Species Concentration

Ionization and Species Concentration

There are times when you need to deal with the concentration of chemical species in solution rather than the concentration of a chemical compound. Perhaps these examples will show why this is important.

When electrolytes dissolve in water, they dissociate into ions. When sodium chloride dissolves in water, sodium and chloride ions are formed. When calcium chloride dissolves in water, calcium and chloride ions are formed. Note that different amounts of chloride ions are formed. Equal concentrations of NaCl and CaCl₂ generate different concentrations of Cl^- ion.

NaCl Na⁺ + Cl^-

CaCl₂ Ca²⁺ + 2 Cl^-

1 M NaCl 1 M Na⁺ and 1 M Cl^-

1 M CaCl₂ 1 M Ca²⁺ and 2 M Cl^-

With weak electrolytes, this issue is complicated by the fact that some of the chemical remains undissociated. For example when HF dissolves in water HF(aq), H⁺(aq) and F^-(aq) are all formed and present in the solution. Depending on the conditions under which the solution is formed, the concentration of each of those chemical species might be different.

When dealing with the concentrations of chemical species, it is customary to use brackets around the formula of the chemical specie as a symbol for the concentration of that specie. For example, [Cl^-] = 2 M means the concentration of chloride ion is 2 M.

Later in this lesson (e.g. equilibrium constants) and in later lessons (e.g. reaction rates) we will need to focus on the concentration of the particular chemical species that are reacting with one another or of particular interest to us for some other reason. Molarity can be used for this. It is simply a matter of specifying that the concentration refers to a particular chemical specie rather than a chemical compound.

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E-mail instructor: Eden Francis

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