The notations(g),(l),(aq)refer to the state of the ion or molecule,
i.e., a gas, a liquid, or in aqueous solution respectively.
Usually, CO2(aq) and H2CO3(aq) are grouped together and expressed
as the sum concentration of CO2(aq)
because it is difficult to distinguish between the two ions.Basically,two of the parameters of the carbon dioxide system(total alkalinity and the total hydrogen ion concentration, pH) can be measured to make an estimate of the carbon dioxide in aqueous solution.