Based on acetic acid $\left(K_{\mathrm{a}}^{\circ }=1.8\times {10}^{-5}\right)$ solution, 

The pH of 0.05 M acetic acid solution is

$\mathrm{pH}=-\log \left\{\left[{\mathrm{H}}^{+}\right]/\mathrm{M}\right\}=-\log \left(9.5\times {10}^{-4}\right)\simeq 3.02$

The pH of 0.05 M acetic acid solution is 3.02

The modern definition of pH is that it is a number that, as the reciprocal of the hydrogen ion concentration or activity logarithm, indicates how acidic or alkaline a solution is.