What are some examples of the properties of bases?

• They might want protons (Bronsted-Lowry definition)
• They might want to donate electrons (Lewis definition)
• They might donate #"OH"^(-)# to solution (Arrhenius definition)
• The conjugate base of a weak acid is a strong base
• The conjugate base of a strong acid is a weak base

A nice example of something that has most of the following properties is #"HSO"_4^(-)#.

This base wants a proton according to the Bronsted-Lowry definition, and it will get that proton by donating electrons according to the Lewis definition, using the lone pairs on the #"O"^(-)#.

It is the conjugate base of #"H"_2"SO"_4#, a strong acid; thus, it is a weak base.

(Since the #"pKa"# of #"H"_2"SO"_4# is about #1000#, it is reasonable to say that most #"H"_2"SO"_4# is actually deprotonated.)

Furthermore, although it might be difficult, it can be deprotonated to get #"SO"_4^(2-)# (the #"pKa"# of #"HSO"_4^(-)# is about #2#). That makes it also a (relatively strong weak) acid by the Bronsted-Lowry definition, because it can donate a proton, AND it makes it a (relatively strong weak) acid by the Lewis definition, because it accepts electrons in order to donate that proton.

(This makes the conjugate base, #"SO"_4^(2-)#, a strong base.)

It is not, however, a base by the Arrhenius definition, but an acid. It donates protons (#"H"^(+)#) to solution upon dissociation, while donating #"OH"^(-)# would require disrupting its resonance structures, which is unfavorable.