pH and pOH: Logarithms, Temperature, and Applications

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What does it mean for a solution to be neutral?

A neutral solution has equal concentrations of hydronium and hydroxide ions: $[\mathrm{H_3O^+}]=[\mathrm{OH^-}]$.

How are pH and hydronium concentration related?

pH is the negative base-10 logarithm of hydronium molarity: $\mathrm{pH}=-\log[\mathrm{H_3O^+}]$.

What does pOH measure?

pOH expresses hydroxide ion molarity on a logarithmic scale: $\mathrm{pOH}=-\log[\mathrm{OH^-}]$.

What is the pH–pOH relationship at 25 °C?

At 25 °C, $\mathrm{pH}+\mathrm{pOH}=14.00$, because $\mathrm{p}K_w=14.00$.

Why can neutral water have a pH other than 7?

The value of $K_w$ varies with temperature. Neutrality requires equal hydronium and hydroxide concentrations, but those equal concentrations—and therefore the pH—can change with temperature.

Quiz questions

1. How is an acidic aqueous solution identified by comparing its hydronium and hydroxide concentrations?Answer: $[\mathrm{H_3O^+}]>[\mathrm{OH^-}]$
2. What is the definition of pH?Answer: $\mathrm{pH}=-\log[\mathrm{H_3O^+}]$
3. At 25 °C, what is the sum of pH and pOH?Answer: 14.00
4. At 25 °C, a solution has a pH of 4. Which classification is correct?Answer: Acidic
5. Pure water at 80 °C has equal hydronium and hydroxide concentrations and a pH of 6.31. What does this indicate?Answer: It is neutral because equal ion concentrations define neutrality at that temperature.