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

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