The pH of solution X  is 8.5. The concentration of hydrogen ions in solution X is- 

pH is a number that represents whether a solution is acidic or not.

It is represented as-

$$\begin{gathered} \mathrm{pH} = -\log \left[{\mathrm{H}}^{+}\right] \\ \mathrm{or} \\ \mathrm{pH} = - \log \left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right] \end{gathered}$$

Now, pH = 8.5

Thus, hydrogen ion concentration is calculated as-

$8.5 = -\log \left[{\mathrm{H}}^{+}\right]$

$\left[{\mathrm{H}}^{+}\right] = {10}^{-8.5}$

Hence, The concentration of hydrogen ions in solution X is ${10}^{-8.5}$.