What are some equilibrium expressions that chemists use for weak acid equilibrium?
Chemists use various equilibrium expressions to describe the equilibrium of weak acid systems. Here are two common equilibrium expressions:
1. The Acid Dissociation Constant (Ka):
The Ka expression relates to the dissociation of a weak acid (HA) and its conjugate base (A-) in water. The equilibrium expression for this dissociation is:
Ka = [H+][A-] / [HA]
In this expression, [H+] represents the concentration of hydrogen ions (protons), [A-] represents the concentration of the dissociated conjugate base, and [HA] represents the concentration of the undissociated weak acid. The value of Ka reflects the relative strength of the acid. A higher Ka value indicates a stronger acid.
2. The Percent Dissociation (%):
The percent dissociation expression represents the fraction of the weak acid molecules that dissociate into ions in solution. It is given by:
% dissociation = (concentration of dissociated acid / initial concentration of acid) × 100
To calculate the percentage dissociation, you need the concentrations of the dissociated acid and the initial concentration of the acid. After determining these values, divide the concentration of dissociated acid by the initial concentration, multiply by 100, and you will have the percent dissociation.
To determine Ka or percent dissociation for a specific weak acid, you'll need to know the initial concentrations of the acid and its dissociated ions, as well as any experimental data such as pH or concentrations of the acid and its conjugate base. By plugging these values into the respective equilibrium expressions, you can calculate the desired parameters.