Why is ortho-nitrophenol less soluble in water than para- and meta-nitrophenols? Which of the given options is the weakest acid: Phenol, Acetic acid, Formic acid, or Benzoic acid?

1. Ortho-nitrophenol is less soluble in water than p- and m-nitrophenols because:

• Ortho-nitrophenol forms intramolecular hydrogen bonds between the hydroxyl group and the nitro group within the same molecule. This internal hydrogen bonding reduces its ability to form hydrogen bonds with water molecules.
• In contrast, para-nitrophenol and meta-nitrophenol cannot form intramolecular hydrogen bonds due to the distance between the groups. They primarily form intermolecular hydrogen bonds with other nitrophenol molecules and with water molecules, leading to greater solubility in water.

Therefore, the correct option is (a).

The final answer is $\boxed{\text{a}}$

2. Which of the following is the weakest acid? To determine the weakest acid, we compare their acid strengths, often indicated by their $\text{pKa}$ values. A higher $\text{pKa}$ value corresponds to a weaker acid.

• a) Phenol ($\text{C}_6\text{H}_5\text{OH}$): $\text{pKa} \approx 10$
• b) Acetic acid ($\text{CH}_3\text{COOH}$): $\text{pKa} \approx 4.76$
• c) Formic acid ($\text{HCOOH}$): $\text{pKa} \approx 3.75$
• d) Benzoic acid ($\text{C}_6\text{H}_5\text{COOH}$): $\text{pKa} \approx 4.2$

Carboxylic acids (b, c, d) are generally much stronger acids than phenols (a) because the carboxylate anion ($\text{R-COO}^-$) is more effectively resonance-stabilized than the phenoxide anion ($\text{C}_6\text{H}_5\text{O}^-$). Among the given options, phenol has the highest $\text{pKa}$ value, making it the weakest acid.

The final answer is $\boxed{\text{a}}$