Asolution 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.