# What is a conjugate base and a conjugate acid?

Dec 16, 2015

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 ${\text{NH}}_{4}^{+}$ the conjugate acid or base of ${\text{NH}}_{3}$? What does that make ${\text{NH}}_{3}$?

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

Jan 6, 2016

The conjugate base is the initial acid LESS a proton ($- {H}^{+}$). The conjugate acid is the initial base PLUS a proton ($+ {H}^{+}$). YOU MUST CONSERVE MASS AND CHARGE (what do I mean here?).

#### Explanation:

The conjugate base of water, ${H}_{2} O$, is $O {H}^{-}$, of $H C l$, $C {l}^{-}$, of ${H}_{2} S {O}_{4}$, $H S {O}_{4}^{-}$, of $H S {O}_{4}^{-}$, $S {O}_{4}^{2 -}$. 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 ${H}_{2} O$, is ${H}_{3} {O}^{+}$, of $H S {O}_{4}^{-}$, ${H}_{2} S {O}_{4}$, of $N {O}_{3}^{-}$, $H N {O}_{3}$.

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, $N {H}_{3}$, and $H {O}^{-}$? These conjugate bases don't exist in water, but they do exist in liquid ammonia.