If [OH−] = 1.0×10^−3 M, what are the pH and pOH of the solution?

• A. pH 3.00; pOH 11.00
• B. pH 11.00; pOH 3.00
• C. pH 7.00; pOH 7.00
• D. pH 12.00; pOH 2.00

Answer: (B)

When you know the concentration of hydroxide, you can find pOH from pOH = −log[OH−]. For [OH−] = 1.0 × 10^−3 M, pOH = −log(1.0 × 10^−3) = 3.0. The pH and pOH are connected by pH + pOH = 14.00 at 25°C, so pH = 14.00 − 3.00 = 11.00. This shows the solution is basic (pH > 7). Therefore, pH is 11.00 and pOH is 3.00. The other possibilities would require different pH/pOH values that don’t match the given [OH−] and the 14-sum relationship.