If the pH of a solution is 3.437, what is the hydrogen ion concentration in mol/L?

By definition, $p H = - {\log}_{10} \left[{H}_{3} {O}^{+}\right]$............
........so $\left[{H}_{3} {O}^{+}\right] = {10}^{- 3.437} \cdot m o l \cdot {L}^{-} 1$
$= 3.656 \times {10}^{-} 4 \cdot m o l \cdot {L}^{-} 1$