If pH equals 4, what is pOH?

If pH equals 4, what is pOH?

• A. 7
• B. 14
• C. 10
• D. 4

Answer: (C)

In water at room temperature, pH and pOH are linked by pH + pOH = 14. This comes from the ion product of water, Kw = [H+][OH−] ≈ 1×10−14, and the definitions pH = −log[H+] and pOH = −log[OH−]. If pH is 4, then pOH = 14 − 4 = 10. That means [H+] = 1×10−4 M and [OH−] = 1×10−10 M, which is consistent with a strongly acidic solution. The other numbers would not match this relationship: 7 would be neutral, 14 would imply a much larger [OH−], and 4 would imply pH equals pOH, which doesn’t occur for this pH in water at standard conditions.

In water at room temperature, pH and pOH are linked by pH + pOH = 14. This comes from the ion product of water, Kw = [H+][OH−] ≈ 1×10−14, and the definitions pH = −log[H+] and pOH = −log[OH−]. If pH is 4, then pOH = 14 − 4 = 10. That means [H+] = 1×10−4 M and [OH−] = 1×10−10 M, which is consistent with a strongly acidic solution. The other numbers would not match this relationship: 7 would be neutral, 14 would imply a much larger [OH−], and 4 would imply pH equals pOH, which doesn’t occur for this pH in water at standard conditions.