You add potassium hydroxide to the solution in Example 18.13. When the [OH^-] reaches 1.9 * 10^-6 M (as you just calculated), magnesium hydroxide begins to precipitate out of solution. As you continue to add KOH, the magnesium hydroxide continues to precipitate. However, at some point, the [OH^-] becomes high enough to begin to precipitate the calcium ions as well. What is the concentration of Mg²+ when Ca²⁺ begins to precipitate?

Example 18.13

The magnesium and calcium ions present in seawater ([Mg²⁺] = 0.059 M and [Ca²⁺] = 0.011 M) can be separated by selective precipitation with KOH. What minimum [OH^-] triggers the precipitation of the Mg²⁺ ion?