How can I find the concentration of ions in a molar solution?

Question

For example, 278 g of CaCl2 is dissolved in 2 liters of water, what is the concentration of the Ca2+ ions and the concentration of the Cl- ions?

Answer 1

To find the concentration of ions, first find the concentration of the compound that the ions are part of. In this example we first find the concentration of CaCl2 since it is made up of Ca²⁺ ions and Cl^- ions.

molar mass CaCl₂ = 111 g/mol

moles CaCl₂ = 278 g x 1 mol/111 g = 2.50 moles

In 2 L the concentration of CaCl₂ = 2.50 moles / 2 L = 1.25 M

To find the concentration of ions, i.e. [Ca²⁺] and [Cl^-] we look at how many of each you get when the molecule dissociates. You get 1 Ca²⁺ ion and 2 Cl^- ions. So, the concentration of ions is ....

[Ca²⁺ ] = 1.25 M CaCl₂ x 1 Ca²⁺ / CaCl₂ = 1.25 M Ca²⁺

[Cl^-] = 1.25 M CaCl₂ x 2 Cl^- / CaCl₂ = 2.50 M Cl^-