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This constraint, also sometimes called mole balance, is a very simple one, and as such it is easily overlooked. When a salt is dissolved in water, the anion and cation are added in stoichiometric proportions. If the dissolution of the salt is the only source of these ions in the solution, then for a salt of composition C_ν+A_ν– we may write:

(3.98)

Thus, for example, for a solution formed by dissolution of CaCl₂ in water, the concentration of Cl^– ion will be twice that of the Ca²⁺ ion. Even if CaCl₂ is not the only source of these ions in solution, its congruent dissolution allows us to write the mass balance constraint in the form of a differential equation:

which just says that CaCl₂ dissolution adds two Cl^– ions to solution for every Ca²⁺ ion added.

By carefully choosing components and boundaries of our system, we can often write conservation equations for components. For example, suppose we have a liter of water containing dissolved CO₂ in equilibrium with calcite (for example, groundwater in limestone). In some circumstances, we may want to choose our system as the water plus the limestone, in which case we may consider Ca conserved and write:

where CaCO_3s is calcite (limestone) and is aqueous calcium ion. We may want to avoid choosing carbonate as a component and choose carbon instead, since the carbonate ion is not conserved because of association and dissociation reactions such as:

Choosing carbon as a component has the disadvantage that some carbon will be present as organic compounds, which we may not wish to consider. A wiser choice is to define CO₂ as a component. Total CO₂ would then include all carbonate species as well as CO₂ (very often, total CO₂ is expressed instead as total carbonate). The conservation equation for total CO₂ for our system would be:

Here we see the importance of the distinction we made between components and species earlier in the chapter. Example 3.9 illustrates the use of mass balance.