Stoichiometry Module: Solutions
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Ionic Concentration

Ionic compounds dissociate completely when dissolved. This means that when one mole of salt (NaCl) is dissolved in water, the resulting solution does not contain any molecules of NaCl, it contains the ions Na+ and Cl-:

With ionic compounds, chemists often talk about ionic concentrations, or the concentrations of the individual ions.

If a 1 M solution of CaCl2 is prepared, what is the concentration of Ca2+? of Cl-?

Ca2+: 1 M Ca2+: 1 M Ca2+: 2 M Ca2+: 2 M
Cl-: 1 M Cl-: 2 M Cl-: 1 M Cl-: 2 M

Calcium chloride is an ionic compound and will dissociate completely:

This chemical equation tells us that a mole of CaCl2 will dissociate to a mole of Ca2+ and two moles of Cl-.

Good! Since CaCl2 dissociates completely, one mole of Ca2+ and two moles of Cl- will form for every mole of CaCl2: