Question

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

For instance, is the entity of `HCl` a formula unit, if we assume that `H` acts as a metal?

Answer 1

Well, certainly we can add `HCl(g)` 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_3O^+`:

`HCl(g) + H_2O(l) rarr H_3O^(+) + Cl^(-)`

In water, hydrolysis of the `H-Cl` molecules occurs to give hydronium ions and halide ions. We represent the hydronium ion as `H_3O^+` or even as `H^+`. As far as anyone knows, this hydronium ion is a cluster of `"2-3 water molecules"` with an extra proton associated with the cluster, i.e. to give `H_7O_3^+` or `H_5O_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 `"acidium ion"` as `H^+` or `H_3O^+`. This is a convenient fiction that works..........and we can speak of and calculate moles of `H_3O^+`.

Capisce?