Weak acid- Strong base titration:
Both NaOH (Strong base) and CH3COOH (Weak acid) are colorless. NaOH was placed in a burette to titrate 25.00 cm³ of 0.1 moldmֿ³ CH3COOH that is poured into a conical flask. A pH electrode was used to measure the pH. During titration there was no color change, neither change in temperature nor smell.
Table2: The volume of NaOH added from the burette to CH3COOH in the flask and pH of the resulting solution.
Data Processing and presentation:
Figure1: effect of adding strong alkali to strong acid on the pH of the resulting solution.
Since the acid is strong, the initial pH before adding alkali was low and equaled 1.2. Due to the addition of alkali the [H3O+] decreased slowly.
OHֿ¹(aq) + H3O+1 (aq) ↔ 2H2O (l)
The pH started to increase slowly till it reached 3. After that point a small addition of strong alkali caused the pH of the titrated solution to jump from 3 to 10. On the graph, this is shown by the vertical region. The jump in pH occurred as a result of the low [H3O+] in this region. A small amount of the strong alkali increased the [OHֿ¹] rapidly thus increasing the pH suddenly.
The addition on alkali continued. Thus, the pH continued to increase slowly. The final pH was close to 12 because it is a strong alkali.
The equivalence point reached when [H3O+] = [OHֿ¹] had a pH of 7. Since both strong alkali and strong acid used were monoprotic with equal concentrations, the volume of the strong alkali needed to reach equivalence point was almost equal to the volume of the strong acid.
Figure2: effect of adding strong alkali to weak acid on the pH of the resulting solution.
The weak acid has a pH of 2.4 and this is the initial pH. Weak acid dissociates partially and has a higher pH than strong acid. As the alkali was added slowly, the [H3O+] decreased.
OHֿ¹(aq) + H3O+1 (aq) ↔ 2H2O (l)
The pH increased slowly till it reached 6. After that a small addition of the strong alkali caused the pH to jump to 11. This is the vertical region on the graph. It is more in the alkaline region than the acidic region. The rapid change is caused by the low [H3O+]. Addition on the strong alkali increased [OHֿ¹] and thus the pH rapidly. Because the addition of the strong alkali continued, the pH continued to increase slowly. The final pH was almost 12 because it is a strong alkali.
The equivalence point is almost 8. It is not equal to 7 because the conjugate base of the weak acid has a tendency to accept a proton from water and OHֿ¹ is produced. The solution turns basic.
Aֿ¹ + H2O ↔ HA + OHֿ
Because both the acid and base are monoprotic with equal concentrations the volume of the string base used was almost equal to the volume of the weak acid.
