How it works
pH is the negative base-10 logarithm of the hydrogen ion concentration in moles per liter. Each whole pH unit is a tenfold change, so pH 3 is ten times more acidic than pH 4.
In water at 25 °C the product [H⁺][OH⁻] equals 1.0 × 10⁻¹⁴, so pH + pOH = 14. Neutral water has pH 7. Lower values are acidic and higher values are basic.
For a strong acid like HCl, [H⁺] equals the acid concentration. Weak acids only partly dissociate, so you need the acid dissociation constant (Ka) to find [H⁺] first.
Formula
pH = −log₁₀[H⁺] [H⁺] = 10^(−pH) pH + pOH = 14 (25 °C)
Questions people ask
What is the pH of 0.01 M HCl?
HCl is a strong acid, so [H⁺] = 0.01 M and pH = −log(0.01) = 2.
Can pH be negative or above 14?
Yes. Very concentrated strong acids can have pH below 0 and concentrated bases above 14, though activity effects make simple concentration-based values less accurate there.
Is neutral always pH 7?
Only at 25 °C. Kw rises with temperature, so pure water at 50 °C has a pH of about 6.6 yet is still neutral because [H⁺] equals [OH⁻].