pH and pOH
Relationship between acidity and basicity at 25°C.
Formula
Variables
Example
pH 3 means pOH 11
How it works
At 25°C, water naturally splits a tiny amount into H+ and OH- ions, and the product of their concentrations is fixed. Because pH is defined as -log10[H+] and pOH as -log10[OH-], adding the two definitions gives pH + pOH = 14 when the ion product of water is 1.0 × 10^-14. This lets you move easily between acidity and basicity in aqueous solutions.
When to use it
Use this when you know either the pH or the pOH of a solution and need the other one. It is especially useful in chemistry problems about acids, bases, and neutralization at 25°C.
Worked example, step by step
Suppose a solution has pOH = 4.6 Use the relationship: pH + pOH = 14 Substitute: pH + 4.6 = 14 Subtract 4.6 from both sides: pH = 14 - 4.6 pH = 9.4 So the solution has pH 9.4.
Common mistakes to avoid
- Using 14 at temperatures other than 25°C without checking whether the problem allows it
- Adding pH and pOH concentrations directly instead of their logarithmic values
- Forgetting that a higher pH means a lower pOH, and vice versa
Did You Know?
pH and pOH always add to 14 in water at room temperature, linking acidity and alkalinity.
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