The pOH Scale HL Only
Specification Reference R3.1.9
Quick Notes
- pOH measures the concentration of OH⁻ ions in solution.
- Calculated using: pOH = –log₁₀[OH⁻]
- [OH⁻] can be found from pOH: [OH⁻] = 10–pOH
- Linked to pH through the equation: pH + pOH = 14 (at 298 K).
- You must be able to interconvert between:
- [H⁺] and pH
- [OH⁻] and pOH
- pH and pOH
Full Notes
What Is the pOH Scale?
The pOH scale is a logarithmic measure of the hydroxide ion concentration [OH⁻] in a solution.
It works in the same way as for pH measuring hydrogen ion concentration [H⁺].
Definitions and Equations
And to work backwards from [OH⁻] we can use:
Note: At 298 K (25 °C): pH + pOH = 14
This relationship allows you to move between pH and pOH easily if you know one value.
Interconverting Values
You may be asked to:
- Calculate pOH from [OH⁻] or vice versa.
- Convert pOH to pH, and then determine [H⁺].
- Go from pH to [OH⁻] using the full sequence:
- Find pOH using pH + pOH = 14
- Then use [OH⁻] = 10–pOH
Summary
- pOH measures hydroxide ion concentration.
- pOH = –log[OH⁻] and [OH⁻] = 10–pOH.
- At 298 K, pH + pOH = 14.
- Be able to convert between [H⁺], [OH⁻], pH, and pOH.