The Acid-Dissociation Constant (Ka) is a measure of the strength of an acid in a solution. It represents the extent to which an acid dissociates (splits into ions) in water, producing hydrogen ions (H+) and the conjugate base. The Ka is defined as the ratio of the concentration of the products (H+ and the conjugate base) to the concentration of the undissociated acid (HA) in the following equilibrium reaction:
HA ⇌ H+ + A-
The units of the Acid-Dissociation Constant (Ka) depend on the concentration units used for the concentrations of the acid (HA), hydrogen ions (H+), and the conjugate base (A-). The most common units for Ka are:
Molarity (M): If the concentrations are expressed in moles per liter (M), the Ka will have units of M^-1. In this case, the Ka value is often written without any units, as it represents a pure number.
Concentration (mol/L): If the concentrations are expressed in moles per liter, the Ka will have units of L/mol.
It's important to use consistent units for all concentrations when calculating or comparing Ka values. Additionally, the pKa is commonly used in chemistry, which is the negative logarithm (base 10) of the Ka. The pKa provides a convenient way to express the acidity or basicity of a compound on a logarithmic scale. Lower pKa values indicate stronger acids, while higher pKa values indicate weaker acids.