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