# What is the entity of an acid? (A formula unit?)

## For instance, is the entity of $H C l$ a formula unit, if we assume that $H$ acts as a metal?

Feb 17, 2017

Well, certainly we can add $H C l \left(g\right)$ to an aqueous solution......

#### Explanation:

In water, we conceive that an acid is a species that increases the concentration of the characteristic cation of the solvent, the hydronium ion, ${H}_{3} {O}^{+}$:

$H C l \left(g\right) + {H}_{2} O \left(l\right) \rightarrow {H}_{3} {O}^{+} + C {l}^{-}$

In water, hydrolysis of the $H - C l$ molecules occurs to give hydronium ions and halide ions. We represent the hydronium ion as ${H}_{3} {O}^{+}$ or even as ${H}^{+}$. As far as anyone knows, this hydronium ion is a cluster of $\text{2-3 water molecules}$ with an extra proton associated with the cluster, i.e. to give ${H}_{7} {O}_{3}^{+}$ or ${H}_{5} {O}_{2}^{+}$. The extra proton is highly mobile, and can tunnel rapidly across the solution. If you have ever played rugby, think of a maul where the forwards cluster together and pass the pill from hand to hand. Of course I am comparing a macro scenario to quantum tunnelling.

Now in solution, we can represent the $\text{acidium ion}$ as ${H}^{+}$ or ${H}_{3} {O}^{+}$. This is a convenient fiction that works..........and we can speak of and calculate moles of ${H}_{3} {O}^{+}$.

Capisce?