A solution with a pH of 9 has a [OH-] concentration of A. 1.0 * 10-14 mol/L. B. 1.0 * 10-9 mol/L. C. 1.0 * 10-5 mol/L. D. 1.0 * 10-7 mol/L.

PH is scale which measures the acidity or alkalinity of a solution. It ranges from a level of 1 to 14. The pH level is expressed as the negative logarithm of the concentration of hydronium ions. To determine the concentration of the hydroxide ions, we use the relation that pH + pOH = 14 where pOH is equal to the negative logarithm of the concentration of the hydroxide ions. We do as follows:

pH + pOH = 14

pOH = 14 - pH

pOH = 14 - 9

pOH = 5

5 = - log [OH-]

[OH-] = 10^-5 M

Therefore, the correct answer would be option C. The concentration of the hydroxide ions in a solution with pH 9 would be 1x10^-5 M.