         

         Observation	Inferences
         (1mk)	(1mk)

   2. You are provided with solid G. Carry out the tests below and record your observations and inferences in the spaces provided.
      1. Using a metallic spatula heat half spatula end full of solid G in a non-luminous flame. Remove it when it ignites.

         Observation	Inferences
         (1mk)	(1mk)

      2. Put the remaining solid G in a boiling tube. Add about 5cm³ of distilled water and shake vigorously. (Keep the content for the next test)

         Observation	Inferences
         (1mk)	(1mk)

      3. Divide the resulting solution into two portions. To the first portion add two drops of acidified potassium Manganate (VII) solution and shake vigorously.

         Observation	Inferences
         (1mk)	(1mk)

      4. Test pH of the second portion using pH indicator paper.

         Observation	Inferences
         (1mk)	(1mk)

CONFIDENTIAL

1. each candidate should have the following
   1. Solid A Measures exactly 2.0g of oxalic acid
   2. Solution B 60cm³ of 0.5m oxalic acid
   3. Solution C -50cm³ of 0.25M sodium hydroxide solution
   4. Solution D -100cm³ of 0.02M acidified KMnO₄ solution
   5. Burrette-50ml
   6. 25 ml pipette
   7. Pipette filler
   8. 250cm³ volumetric flask
   9. 50ml measuring cylinders
   10. Thermometer (−10°C−110°C)
   11. 2-conical flask
   12. 100ml plastic beaker
   13. Accessible to about 500cm³ of distilled water
   14. Means of labeling
   15. Solid G – 0.5g of oxalic acid
   16. 10cm³ of solution F
   17. A boiling tube
   18. Test tube holder
   19. The test tubes
   20. 10cm³ measuring cylinder
   21. Filter paper and filter funnel
   22. Metallic spatula
   23. Boiling tube
2. Accessible to the following
   1. Source of heat.
   2. 2M sodium hydroxide solution
   3. 2M ammonia solution.
   4. 2M Nitric (v) acid
   5. 0.5M Barium chloride solution
   6. Universal indicator paper and a chart
      N/B
      • All the bench solutions supplied with a dropper
      • Solution F is a mixture of Al₂(SO₄)₃ and Cu(No₃)₂ in the ratio of 1.1
      • 0.5M oxalic acid is prepared by dissolving 63g in distilled water and making it to one litre
      • 0.25M NaOH is prepared by dissolving 10g of sodium hydroxide in water and making it to one litre
      • 0.02M acidified KMnO₄ Dissolve 3.2g of KMnO₄ crystals in about 500cm³ of water then acidify with about 400cm³ of 1M H₂SO₄ and make up to a litre
      • 2M NH₄OH Measuring 112cm³ of Ammonium solution make it up to a litre
      • 0.5M Barium Chloride,Dissolve 104g in a little water then make up to 1 litre
      • 2M HN0₃ Dissolve 125cm³ of conc.HN0₃ acid in a little water and then make it a litre

MARKING SCHEME

Question 1

Procedure 1(a) – Table 1

1. Complete table ½ mark
   • Final temperature must be lower than the initial temperature, otherwise penalize fully
   • For initial temperature values>40⁰ or < 10⁰ are treated as unrealistic values.
2. Accuracy
   Compare school value (initially value) with students value, and if within ±2⁰C award
   ( ½ mk) otherwise zero.
   1. ∆T =Final temperature – initial temperature.✓1
   2.  
      1. ∆H = MC∆T
                 =40 x 4.2 x ∆T✓1
         Conditions
         • Accept an error of ± 2 units in the 3^rd  digit  if  answer is in joules or 3^rd  d.f if in kilojoules, other wise penalize( ½ mk)
         • Award 1 mark for correct substitution and ignore the formula.
         • Penalize ( 1/2mark) for wrong units shown.
      2. 2g.    ans in c(i) above
         126g     ✓1
         Ansc(i) × 126 ✓1
                  2
         = C.A J/mol.✓1
         Conditions
         • Wrong units given or omitted in final answer, penalize ( ½ mark)
         • Accept arithmetic error ± 2 units
         • Correct sign (+ve) must be shown for ∆H,
           
           

Procedures II(b)- Table
As table 1 but the final temperature must be higher than the initial temperature
Calculation

1. ∆H =Final temperature - Initial temperature ✓1
2.  
   1. ∆H =80 x 4.2 x ∆T
      =C.A.J✓1
      Conditions
      
      • Accept an arror of ±2 in the 4^th digit
   2. Moles reacting = 0.5 × 40 = 0.02 moles ✓1
                                    1000
      ∆H₂ = Ans b(i) × 1 =C.A.J ✓1
                        0.02
3. ∆H₃ =∆H₁ - ∆H₂ ✓1
   =C.A.J✓1
   
   Conditions
   • Negative value must be shown on correct answer, otherwise penalize ( ½ mk)
   • Penalize ( ½ mark) for wrong answer .
   • For correct substitution without formular.
     You will credit ( 1 ½ mk as step II
     NB Capital J and small k MUST  be used

Procedure II Table III

Marks must awarded as follows

1.  
   1. Complete table  – award ½ mark.
   2. Incomplete table with two titrations done – award ½ mark
   3. With one titration- award 0 mark
      Penalties
      • Wrong arithmetic
      • Inverted  table
      • Burette reading beyond 50.0cm³
        NB Penalize each maximum of ½ mark.
2. Decimal place award 1 mark
   • Only 1 d.p used consistently, otherwise penalize fully i.e award 0mark
3. Accuracy award 1 mark
   • Compare the candidate’s titre values with the school value. It must be within + 0.1 school                         value.
4. Principle of averaging –(1mk)
   1. 3 Consistent titres values done, average award a mark.
   2. 3 titre values are attained but only 2 are possible and are averaged- award (1mk)                           but if 3 titres are possible but only 2 are averaged- award 0 mark.
5. Final answer – 1mark.
   Compare the candidate’s correct average titre value with the school value.

2. Calculations
    = 0.2 × averagetitre ✓½ mk
               1000
   = C.A ½ mk)
   Penalties
   • wrong transfer of average titre penalize ½ mark
   • An arithmetic error which is beyond ±2 units is omitted penalize ( ½ mk)
3. Moles of B in 15cm³ ⇒ 15 × 0.5 ✓¹/₂  
                                            1000
                                     =7.5 x 10^-3mol
                         250cm³ 7.5 x 10^-3 mol
                          25cm³              ?
                          = 7.5 × 10¯³ × 25  ✓½ 
                                         250
4. Ans(c)  ✓2
   Ans(d)