[jwxie]

According to my textbook
"As in the case of acetic acid, most of the NH3 molecules remain non-ionized at equilibrium and ammonia is therefore a weak base."

Why do we even mention NH3?

I know we usually called those OH- bases. So there is another type of base? (base on NH3-)

I don't get that statement.

[MrTeo]

I know we usually called those OH- bases. So there is another type of base? (base on NH3-)

There are more definitions of acid/base and each one of them covers a different area of substances. When you talk about dissociation with the production of OH^- you think at the Arrhenius definition of base, which is sometimes correct, but fails to explain a wide number of phenomena, such as the basic behaviour of NH₃ in our case That's why there's a better definition (which also includes Arrhenius acids and bases), created by Brönsted and Lowry (if I remember well they worked independently and got the same results more or less at the same time): an acid is a chemical species which can lose a proton (H⁺) and a base can gain (or "accept") it. Considering the ammonia behaviour in water we can easily see that it's a base according to this theory (and in fact a NH₃ solution is slightly basic):

NH₃+H₂O ⇋ NH₄⁺+OH^-

And as your book says "most of the NH3 molecules remain non-ionized at equilibrium and ammonia is therefore a weak base", which simply means that the equilibrium I've written is shifted towards left

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=bronsted-lowry-theory