Here are the solutions to the questions: ii) Phenolphthalein is a weak acid indicator. In acidic solutions, the equilibrium $\text{HIn}(aq) \rightleftharpoons \text{H}^+(aq) + \text{In}^-(aq)$ shifts to the left, and the indicator exists predominantly in its undissociated, colourless form ($\text{HIn}$). As a base is added during the titration, $\text{H}^+$ ions are neutralized, causing the pH to increase. At the end point, when the solution becomes slightly alkaline (pH 8.2-10), the equilibrium shifts to the right. The $\text{HIn}$ dissociates to form $\text{In}^-$, which is the red/pink coloured form. This shift causes the solution to change from colourless to red/pink. iii) Phenolphthalein is suitable for titrations where the equivalence point occurs in the pH range of 8.2 - 10. This is characteristic of a titration between a weak acid and a strong alkali. Acid: Ethanoic acid* ($\text{CH}_3\text{COOH}$) Alkali: Sodium hydroxide* ($\text{NaOH}$) iv) To calculate the pH of the solution: Step 1: Calculate the pOH from the given $\text{OH}^-$ ion concentration. $$[\text{OH}^-] = 0.0025 \text{ M}$$ $$\text{pOH} = -\log_{10}[\text{OH}^-]$$ $$\text{pOH} = -\log_{10}(0.0025)$$ $$\text{pOH} \approx 2.60$$ Step 2: Calculate the pH using the relationship $\text{pH} + \text{pOH} = 14$. $$\text{pH} = 14 - \text{pOH}$$ $$\text{pH} = 14 - 2.60$$ $$\text{pH} = \boxed{11.40}$$