Weak Acid Solutions

A weak acid only partially dissociates in water, leaving most dissolved molecules in their original form and generating relatively few hydronium ions. Unlike strong acids that completely ionize, weak acids establish an equilibrium between the molecular and ionic forms. Common examples include acetic acid in vinegar and formic acid in ant venom, where only about 1% of molecules ionize under typical conditions.

The Ka value expresses the ratio of product to reactant concentrations at equilibrium. By substituting equilibrium concentrations into the Ka expression and solving for hydronium ion concentration, you can determine pH using the negative logarithm. For example, hydrocyanic acid with Ka = 4.9 × 10−10 yields a hydronium concentration of 8.6 × 10−6 M, corresponding to a pH of 5.07.

An ICE table tracks Initial, Change, and Equilibrium concentrations of reactants and products during a weak acid dissociation. It systematically organizes how concentrations shift as the system reaches equilibrium, with changes denoted by x. This method simplifies substitution into the Ka expression and helps verify whether simplifying assumptions, such as neglecting x compared to initial concentration, are valid.

The assumption that x is negligible is valid when x is less than 5% of the initial acid concentration. For instance, in a 0.15 M hydrocyanic acid solution, x equals 8.6 × 10−6 M, which is only 0.0057% of 0.15 M, well below the 5% threshold. This simplification allows you to approximate (initial concentration − x) as simply the initial concentration, streamlining calculations.

Convert pH to hydronium ion concentration using the antilog function, then construct an ICE table with this equilibrium value. Calculate the change in concentration from the initial acid concentration and hydronium concentration. Finally, substitute all equilibrium values into the Ka expression to solve for the dissociation constant, as demonstrated with nitrous acid where pH 2.34 yields Ka = 4.6 × 10−4.

Water autoionization produces only 1 × 10−7 M hydronium ions, which is typically much smaller than the hydronium concentration generated by the weak acid itself. Since this contribution is negligible compared to the acid's ionization, it is treated as approximately zero in ICE tables. This simplification does not significantly affect the accuracy of Ka or pH calculations for weak acid solutions.

Weak acids are abundant in nature and everyday products. Acetic acid is the main ingredient in vinegar, formic acid causes the sting of ant bites, and hydrocyanic acid is a Brønsted acid that donates protons to water. Weak acids are also responsible for the tangy taste of citrus fruits and unpleasant smells in body odor, making them chemically and biologically significant compounds.