Answer
The hydronium ion concentration of a solution with pH = 8.37 is equal to: $$4.3 \times 10^{-9} \space M$$
Work Step by Step
Using the equation for $[H_3O^+]$ based on pH:
$$[H_3O^+] = 10^{-pH} = 10^{-8.37} = 4.3 \times 10^{-9} \space M$$