Which one of the following is a weak acid?

A. HCl (Hydrochloric acid)
B. CH?COOH (Acetic acid)
C. H?SO? (Sulfuric acid)
D. NaOH (Sodium hydroxide)

Answer - B. CH?COOH (Acetic acid)

Explanation: 

A weak acid is an acid that does not completely break into ions when dissolved in water. Only a small number of its molecules release hydrogen ions (H?), while many molecules remain unchanged in the solution.

Let’s look at each option:

• HCl (Hydrochloric acid): It is a strong acid because it almost fully separates into H? and Cl? ions in water.
• CH?COOH (Acetic acid): This acid partially ionizes in water, meaning only some molecules release hydrogen ions. Because of this limited ionization, it is considered a weak acid.
• H?SO? (Sulfuric acid): Sulfuric acid is a strong acid and dissociates almost completely in water.
• NaOH (Sodium hydroxide): This is not an acid. It is a strong base.

Therefore, the correct option is Acetic acid (CH?COOH) because it ionizes only partially in water and behaves as a weak acid.