What is a conjugate base and a conjugate acid?

It's just a relationship between a base and whatever happens to it if you add a proton, or an acid and whatever happens to it if you remove a proton.

Generically, it can written as:

`stackrel("acid")(overbrace("HA")) rightleftharpoons "H"^(+) + stackrel("conjugate base")(overbrace("A"^(-)))`

`stackrel("base")(overbrace("B")) + "H"^(+) rightleftharpoons stackrel("conjugate acid")(overbrace("BH"^(+)))`

So, what do you think? Is `"NH"_4^(+)` the conjugate acid or base of `"NH"_3`? What does that make `"NH"_3`?

Can water be both? i.e. Is water amphoteric? Why or why not?

The conjugate base of water, `H_2O`, is `OH^-`, of `HCl`, `Cl^-`, of `H_2SO_4`, `HSO_4^-`, of `HSO_4^-`, `SO_4^(2-)`. For each example I have simply removed `H^+` from the starting formula and conserved mass and charge in each case.

Likewise the conjugate acid of `H_2O`, is `H_3O^+`, of `HSO_4^-`, `H_2SO_4`, of `NO_3^-`, `HNO_3`.

In each case, I have simply removed OR added a proton (`H^+`) to give the conjugate base or conjugate acid.

What are the conjugate bases of ammonia, `NH_3`, and `HO^-`? These conjugate bases don't exist in water, but they do exist in liquid ammonia.