Question

Determine the concentrations of Na2CO3, Na , and CO32– in a solution prepared by dissolving 1.28 × 10–4 g Na2CO3 in 2.00 L of water. Express all three concentrations in molarity. Additionally, express the concentrations of the ionic species in parts per thousand (ppt). Note: Determine the formal concentration of CO32–. Ignore any reactions with water.

Answer 1

Note: However, Na₂CO₃ does not exist in solution, this salt ionizes in water to Na⁺, and the carbonate anion, CO₃^2-. Therefore, the solution is not 0.603 * 10^-6 M Na₂CO_3. In fact it is nearly 0 M Na₂CO₃ because the sodium carbonate has ionized and reacted with water.

The solution, instead, is 1.206 *10^-6 M  in Na⁺ and 0.603 *10^-6 M  in Cl^–. The formal concentration of Na2CO3 however, is 0.603 *10^-6 F because it represents the total amount of Na2CO3 in solution.

Hence, in this case the formal concentration unit is used to account for the fact that there is Na2CO₃ , though it does not exist as such in this form.