Chemistry Paper 3 Questions and Answers with Confidential - Form 3 Mid Term 2 Exams 2021

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INSTRUCTIONS

  • Answer all the questions in the spaces provided
  • Spend the first 15 minutes of the 2¼ hours to read the question paper and ensure you have all the chemicals and apparatus that you may need.
  • All working must be clearly shown where necessary.
  • Mathematical tables and silent calculators may be used.
  • Answer all the questions in English.
    For Examiners use only.

Q1

You are provided with

  • Metal carbonate M2CO3 solid Q which weighs exactly 5.0g
  • 2M hydrochloric acid solution P
  • Sodium hydroxide solution R made by dissolving 40g of the solid in a litre of solution.

You are required to determine

  • The relative formula mass (RFM) of M2CO3 and hence the relative atomic mass (RAM) of M.

Procedure

  • Measure accurately using a measuring cylinder 100cm3 of solution P into a clean 250cm3 conical flask.
  • Add all the 5.0g of solid Q,shake well and wait for effervescence to stop.
  • Put the solution into a 250ml clean beaker.
  • Label the resulting solution S.
  • Pipette 25cm3 of solution R into a clean conical flask and add 2-3 drops of phenolphthalein indicator
  • Fill the burette with solution S and titrate against solution R.
  • Record the results in the table below
  • Repeat the titration two more times and record in the table. (4mks)
       I   II   III 
     Final burette reading       
     Initial burette reading       
     Volume of S used(cm3)      

    1. Calculate the average volume of solution S used (1mk)
    2. Calculate
      1. Moles of sodium hydroxide solution R used (2mks)
      2. Moles of hydrochloric acid solution S in the average volume used (2mks)
      3. Moles of HCl solution S in 100cm the of solution. (2mks)
      4. moles of hydrochloric acid solution P in the 100cm2 of the original solution (2mks)
      5. Moles of HCl solution P that reacted with solid Q (2mks)
      6. Moles of M2CO3 solid Q that reacted (2mks)
      7. The RFM of M2CO3 (2mks)
      8. The RAM of metal M (1mk)
        (Na=23 O=16, H=1)

Q2

Carry out the tests on the substances given and record your observations and inferences in the spaces provided.

  1.  
    1. Put half of solid A in a boiling tube, add about 3cm3 of distilled water shake and retain the solution
        Observation     Inference 
                  ½mk           ½mk 
    2. To the solution above, add a few drops of ammonia solution then add in excess
        Observation     Inference 
                  1mk           1mk 
    3. Scoop the rest of solid A in a clean metallic spatula and burn on a non-luminous flame.
        Observation     Inference 
                  ½mk           ½mk 
  2.  
    1. Put ½ of solid B in a test tube and add about 1cm3 of distilled water
        Observation     Inference 
                  ½mk           ½mk 
    2. Put the remaining solid in a test tube and add about 1cm3 of dilute hydrochloric acid.
        Observation     Inference 
                  ½mk           ½mk 
  3.  
    1. Put ½ of solid C in a test tube,heat gently then strongly observing the colour changes in the solid when heating and after cooling. Test any gas produced with wet litmus papers and a glowing splint.
        Observation     Inference 
                  3mk           1½mk 
    2. Put the remaining solid C in a test tube and add about 2cm3 of distilled water,shake well. Add ammonia solution drop wise then in excess
        Observation     Inference 
                  1mk           1mk 
  4. Put solid D in a test tube and add 2cm3 of distilled water. Shake well then add ammonia solution drop wise then in excess.
      Observation     Inference 
                1mk           1mk 
  5. Repeat procedure in (d) above with solid E.
      Observation     Inference 
                1mk           1mk 
  6. Divide solution F into 3 portions.
    1. Test the PH of the first portion with universal indicator.
        Observation     Inference 
                  ½mk           ½mk 
    2. in the second portion drop a piece of magnesium ribbon and test for any gas produced with a burning splint.
        Observation     Inference 
                  1mk           ½mk 
    3. To the third portion add a ¼ spatula of sodium hydrogen carbonate.
        Observation     Inference 
                  ½mk           ½mk 

CONFIDENTIAL

In addition to the fittings and chemicals found in a chemistry laboratory, each student will require the following.

  1. 120cm3 of solution P-2M HCL
  2. Accurately weighed 5.0g of solid Q-Na2CO3
  3. About 80cm3 of solution R-40g/litre NaoH.
  4. 100ml measuring cylinder
  5. 10ml measuring cylinder
  6. 3 conical flasks
  7. 250ml beaker
  8. 1 label
  9. 25.0ml pipette
  10. burette
  11. tile,funnel,stand,clamp and boss
  12. distilled water
  13. About 8 test tubes
  14. Metallic spatula – clean
  15. 2 boiling tubes
  16. PH chart
  17. 2cm length magnesium ribbon(cleaned)
  18. 2 wooden splints
  19. ½ spatula of sodium hydrogen carbonate
  20. one red and one blue litmus papers
  21. Test-tube holder
  22. ½ spatula of each of the solids below
    • A – Na2SO4
    • B – Na2CO3
    • C – Zn(NO3)2
    • D – Pb(NO3)2
    • E – CuSO4.5H2O
  23. Solution F-10cm3 1M HCl

Access to

  1. Means of heating
  2. Phenolphthalein indicator
  3. 2M ammonia solution
  4. 1M HCl
  5. Universal indicator


MARKING SCHEME

Q1

  1. Table 3 titrations - 1mk
            2 titrations - ½mk
            1 titration - 0mk
  2. Decimals 1dp or 2dp used consistently - 1mk
    (if 2dp,the 2nd should be a 0 or 5)
  3. Accuracy -Compare with any of SV within ± 0.1 - 1mk
                                                                 ±0.2 - ½mk
  4. Principles of averaging
    2 or 3 consistent values averaged - 1mk
    penalize ½mk for arithmetic error
  5. Final answer ____ 1mk
    Within ±0.1 of SV - 1mk
    ±0.2 of SV ______ ½mk
  1. See 4 and 5 above (≈ 23.5cm3)
  2.  
    1. 40g/40 = 1M
      No of moles = 1 x 25 = 0.025 moles of NaOH
                             1000
    2. NaOH + HCl →NaCl + H2O
      Mole ratio
      NaOH : HCl
      1 : 1
      0.025 : ? = 0.025 moles of HCl
    3. 0.025 moles = X (volm from (a) above)
             ?           = 100cm3
      0.025 x 100 = 2.5 moles of HCl
             X              X
    4. 2 moles _____ 1000cm3
           ?      ____ 100cm3
      = 100 x 2 = 0.2 moles of P
           1000
    5. (0.2 − 2.5/X) moles of HCl
    6. M2CO3 + 2HCl → 2MCl + CO2 + H2O
      Mole ratio
      M2CO3 : HCl
      1 : 2
       ? __ 0.2 – 2.5/x
      = (0.2 – 2.5/X) × ½ = moles of Q
    7. RFM =           5.0        
                  (0.2- 2.5/X)×½
    8. 2M + 60 =         5.0        
                       (0.2- 2.5/X)×½
      M = ½(        5.0        −60)
                 (0.2- 2.5/X)×½

Q2  

  Observation     Inference 
(a)(i)Dissolves to form a Colourless solution             presence of a soluble salt 
  Award absence of coloured ions            
 (ii)No white ppt formed   Absence of Pb2+, Al3+,Zn2+
 (iii) Burns with a yellow flame  Presence of Na+
(b)(i)Dissolves to form a Colourless solution           As (a)(i) above
(ii) Effervescence is produced    Presence of a CO3-2

(c)(i) -solid changes from white to
          Yellow and back to white on cooling 
-Brown gas is produced
-Turns wet blue litmus paper red and red remains red

-Relights a glowing splint 

-Presence of Zn2+ 
-Presence of NO3
-Presence of an acidic gas


-Presence of O2 ½ each for any 3     

(ii) - Dissolves to form a colourless solution 
-White ppt formed, soluble in excess.

-Presence of a soluble salt
-Presence of Zn2+
(d) White ppt formed insoluble in excess  Presence of Pb2+ ,Al3+
(e) Blue ppt formed,soluble in excess to form a deep blue solution          Presence of Cu2+

(f)(i) PH 1 or 2 

Presence of a strong acid
(ii) Produces effervescence and a gas that burns with a pop sound  -Presence of an acidic solution
-Allow presence of H+
(iii) Produces effervescence and a colourless gas         -Presence of an acidic solution
-Allow presence of H+

 


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