Ammonia is considered as a base because Nitrogen atom in it contains lone pair of electrons hence it can donate electrons to other atoms therefore it can be considered as base. Due to the presence of a lone pair on NH3. It abstracts a proton or H+ ion released by an acid.
Is ammonia a strong or weak?
Ammonia is a typical weak base. Ammonia itself obviously doesn't contain hydroxide ions, but it reacts with water to produce ammonium ions and hydroxide ions. However, the reaction is reversible, and at any one time about 99% of the ammonia is still present as ammonia molecules.
Is ammonia a strong or weak acid?
Both ammonia is a weak base and ammonium ion is a weak acid. Many, even most, acid/base conjugate pairs are like that. We should be using comparative instead of absolute adjectives in the rule about conjugate acid-base strengths: A weaker acid has a stronger conjugate base, not necessarily a totally strong one.