# Question #bde65

Jun 18, 2015

Because when you add sulfuric acid to water, the protonation of water molecules takes place.

#### Explanation:

Hydrogen sulfate, or bisulfate ion, as you'll sometimes see it called, cannot act as a conjugate base in solution because sulfuric acid, ${H}_{2} S {O}_{4}$, is a strong acid that dissociates completely.

In other words, in the first dissociation of sulfuric acid, the equilibrium that forms hydronium ions, ${H}_{3} {O}^{+}$ and hydrogen sulfate lies so far to the right that, for all intended purposes, you can consider that no sulfuric acid molecules remain undissociated.

${H}_{2} S {O}_{4 \left(a q\right)} + {H}_{2} {O}_{\left(l\right)} r i g h t \le f t h a r p \infty n s {H}_{3} {O}_{\left(a q\right)}^{+} + H S {O}_{4 \left(a q\right)}^{-}$, ${K}_{a 1} = 2.4 \cdot {10}^{6}$

Notice the very large value of the acid dissociation constant, ${K}_{a}$. In theory, $H S {O}_{4}^{-}$, being the conjugate base of sulfuric acid, can pick up a proton to reform sulfuric acid. However, for practical purposes, the dissociation of sulfuric acid can be seen as going to completetion because of the very large ${K}_{a}$.

${H}_{2} S {O}_{4 \left(a q\right)} + {H}_{2} {O}_{\left(l\right)} \to {H}_{3} {O}_{\left(a q\right)}^{+} + H S {O}_{4 \left(a q\right)}^{-}$

So, when you place sulfuric acid in water (never do the other way around!), all the sulfuric acid molecules will lose their proton, which is then picked up by water molecules $\to$ hydronium is formed.

Once this happens, you can consider that the solution no longer contains ${H}_{2} S {O}_{4}$ molecules. Hydrogen sulfate goes on to act as a weak acid, donating its proton to form bisulfate ions, $S {O}_{4}^{2 -}$.

However, hydrogen sulfate is a weak acid and thus cannot dissociate completely.

$H S {O}_{4 \left(a q\right)}^{-} + {H}_{2} {O}_{\left(l\right)} r i g h t \le f t h a r p \infty n s {H}_{3} {O}_{\left(a q\right)}^{+} + S {O}_{4 \left(a q\right)}^{2 -}$, ${K}_{a 2} = 1.0 \cdot {10}^{- 2}$

This means that your solution will contain both hydrogen sulfate and bisulfate ions.