What are the concentrations of hydroxide and hydronium ions in a solution with a pH of 8.3? 2.9 × 10–5 M H3O+ and 3.4 × 10–10 M OH–
3.1 × 10–6 M H3O+ and 3.2 × 10–9 M OH–
5.0 × 10–9 M H3O+ and 2.0 × 10–6 M OH–
6.3 × 10–11 M H3O+ and 1.6 × 10–4 M OH–

5.0 x 10-9 M H3O+ and 2.0 x 10-6 M OH-
Explanation:
To find H3O+, do the following:
pH = -log[H3O+]
-pH = log[H3O+]
antilog -(8.3) = antilog log (antilog and log cancel out) [H3O+]
5.0 x 10^-9 = [H3O+]
To find OH, you need to know pOH
pOH = 14 - pH
pOH = 14 - 8.3
pOH = 5.7
To find OH-, use a similar formula to the one above:
pOH = -log [OH-]
-pOH = log [OH-]
antilog - (5.7) = antilog log (antilog and log cancel out) [OH-]
2.0 x 10^-6 = [OH-]