# Arrange the following acid in increasing order of strengths (HCO_3 )^- , (H_3 O )^+ , (HSO_4) ^ - , HSO_3 F?

Jun 18, 2016

$\text{Weakest to strongest: } H C {O}_{3}^{-} , H S {O}_{4}^{-} , H S {O}_{3} F$

#### Explanation:

We cannot consider the hydronium ion, ${H}_{3} {O}^{+}$, as it is an hypothetical entity, whereas the the other species are stuff you could put in a bottle and characterize. We conceive that when an acid is put into water, a strong acid protonates water to form the hydronium ion, ${H}_{3} {O}^{+}$.

I am breaking my own rule, and not looking at metrics, which I should properly do, but here we can use induction.

The strength of an acid relates to how far the following equilibrium lies to the right:

$H A \left(a q\right) + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + {A}^{-}$

For stronger acids, the equilibrium lies further to the right. Again this reaction illustrates my reluctance to consider the acidity of ${H}_{3} {O}^{+}$, the acidium ion in water, as it is the result of acid-base behavior in water.

We have a choice between anions and neutral species, and, reasonably, the anions, bicarbonate, and bisulfate should be weaker acids than the neutral species.

So the strongest acid is fluorosulfonic acid, $H S {O}_{3} F$, then bisulfate, $H S {O}_{4}^{-}$, and then bicarbonate, $H C {O}_{3}^{-}$. I urge you to look up $p {K}_{a}$ values for these acids.