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When ionic salts are dissolved in water, they completely dissociate because they are strong electrolytes. Sometimes, chemists (and even biologists!) are interested in the concentration of a specific ion or total ion concentration.

The stoichiometry (a ratio of quantities) in the dissolution process determines the concentration of ions in solution. For example, let's consider when 1 mole of NaCl dissolves in 1 L of water. Since concentration is typically expressed in molarity (M or mol/L), then the concentration of NaCl would be 1 mole NaCl / 1 L = 1 M. NaCl will completely dissociate into Na ^+ and Cl ^- . Considering stoichiometry, for every one NaCl, you get one Na ^+ cation and one Cl ^- anion. In this simple example, the concentration of both Na ^+ and Cl ^- are each 1 M as well.
Now let's look at a salt such as SrCl_2 . If we dissolve 1 mole of SrCl_2 in 1 L of water, the concentration of SrCl_2 will again be 1 M. However, when SrCl_2 dissociates, one Sr^{2+} cation and two Cl^- anions are produced. Using this stoichiometry, one can calculate the Cl^- concentration and total ion concentration by the following: [Cl^-] = \frac{1 mol SrCl_2}{1 L} \frac{2 mol Cl^-}{1 mol SrCl_2} = 2 \text{ M } Cl^-
[\text{total ion}] = \frac{1 mol SrCl_2}{1 L} \frac{3 \text{mol ions}}{1 mol SrCl_2} = 3 \text{ M } Cl^-

Check out the video below for more examples: