Final answer:
The composition difference between a weak base and its conjugate acid is the addition or removal of one H+ ion. For a weak base to form its conjugate acid, a hydrogen ion is added; conversely, removing an H+ from the conjugate acid yields the base.
Step-by-step explanation:
The difference in composition between a weak base and its conjugate acid is one H+. A base and its conjugate acid are linked by the gain or loss of a single hydrogen ion. For example, when water (H₂O) acts as a base, it can accept an H+ ion to become hydroxonium ion (H₃O+), which is its conjugate acid. Conversely, a hydroxide ion (OH⁻), which is a common weak base, would join with one H+ to become water, thus water is the conjugate acid of OH⁻. This relationship is part of the broader concept of conjugate acid-base pairs in which the base forms a conjugate acid by acquiring an H+ ion.
In the context of acid-base chemistry, a weak base like NH₃ (ammonia) will accept an H+ to form NH₄+ (ammonium), its conjugate acid. Similarly, a substance such as the acetate ion can also serve as a base by accepting an H+ ion to form acetic acid. The transformation from a base to its conjugate acid or vice versa always involves the transfer of a single H+ ion.