Calculate Concentration of Ions in Solution

This worked example problem illustrates the steps to calculate the concentration of ions in an aqueous solution in terms of molarity. Molarity is one of the most common units of concentration. Molarity is the number of moles of a substance per unit volume. 

Question

a. State the concentration, in moles per liter, of each ion in 1.0 mol Al(NO₃)₃.
b. State the concentration, in moles per liter, of each ion in 0.20 mol K₂CrO₄.

Solution

Part a. Dissolving 1 mol of Al(NO₃)₃ in water dissociates into 1 mol Al³⁺ and 3 mol NO^3- by the reaction:

Al(NO₃)₃(s) → Al³⁺(aq) + 3 NO^3-(aq)

Therefore:

concentration of Al³⁺ = 1.0 M
concentration of NO^3- = 3.0 M

Part b. K₂CrO₄ dissociates in water by the reaction:

K₂CrO₄ → 2 K⁺(aq) + CrO₄^2-

One mol of K₂CrO₄ produces 2 mol of K⁺ and 1 mol of CrO₄^2-. Therefore, for a 0.20 M solution:

concentration of CrO₄^2- = 0.20 M
concentration of K⁺ = 2×(0.20 M) = 0.40 M

Answer

Part a.
Concentration of Al³⁺ = 1.0 M
Concentration of NO^3- = 3.0 M

Part b.
Concentration of CrO₄^2- = 0.20 M
Concentration of K⁺ = 0.40 M