pH value is a numeric scale used to indicate the acidity or basicity of an aqueous solution. The calculation of pH value is pH = -log[H3O+]. [H3O+] is the concentration of hydrogen ion (or hydronium ion) in the solution, in the unit of mole/L (or M). Given that the concentration of hydronium ion is [H3O+]=3.16x10-9M, therefore pH of this solution is pH = -log(3.16x10-9) = -(-8.5) = 8.5. pH value is larger than 7, so this solution is basic.
