pH Calculator
Calculate pH, pOH, hydrogen ion concentration [H⁺] and hydroxide ion concentration [OH⁻] from any given value. Free chemistry pH calculator for acid-base solutions with charts and detailed breakdowns.
About This Calculator
The pH Calculator computes pH, pOH, hydrogen ion concentration [H⁺], and hydroxide ion concentration [OH⁻] from any single input value. It uses the fundamental formula pH = -log₁₀[H⁺] and the water dissociation relationship pH + pOH = 14.
The pH scale ranges from 0 to 14, measuring how acidic or basic a solution is. Acids donate H⁺ ions (lower pH), while bases donate OH⁻ ions (higher pH). This calculator supports the worldwide chemistry standard and works for any aqueous solution at 25°C.
Frequently Asked Questions
What is the pH formula?
pH = -log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in moles per liter. For example, if [H⁺] = 1 × 10⁻⁴ M, pH = 4. A tenfold change in [H⁺] changes pH by exactly 1 unit because the scale is logarithmic.
Why is the pH scale from 0 to 14?
The scale spans 14 orders of magnitude of [H⁺]. Pure water has [H⁺] = 1 × 10⁻⁷ M (pH 7). Strong acids approach 1 M [H⁺] (pH 0), and strong bases approach 1 × 10⁻¹⁴ M [H⁺] (pH 14). The self-ionization constant of water Kw = [H⁺][OH⁻] = 1 × 10⁻¹⁴ at 25°C.
How do I calculate [H⁺] from pH?
To find [H⁺] from pH, use the inverse formula: [H⁺] = 10⁻ᵖᴴ. For pH 7, [H⁺] = 1 × 10⁻⁷ M. For pH 3, [H⁺] = 1 × 10⁻³ M = 0.001 M. Each unit decrease in pH means a 10× increase in hydrogen ion concentration.
What is pOH and how is it related to pH?
pOH is the negative logarithm of the hydroxide ion concentration: pOH = -log₁₀[OH⁻]. In water at 25°C, pH + pOH = 14. So if pH is 5, pOH is 9, and [OH⁻] = 10⁻⁹ M. This relationship comes from the water dissociation constant Kw.
How do strong and weak acids differ?
Strong acids (HCl, H₂SO₄, HNO₃) completely dissociate in water, so [H⁺] equals the acid concentration. Weak acids (acetic acid, citric acid) only partially dissociate, requiring the acid dissociation constant Ka to calculate [H⁺]. This calculator works for any [H⁺] regardless of the acid's strength.