Chemistry Paper 3 Questions and Answers with Confidential - Momaliche Joint Mock Exams 2021/2022

INSTRUCTIONS TO CANDIDATES

• Fill in your details above.
• Write your answers in ENGLISH
• Answer all the questions in the spaces provided in the question paper.
• You are not allowed to start working with the apparatus for the first 15 minutes of the 2 ¼ hours allowed for this paper.
• All working must be clearly shown.
• Mathematical tables and electronic calculators may be used.

Question 1.
You are provided with:

• Hydrochloric acid, solution A
• 0.4M Sodium hydroxide, Solution B
• 0.1g of divalent metal C

You are required to determine;

1. Molar enthalpy change for the reaction between metal C and hydrochloric acid.
2. the Molarity of Acid A.

Procedure I
Using a measuring cylinder, place 100cm³ of acid A in a 250ml plastic beaker. Record its temperature as T1. put metal C into the beaker and stir using the thermometer. Record the highest temperature attained as temperature T2 in table I below.
(Label this solution as F and preserve it for procedure II)
Table I

1. Determine the temperature change, ΔTºC (1/2mk)
2. How many moles of C were used in the experiment (C=24.0) (1mk)
3.      
   1. Calculate the enthalpy change for the reaction. (1mk)
      (s.h.c = 4.2kJkg^-1k^-1, density of solution=1g/cm³
   2. Calculate the molar enthalpy change for the reaction (1mk)

Procedure II
Fill the burette with solution F. Pipette 25cm³ of solution B into a conical flask. Add 3 drops of phenolphthalein indicator. Run the solution in the burette into the conical flask until the pink colour just disappears. Record your readings in the table II below. Repeat the above procedure to obtain concordant results and complete the table.
Table II

(4mks)

1. Determine the average volume of solution F used (1mk)
2. Calculate:
   
   1. The number of moles of solution B used. (1mk)
   2. The number of moles of hydrochloric acid in solution F that reacted with 25cm³ of solution B. (1mk)
   3. The number of moles of hydrochloric acid in 100cm³ of solution F. (1mk)
   4. The initial number of moles of hydrochloric acid in 100cm³ of solution A. (1mk)
   5. The molarity of Hydrochloric acid, solution A. (1mk)

Question 2
You are provided with:

1. Sodium thiosulphate containing 40g/litre, solution D.
2. 2M Hydrochloric acid, solution E. You are required to:
   Determine the rate of reaction between sodium thiosulphate and Hydrochloric acid.

Procedure:
Into a 100ml glass beaker, place 20cm³ of D. Using a pencil, Mark a cross (X) on a white paper. Place a beaker containing solution D on the cross
X. Add 20cm³ of solution E into solution D and at the same time start a stop watch.
Shake the beaker and immediately place it on the cross. Observe the cross (X) through the solution (from the top) and record the time (t) in seconds taken for the cross to be longer visible.

Repeat the procedure using the other solutions of E diluted with water as indicated in the table III below.
Table III (5mk)

1. Plot a graph of 1/time (y-axis) against volume of solution E. (3mks)
2.    
   1. From the graph, determine the time taken for the cross (X) to be invisible at 16.5cm³ of solution E. (1mk)
   2. If the volume of solution E in b (i) above was diluted using 3.5cm³ of water, what would be the concentration of E in the mixture in moles/litre. (1mk)
3. Explain the shape of the graph. (1mk)

Question 3.
Procedure:
You are provided with solid G and H. Carry out the tests and record your observation and inferences in spaces provided.

1. Place all solid G in a clean boiling tube. Add about 10cm³ of distilled water and shake well. Divide the solution into 4 portions.
   
   1. To the first portion add sodium hydroxide dropwise till in excess.
      

      Observations	Inferences

   2. To the second portion add aqueous ammonia dropwise till in excess.
      

      Observations	Inferences

   3. To the third portion add 3 drops of dilute hydrochloric acid, solution E.
      

      Observations	Inferences

   4. To the fourth portion, add 3 drops of Lead (II) nitrate solution followed by dilute nitric (IV) acid.
      

      Observations	Inferences

2.    
   1. Using a clean metallic spatula, ignite about one third of solid H in a Bunsen burner flame.
      

      Observations	Inferences

   2. Put the remaining solid H in a clean boiling tube tube. Add about 8ml distilled water and shake well. Divide the solution into three portions.
      

      Observations	Inferences

      1. Determine the pH of the solution using universal indicator paper.
         

         Observations	Inferences

      2. To the second portion, add 2drops of acidified Potassium Manganate (VII) solution.
         

         Observations	Inferences

      3. To the third portion add sodium hydrogen carbonate solid.
         

         Observations	Inferences

CONFIDENTIAL
The following equipments and apparatus should be provided all candidates.

1. In addition to the fittings and apparatus found in a chemistry laboratory, each candidate will require the following:
   
   • About 120cm³ of solution A
   • About 100cm³ of solution B
   • One pipette, 25ml
   • One burette, 0-50ml
   • At least 2 conical flasks, 250ml
   • A 100ml measuring cylinder
   • A 10ml measuring cylinder
   • Thermometer
   • 250ml plastic beaker
   • 100ml glass beaker
   • About 120cm³ of solution D
   • About 90cm³ of solution E
   • 0.1g of solid C
   • Stop watch
   • About 2g of solid G
   • About 2g of solid H
   • Metallic spatula
   • About 0.5g of sodium hydrogen carbonate solid
   • Six dry test tubes
   • One boiling tube
   • One white plain paper, to mark an X on
2. Accessto:
   
   • Source of heat
   • Distilled water
   • Universal indicator paper and pH chart range 1-14
   • Acidified KMnO₄ solution ,dissolve 3.2g KMnO₄ in 400ml of 2M H₂SO₄ and dilute to 1L
   • Pb (NO₃)₂ solution,
   • 2M sodium hydroxide
   • 2M ammonia solution
   • Phenolphthalein indicator
3. Additionalinformation
   
   • Solution A-0.6M HCl acid
   • Solution B-0.4M NaOH(aq)
   • Solution D- Made by dissolving 40g of sodium thiosulphate per litre of solution.
   • Solution E- 2M HCl(aq) acid
   • Solid C- Magnesium granules or powder
   • Solid G- Aluminium sulphate
   • Solid H- Maleic acid
   • Dilute HNO₃ 2M

MARKING SCHEME

1. Table I
   

   Final temperature (ºC) T2	20.0
   Initial temperature (ºC) T1

   CT √ ½
   DP √ ½ (Accept whole numbers)
   A√ ½ (Teachers initial temperature) ±2
   
   1. ΔT = Final Temperature – initial Temperature
      = 20.0 – 17.0 = 3.0 √ ½
   2. 0.1 = 0.004167 moles computation √ ½ Answer √ ½
      24 
   3.    
      1. m= 100cm³ x 1g/cm³ = 100g
         Q = 100 x 4.2 x Ans(a) √ ½
                         1000
         = Ans kJ √ ½
      2. Ans in c)i)/Ans in b) x 1 √1 NB: Penalise 1mk for wrong units. TABLE II
         

         	I	II	III
         Final burette readings (cm3)	20.4	20.6	21.0
         Initial burette readings (cm3)	0.0	0.0	0.0
         Volume of solution F used (cm3)	20.4	20.6	21.0

         Complete table - 1mk Conditions
         3 readings (at least 2 consistent readings) – 1
         2 readings and all consistent ½
         1 reading – 0
         2 in consistent readings – 0
         Penalties
         
         • Wrong Anthrmetic
         • Inverted table.
         • Un realistic readings.
           NB: For each penalize ½ mk up to a maximum of ½ mk
           Decimal point – 1 mk
         • Accept either 1 or 2 d.p used consistently otherwise penalize fully.
         • If two d.p used the 2nd d.p must be either be ‘o’ or ‘5’
         • Accept inconsistency of 0 i.e 0.0 or 0.00 for initial reading
           Accuracy 1mk
         • Compare any one of students readings with the school titre value
         • If at least 1 reading with ± 0.1 √ 1
         • If within ± 0.2 √ ½
         • If not within ± 0.2 √ 0 Principles of averaging
           1. 20.4 + 20.6 = √
                      2 
              Final Answer 1mk
              Final average accuracy if within ± 0.1 of sch. Average 1 mk, if within ± 0.2 of sch value ½ mk Compare the average value with the teachers average value.
              
              • If within ± 0.1 – 1mk
              • If not within ± 0.1 – 0mk
                Total marks 5 mks
           2.    
              1. the no. of moles of B
                 25 x 0.4 = 0.01 moles √ computation ½ mk
                   1000 √ Ans ½
              2. the no. of moles of acid in F
                 mole ratio = 1:1 √ mole ration ½ mk
                 = 0.01 moles √ ½ mk
              3. moles of acid in 100 cm3 of F
                 100 x 0.01 = 0.04878 moles √ computation ½ mk
                      20.5 √ Ans ½
              4. Initial no. of moles = moles reacted with solid C+ moles reacted with NaOH
              5. Morality of A.
                 1000 x ans iv = √ computation ½ mk
                       100
2. TABLE III 5mk
   

   Experiment	1	2	3	4	5
   Volume of solution D (cm3)	20	20	20	20	20
   Volume of solution E (cm3)	20.0	17.5	15.0	12.5	10.0
   Volume of water (cm3)	0.0	2.5	5.0	7.5	10.0
   Time taken for X to disappear	20	27	34	43	50
   1/time (sec-1)

   √ compete table 1mk
   
   • Reject readings in mins.
   • Filled table and correct computation – 1
     √ Decimal points consistency 1mk
   • for time taken NO dp
     √ Accuracy 1mk
   • Tied to school values 1st reading at 0 cm3 of water ± 5 sec.
     √ Trend 1mk
   • Increase in time continuously.
   • Calculations of ¹/_t
   • Accept ¹/_t to 4th d.p divided fully
   • Reject ¹/_t in fraction.
   • All correctly done 1mk, 4 correctly done ½ mk, otherwise zero
     
     1. GRAPH (See the graph paper)
        √ Plotting 1mk
        
        • 5 correct plots 1mks
          5 plotted, 4 correct plots – ½ mk. 5 plotted, 1-3 wrong plots – 0mk
          √ Scale ½ mk
          √ Labelling ½
          √ Straight line (Line of best fit) 1mk
     2.    
        1. ¹/_T = 3.75 x 10-2 sec
           = t = 26.67secs Accept ± -2
           Showing on graph ½ mk, correct answer ½ mk
        2. C₁ V₁ = C₂ V₂
           2x 16.5 = C₂ x 20
           C₂ = 2 x 16.5√ = 1.65 M√ computation √ ½
                       20 Ans √ ½
           Graph: Appropriateness of scale ½ mk: Labeling both axes ½ mk: Plotting ,all 5 points correctil 1mk, 4 pionts ½ mk, otherwise zero. Alowed deviation 1mm vertical and horizontal
     3. The graph is a straight line. This indicates that the rate of reaction is directly proportional to the concentration of the acid solution E √ (1mk)
        OR (words to the relationship of diluting, decrease in the time, increase in reciprocal)
        NB: Any values given here as readings are only examples, learners work is marked as per his/her reading.
3. 
   

   Observations	Inferences
   a) .i) White precipitate √ ½ soluble in excess √½ ii) White precipitate √ ½ insoluble in excess √½ iii) No white precipitate √ ½ iv) White precipitate, √ ½ insoluble in dilute nitric acid. b)i) solid melts. ½ √ burns with yellow smoky/sooty/luminous flame √ ½ II i) PH = 1 or 2 √ ½ ONLY ONE VALUE ii) Purple KMnO4 decolourises √ iii) Effervescence /hissing sound. √ ½	Al 3+, Pb2+ , Zn 2+ √ ( 3ions–1mk, 2ions- ½mk, 1ion–Omk Penalize full for contradictory ion) Al 3+, Pb2+ √ (2ions -1mk, 1ion -½ mk) Al 3+ confirmed √ Or Pb2+ absent. Reject if not mentioned in a(i) and (ii)above.  SO42- , Cl- √ Twomentioned–1mk One mentioned – ½ mk  C = C , C = C- 2 group – 1 √ 1 group – ½  R – COOH/H+ √ ½ C = C , C = C √ 2 group – 1 1 group – ½ Acidic substance/R- COOH/H+ √ ½