Chemistry Paper 3 Questions and Answers - Momaliche Joint Pre Mock Exams 2022

QUESTIONS

1. You are provided with
2. Solid A: 0.31 g of a carbonate (MCO₃).
3. Solution B: l.1M hydrochloric acid
4. Solution C: 0.2M sodium hydroxide.
5. Phenolphthalein indicator

You are required to:

1. Determine the molar mass of the carbonate
2. Determine the relative formula mass and hence formula of the carbonate.

PROCEDURE

• Measure 50 cm³ of solution B using a measuring cylinder. Transfer the entire solid A provided into a 250cm³ volumetric flask. Transfer 50 cm³ solutions B into 250cm³ volumetric flask containing solid A and swirl the contents until the entire Solid dissolves and no more effervescence occurs. Add more distilled water up to the 250cm³ mark and label this solution D.
• Pipette 25.0cm³ of solution D and transfer to a conical flask. Add two drops of phenophthalein indicator and titrate with solution Cfrom the burette. Record your results in table I below.
• Repeat the titration to get two more concordant values.

TABLE 1

(4 mks)

1. Calculate;
   1. The average volume of solution C used. (1mk)
   2. The moles of solution C in the volume in (i) above. (1mk)
   3. The moles of D that reacted with C. (1mk)
   4. The moles of hydrochloric acid in 250 cm³ of solution D (1mk)
   5. The moles of hydrochloric acid in 25cm³ of B. (1mk)
   6. Calculate the moles of HCl which reacted with the carbonate (1mk.. (1mk)
   7. Calculate the moles of the carbonate that reacted with the acid (1mk)
   8. Determine the relative formula mass of the carbonate and the value of M. (1mks)

2. You are provided with 2.5g of a hydrous sodium carbonate labeled F. You are required to determine the enthalpy of solution of solid F.
   Procedure
   Using 50m1 measuring cylinder place 25cm³ of water into l00ml plastic beaker. Stir the water gently with a thermometer and take its temp after every half-minute. Record the reading in the table below. At exactly 2 minutes add all solid F to the water at once. Stir well with the thermometer as you take the temperature ofto the mixture after every half- minute upto the 4thminute.
   Table III
   

   Time (min)	0	½	1	1½	2	2½	3	3½	4
   Temp ºC					X

   1. On the grid provided plot a graph of temp against time. (3mks
   2. From the graph determine the change in temp (∆T) ( 1mk)
   3.    
      1. Calculate the number of moles of solid F used in the experiment (Na= 23,C=12, 0=16) (1mk)
      2. Calculate the molar enthalpy of solution. (Density of solution 1gcm^-3, specific heat capacity of solution is 4.2Jg^-1K^-1 (2mks)
3. You are provided with solid D. Carry out the following tests and write down all the observations and Inferences.    
   1.     
      1. Place a half spatulafull of solid in a dry test tube and heat gently then strongly.Test any gas produced using red and blue litmus papers.
         

         Observations	Inferences

      2. Place the remaining solid D in a boiling tube and add about 10cm³ of water shake
         Vigorously then divide the mixture into four portions.
         

         Observations	Inferences

      3. To the 1st portion add four drops of barium chloride solution
         

         Observations	Inferences

      4. To the 2nd portion add sodium hydroxide solution drop wise till in excess.
         

         Observations	Inferences

      5. To the third portion add aqueous ammonia solution drop wise till excess about 1cm³
         

         Observations	Inferences

      6. To the fourth portion add 3 drops of sodium chloride
         

         Observations	Inferences

   2. You are provided with substance E. Carry out tests on it.
      1. Place about one thirdof solid E on a metallic spatulaand ignite it in a flame.
         

         Observations	Inferences

      2. Place the remaining solid E boiling tube add about 5 cm³ of distilled water. Shake the contents and divide into 3 portions.
         1. To portion one add 3 drops of Universal indicator
            

            Observations	Inferences

         2. To second portion all sodium carbonate provided
            

            Observations	Inferences

         3. To third portion add 2 drops of acidified potassium manganate (VII) solution. Warm the mixture
            

            Observations	Inferences

CONFIDENTIAL
Each candidate will require

1. Solution B about 60 cm³ of 1.1 M hydrochloric acid solution.
2. Solution C about 100 cm³ of 0.2M sodium hydroxide solution.
3. One 250 ml volumetric flask.
4. One 100 ml measuring cylinder
5. 50 ml Burette
6. 25ml pipette.
7. 2 conical flasks
8. Retort stand
9. Filter funnel
10. 2.5g of solid F
11. 500ml of distilled water
12. 100ml plastic beaker
13. Thermometer
14. 1 spatula-ful of solid D
15. ½ spatula of solid E
16. Six test tubes in a rack
17. Red litmus paper
18. Blue litmus paper
19. Metallic spatula
20. 1 lable

ACCESS TO

• 1M NaOH
• 1M NH₄OH
• Barium chloride soln. (BaCl₂)
• 0.5M NaCl
• Universal indicator soln.(pH 4 to 11)
• Sodium carbonate solid
• KmnO₄.
• Phenolphthalein indicator
• Source of heating

NOTES

1. Solid A — Exactly 0.31 of Zinc carbonate.
2. Solid D-AluminiumSulphate
3. Solid E-Maleic acid
4. Solid F-anhydrous sodium carbonate
5. BaCl₂-dissolve 4g of solid in 1dm³ of solution.

MARKING SCHEME

1. Titration table ………(5mks)
   Complete table CT ……………. (1mk)
   Conditions
   1. No arithmetic error/subtraction
   2. No table inversion
   3. No values beyond 50cm³ unless explained
   4. No unrealistic values (titre) values above 100cm³ or below 1cm³
      • Decimal D ………..(1mk)
      • All values in the table MUST be written to 1d.p. throughout or 2 d.p throughout with 2nd d.p. value being 0 or 5.
      • Accuracy (AC) ……………..(1mk)
      • Anyone of the candidate’s titre value is compared to the teacher’s (s.v.) within
        ±0.1………………..(1mk)
        Or± 0.2 ………………….( ½ mk)
        Beyond ± 0.2……………………. (0mk)
      • Principles of averaging (PA) …(1mk)
      • Values averaged MUST be shown and MUST be within ±0.2 of each other. (1mk)
      • Final averaged value FA ………(1mk)
      • If correct averaging is done, the answer obtained is credited as the accuracy mark on S.V
        ±0.1………………..(1mk)
        Or ± 0.2 ………………….( ½ mk)
        Beyond ± 0.2……………………. (0mk)

1.      
   1. Calculate:
   2. Ans. in (i) above x 0.2√½ = ans. a (ii) √½ mk
                    1000
   3. Ratio of moles of D:C = 1:1 √½
      Same as ans. as (ii) above √½
   4. 250 x ans. a (iii)√½= ans. a(iv) √½
               25
   5. 50 x 1.1[ans. a (iv)] √½= ans a(v) √½
        1000

Question 2
Table III

Notes on marking table
CT = Complete table ….1mk
Table to be filled completely
If at time 0min a student records temperature as 0 or any reading is more than 40 penalize half mark.
D.P = decimal point ……. ½mk
All values to be recorded as whole numbers OR with a decimal as 0 or 0.5 only if any other figure is used award 0mk for d.p
Accuracy.Consider reading at time 0min if 2ºC of school value award ½mk.\if beyond award 0mk
Trend 1 mk
For time t= 0 to 1½ values to be constant. ½ mk
For time t = 2 ½ to 4 ½mk
Values to be constant and higher than those of t = 0 to t = 1 ½ .the last value can drop by 1ºC
Sub total for table III is 3mks

Graph

1.           
   1. Labeled axes …. ½mk
      Axes to be labeled with quantities and unit i.e Temp in ºC and time in min.
   2. Scale . ½mk
      Plot to cover ²/₃ of the given grid.
   3. Plots ……1mk
      To be plotted accurately
   4. Plots for t =0 to 1 ½ to bejoined by straight line …. ½mk
      t = 2½ to 4 to be joined by straight line. The two lines should not be joined together …. ½mk
2. ΔT = 27 – 23 = 4ºC ½ mk
3.        
   1. anhydrous sodium Carbonate
      Formula is Na2CO₃
      RFM Na2CO₃ = 23 x² + 12 + 16 x 3 = 106
      Molesof Na2CO₃= 2.5 = 0.0236mol. ½mk
      106 ½mk
      Ans. to 3s.f or 4s.f not 1 or 2s.f
   2. Heat change = mass x specific heat cap. X temp change
      = 25cm³ x 1gcm^-3 x 4.2J^g-1K^-1 x 4k
      = 420J (1mk)
4. Molar heat change = 0.420  ½ mk
                                    0.0236
   ΔH = -17.797KJmol-1 ½ mk
   If –ve sign is missing award 0 for final answer and units.
   Total marks for Q2 = 10mks

3. (15mks)
   
   1. 
      

      Observations	Inferences
      (a) Colourless liquid forms on sides of test tube ½ – Moist blue litmusturns red. Moist red litmus remained red ½mk -Colourless gas (b) Dissolves colourless solution (c) Whit ppt. forms ½mk (d) Whit ppt. ½mk soluble in excess ½mk (e) White ppt.½mk insoluble in excess ½mk (f) No white ppt. forms ½mk	Water of crystallization or hydrated. ½mk Acidic gas ½mk (2mks) -SO32-, SO42-, SO32- Soluble substance polar compound (1mk) SO42-, CO32- or SO32-(½mk each) Al3+, Zn2+ or Pb2+ ½ mk each Al3+, ½ mk or Pb2+½ mk Al3+, present or Pb2+ absent ½mk

   2. 
      

      Observations	Inferences
      (a) (i) Burns with non smoky bright flame (1mk) (ii) Soln. pH = 4 ½mk (b) Bubbles evolved. Effervescence ½mk (c) KMnO4 decolourized after sometime not immediately	Saturated hydrocarbon 1mk C = C absent (2mks) present Weak acid or –COOH group ½mk H+ or –COOH present ( ½mk) Weak reducing agent (1mk)