Understanding pH and pOH
Quick Answer: pH measures the acidity or basicity of an aqueous solution. It is the negative logarithm (base 10) of the hydrogen ion concentration: pH = −log[H+]. The sum of pH and pOH always equals 14 at 25°C.
Key Formulas
pH = −log10[H+] | pOH = −log10[OH−] | pH + pOH = 14 (at 298 K)
The ionic product of water, Kw = [H+][OH−] = 1.0 × 10−14 at 25°C. Acidic solutions have pH < 7, basic solutions pH > 7, and neutral solutions pH = 7.
Solved Examples
[H+] = 0.1 = 10−1
pH = −log(0.1) = 1.00
pOH = 14 − 1 = 13.00
[OH−] = 0.01 = 10−2
pOH = −log(0.01) = 2.00
pH = 14 − 2 = 12.00
[H+] = [OH−] = 10−7
pH = 7.00, pOH = 7.00
Neutral
FAQs
Yes, for strong acids with concentration > 1 M (e.g., 10 M HCl has pH = −1). The typical 0–14 range applies to dilute solutions.
Human blood has a tightly regulated pH between 7.35 and 7.45. Values outside this range indicate serious medical conditions.
Kw increases with temperature. At 100°C, neutral pH is about 6.14, not 7. The pH + pOH = pKw relationship still holds.
A buffer resists pH changes when small amounts of acid or base are added. It consists of a weak acid and its conjugate base (or weak base and its conjugate acid).