Gym · Equilibrium & acid–base

Weak-acid pH: solve the ICE table for [H⁺]

A weak acid only partly ionizes, so its pH is not simply −log of its concentration — you have to solve the equilibrium. Each drill gives a weak acid, its Kₐ, and how concentrated the solution is; you find the pH the real way, from the ICE table's mass-action root (the same solver the acetic-acid lesson uses). Type your answer (rounded to three significant figures). If you slip, the drill names the mistake — treating the weak acid as strong (pH = −log[HA]₀), confusing Kₐ with the hydrogen-ion concentration (that gives the pKₐ), or forgetting the square root in [H⁺] = √(Kₐ·[HA]₀). The arithmetic is machine-checked; the equilibrium model is disclosed, not proved, and Kₐ is a sourced measured value.

Model: One dominant equilibrium: only the acid's ionization matters, and water's own autoionization is neglected ([H⁺]₀ = 0). Valid when the acid's [H⁺] ≫ 10⁻⁷. Activities are approximated by molar concentrations — the ideal-dilute-solution model, which is why Kₐ is written with concentrations. The arithmetic is machine-checked and the answer is reported to 3 significant figures.