Chemistry Paper 3 WASSCE (PRIVATE CANDIDATE), 2019

Question 1

In a titration experiment, 21.60 cm3 of H2SO4 (aq) was required to completely react with 25.0 cm3 of NaOH (aq) containing 2.00 g of the alkali in 500 cm3 of solution.

  1. Write a balanced equation for the reaction.
  2. Calculate the concentration of:
    1. NaOH (aq) in moldm-3;
    2. H2SO4 (aq) in moldm-3;
    3. H2SO4 (aq) in gdm-3.
  3. State three precautions that should be taken during the titration. [H = 1.0, O = 16.0, Na = 23.0, S = 32.0]
[17 MARKS]

Observation

In part (a), majority of the candidates could not write a balanced equation for the reaction.
In part (b), majority of the candidates could not calculate the concentration of H2SO4 in gdm-3 correctly.
In part (c), majority of the candidates stated three precautions that should be taken during the titration.
The expected answers include:

(a) 2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l)
    (b)	(i)	ρ = m/v

	    = 2.0/0.5
	    = 4.0 gdm-3

	Molar mass of NaOH = 23 + 16 + 1
                  = 40 gmol-1

                  C = ρ/M
		     = 4.0
		       40.0
		   = 0.100 moldm-3
        
ALTERNATIVE METHOD Chem (C).
  • Rinse burette with the solution of acid.
  • Rinse pipette with the solution of base.
  • Expel air bubbles from burette and pipette.
  • Avoid splashes of solution on the walls of conical flask.
  • Remove the funnel before titration
  • Do not blow out the remaining drop of solution from the tip of pipette.
  • Read the burette and pipette at eye level / avoid parallax error.
  • 2 or 3 drops of the indicator should be used.
  • Titration mixture should be swirled gently to avoid splashing solution.
  • Pipette the base to the mark.
  • Avoid spillage of solution from pipette when transferring the base.
  • Clamp the burette in a vertical position.
  • Avoid leakage of burette.
  • Place the conical flask on a white surface (tile or filter paper).