Chemistry Paper 3 Questions and Answers - Form 3 End Term 2 Exams 2022

QUESTIONS
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)

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

   2. To the solution above, add a few drops of ammonia solution then add in excess
      

      Observation	Inference

   3. Scoop the rest of solid A in a clean metallic spatula and burn on a non-luminous flame.
      

      Observation	Inference

2.          
   1. Put ½ of solid B in a test tube and add about 1cm3 of distilled water
      

      Observation	Inference

   2. Put the remaining solid in a test tube and add about 1cm3 of dilute hydrochloric acid.
      

      Observation	Inference

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

   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

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

5. Repeat procedure in (d) above with solid E.
   

   Observation	Inference

6. Divide solution F into 3 portions.
   
   1. Test the PH of the first portion with universal indicator.
      

      Observation	Inference

   2. In the second portion drop a piece of magnesium ribbon and test for any gas produced with a burning splint.
      

      Observation	Inference

   3. To the third portion add a ¼ spatula of sodium hydrogen carbonate.

MARKING SCHEME

Q1

1. Table 3 titrations - 1mk
   2 titrations - 1/2mk
   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 - 1/2mk
4. Principles of averaging
   2 0r 3 consistent values averaged - 1mk
   :- penalize 1/2mk for arithmetic error
5. Final answer ____ 1mk
   Within + 0.1 of SV - 1mk
   + 0.2 of SV ______ 1/2mk
   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 = 2.5moles 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) x ½ = moles of Q
      7. RFM =     5.0     
                    ( 0.2- ^2.5/x) x ½
      8. 2M + 60 =     5.0      
                         (0.2- ^2.5/x)x ½
         M = ½(         5.0        - 60)
                     (0.2- ^2.5/x)x ½

Q2