This example problem demonstrates how to calculate the molarity of ions in an aqueous solution.
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?
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
Msolute = number of moles of CuCl2/Volume
Msolute = 0.07 mol/(600 mL x 1 L/1000 mL)
Msolute = 0.07 mol/0.600 L
Msolute = 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
The molarity of Cl- ions in solution is 0.24 M.