$$CO_2 + H_2O ⇌ H_2CO_3$$
The pKa value is the negative logarithm of the acid dissociation constant, which is a measure of the tendency of an acid to donate a proton. In the case of carbonic acid, the acid dissociation constant is:
$$K_a = \frac{[H ^+][HCO_3 ^-]}{[H_2CO_3]}$$
The pKa value can be used to calculate the pH of a solution of carbonic acid. The pH is a measure of the acidity or basicity of a solution, and is defined as the negative logarithm of the hydrogen ion concentration. The pH of a solution can be calculated using the following equation:
$$pH = - \log[H^+]$$
The pH of a solution of carbonic acid can be calculated using the pKa value and the concentration of carbonic acid. For example, a solution of carbonic acid with a concentration of 0.01 M has a pH of 5.35.
