Question

What is the `"pH"` for `2.5 xx 10^(-6) "M"` `"HCl"`?

Answer 1

`pH=5.60`

Explanation:

HCl is a strong acid that fully ionizes when dissolved in water according to the following equation:

`HCl + H_2O -> H_3O^+ + Cl^-`

Having 1:1 ratio `=> [H_3O^+] = 2.5xx10^-6\ M`

`pH = -log [H_3O^+]`
`pH = -log(2.5xx10^-6)`
`pH = 5.60`

Note that the contribution of the autoionization of `H_2O` to `[H_3O^+]` is small (in the range of `10^-7 M`) that is why it is neglected. If it was not, it may contribute about one-tenth or so to the `"pH"`.