Answer
- pH is the negative logarithm of the hydronium concentration:
$pH = -log(H_3O^+)$
- The pH is used as a concentration measure because it is easier to represent, if compared to the concentration in $mol/L$ (M).
Work Step by Step
The pH is, most of the time, represented by a number between 0 and 14, and the concentration of $H_3O^+$ in mol/L is represented by a number between $1$ and $10^{-14}$.
So, for example, a solution with a $[H_3O^+] \approx 5.0 \times 10^{-4}M$, can be represented by the pH scale with $pH \approx 3.3 $, which is, clearly, easier to write and represent.
