pH

Meaning: pH expresses acidity on a logarithmic scale. Strictly it uses H⁺ activity, but the MCAT works with the molar concentration of H⁺ (H₃O⁺). In pure water at 25 °C, [H⁺] = [OH⁻] = 10⁻⁷ M, so pH 7 is neutral, lower values are acidic, and higher values are basic.

Equation: pH = −log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in mol/L (M) and pH is unitless. At 25 °C, pH + pOH = 14.

On the MCAT: Expect conversions between [H⁺] and pH, often by estimation: [H⁺] = 3 × 10⁻⁴ M gives pH ≈ 3.5. Passages also ask for the pH of strong acid or base solutions, points on a titration curve, or buffer pH from pH = pKa + log([A⁻]/[HA]), where A⁻ is the conjugate base and HA the weak acid. You may also have to predict the direction of change after dilution or after adding acid.

Common confusion: Treating pH as linear. pH 3 is 100 times more acidic than pH 5, not twice as acidic. Also, 7 is neutral only at 25 °C because Kw changes with temperature, and diluting an acid moves its pH toward 7 without carrying it past 7.