# An acidic solution has a pH of 4.02. What is the H_3O^+ concentration of this solution in M?

${H}_{3} {O}^{+} = 9.55 \times {10}^{-} 6 \cdot m o l \cdot {L}^{-} 1$
By definition, $p H = - {\log}_{10} \left[{H}_{3} {O}^{+}\right]$
And thus if $p H = 4.02$, $\left[{H}_{3} {O}^{+}\right] = {10}^{- 4.02}$ $=$ $9.55 \times {10}^{-} 6 \cdot m o l \cdot {L}^{-} 1$.