pOH and pKw

In-depth explanation of pOH and pKw

pOH is defined as the negative logarithm of the hydroxide concentration, pOH = -log[OH-], and it mirrors pH for basic solutions. Because Kw equals [H+][OH-], taking negative logarithms gives pH + pOH = pKw, which is 14.00 at 25 C. This relation lets a chemist find the hydrogen ion concentration of a base by first calculating its hydroxide concentration. The sum changes with temperature, so pKw must be looked up when the work is not done at 25 C. The approach is especially direct for strong bases, where the hydroxide concentration equals the base concentration.

Going deeper: pOH and pKw

Key ideas: pOH = -log[OH-] and pH + pOH = pKw, equal to 14.00 at 25 C. For a base, find pOH first from [OH-], then subtract from 14. The same rule p(x) = -log(x) applies to Ka, Kb and Kw, so pKa + pKb = 14 for a conjugate pair.

Common mistakes and connections: Students use 14 at all temperatures, but pKw is 13.0 at about 60 C. Another slip is reporting pH for a base directly from pOH. A solution is basic when pOH is less than pH, not simply when pH exceeds 7 at nonstandard temperatures. Useful for titrations of bases.

Visualization of pOH and pKw

At 25 C pH + pOH = 14, so the line falls one unit for each unit of pH; 0.010 M NaOH has pOH 2 and so sits at pH 12. The interactive lesson shows this as a live animation or chart.

Worked example and use case

A 0.010 M NaOH solution has pOH 2.00 and therefore pH 12.00.

Where pOH and pKw fits in Acids, Bases & Electrochemistry

pOH and pKw is topic 8 of 105 in Acids, Bases & Electrochemistry.

Related topics

• Amphoteric SubstancesIntermediate
• Autoionization of WaterIntermediate
• pH ScaleBeginner
• Strong AcidsBeginner
• Strong BasesBeginner
• Weak AcidsIntermediate

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