Solve the Problem $\mathrm { pH }$ Of a Solution Ranges from 0 to 14

Solve the problem.
-The $\mathrm { pH }$ of a solution ranges from 0 to 14 . An acid has a pH less than 7 . Pure water is neutral and has a pH of 7 . The $\mathrm { pH }$ of a solution is given by $\mathrm { pH } = - \log \mathrm { x }$ where $\mathrm { x }$ represents the concentration of the hydrogen ions in the solution in moles per liter. Find the hydrogen ion concentration if the $\mathrm { pH } = 8.4$ .

A) $3.98 \times 10 ^ { - 9 }$
B) $3.98 \times 10^{ - 8}$
C) $2.51 \times 10 ^ { - 8 }$
D) $2.51 \times 10 ^ { - 9 }$