Molar Concentration of Ions Example Problem

This example problem demonstrates how to calculate the molarity of ions in an aqueous solution.

Molar Concentration of Ions Problem

A solution is prepared by dissolving 9.82 g of copper chloride (CuCl₂) in enough water to make 600 mL of solution. What is the molarity of Cl^- ions in solution?

Solution:

To find the molarity of the ions, the molarity of the solute and the ion to solute ratio must be found.

Step 1 - Find the molarity of the solute

From the periodic table:

atomic mass of Cu = 63.55
atomic mass of Cl = 35.45

atomic mass of CuCl₂ = 1(63.55) + 2(35.45)
atomic mass of CuCl₂ = 63.55 + 70.9
atomic mass of CuCl₂ = 134.45 g/mol

number of moles of CuCl₂ = 9.82 g x 1 mol/134.45 g
number of moles of CuCl₂ = 0.07 mol

M_solute = number of moles of CuCl₂/Volume
M_solute = 0.07 mol/(600 mL x 1 L/1000 mL)
M_solute = 0.07 mol/0.600 L
M_solute = 0.12 mol/L

Step 2 - Find the ion to solute ratio

CuCl₂ dissociates by the reaction

CuCl₂ → Cu²⁺ + 2Cl^-

ion/solute = # moles of Cl^-/# moles CuCl₂
ion/solute = 2 moles of Cl^-/1 mole CuCl₂

Step 3 - Find ion molarity

M of Cl^- = M of CuCl₂ x ion/solute
M of Cl^- = 0.12 mol CuCl₂/L x 2 moles of Cl^-/1 mole CuCl₂
M of Cl^- = 0.24 moles of Cl^-/L
M of Cl^- = 0.24 M

Answer

The molarity of Cl^- ions in solution is 0.24 M.