pH Calculator — Calculate pH, [H⁺] & [OH⁻] ConcentrationpH = −log[H⁺] · pOH = −log[OH⁻] · pH + pOH = 14 · Acids · Bases · Buffers
Use this free pH Calculator to instantly compute the pH of any aqueous solution from its hydrogen ion concentration [H⁺], or conversely determine the [H⁺] concentration from a known pH value— using the standard pH formula: pH = −log₁₀[H⁺] and its inverse [H⁺] = 10⁻ᵖᴴ. This calculator also computes: pH from [H⁺] in mol/L (molarity) · [H⁺] from pH value · pOH = −log[OH⁻] and [OH⁻] concentration · pH + pOH = 14 (at 25°C) relationship · acid/base classification — strongly acidic, weakly acidic, neutral, weakly basic, or strongly basic — across the full pH scale from 0 to 14.
This online pH calculator is trusted across every area of chemistry, biology, and environmental science: A-Level, AP Chemistry, IB Chemistry, JEE, and NEET acid-base equilibrium problems, strong acid and strong base pH calculation (HCl, H₂SO₄, NaOH, KOH), weak acid and weak base pH using Ka and Kb dissociation constants, buffer solution pH calculation (Henderson-Hasselbalch equation), water quality testing — drinking water, swimming pools, aquariums, and soil pH, pharmaceutical formulation and drug stability pH analysis, and environmental science — acid rain, ocean acidification, and wastewater pH. Key pH reference values: pure water = 7.0 (neutral), stomach acid = 1.5–3.5, blood = 7.35–7.45, vinegar = 2.4, baking soda solution = 8.3 — making this tool essential for any acid-base chemistry calculation.
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pH Calculator — Hydrogen Ion Concentration, pOH, and Buffer Problems
pH is the negative logarithm of hydrogen ion concentration: pH = -log[H⁺]. Pure water at 25°C has [H⁺] = 10⁻⁷ M and pH = 7.00. A strong acid solution of 0.01 M HCl has [H⁺] = 0.01 M and pH = 2.00. The logarithmic scale means a one-unit pH change represents a 10-fold change in [H⁺]. A solution at pH 3 is 10 times more acidic than pH 4 and 1,000 times more acidic than pH 6. The pH calculator converts between [H⁺], [OH⁻], pH, and pOH with the water autoionization equilibrium Kw = [H⁺][OH⁻] = 10⁻¹⁴ at 25°C.
Strong acid and strong base pH calculations are straightforward because they fully dissociate. For weak acids and bases, the equilibrium constant Ka or Kb must be used. Acetic acid (Ka = 1.8 × 10⁻⁵) at 0.1 M produces [H⁺] = √(Ka × C) = 1.34 × 10⁻³ M, giving pH = 2.87 — considerably less acidic than a strong acid at the same concentration (which would give pH 1). The calculator handles both strong and weak acid/base calculations with the appropriate equilibrium treatment.
Buffer solutions resist pH changes upon addition of small amounts of acid or base, operating through the Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]). A buffer made from 0.1 M acetic acid and 0.1 M sodium acetate has pH = 4.74 = pKa of acetic acid. The maximum buffering capacity is at pH = pKa, where the buffer contains equal concentrations of acid and conjugate base. The calculator solves Henderson-Hasselbalch in both directions — computing pH from buffer composition and computing required composition to achieve a target pH.