Magnesium oxide (MgO) has very low solubility in water. Its solubility product constant (Ksp) at 25 °C is 5.61 x 10^-12. This means that the maximum concentration of Mg²⁺ and OH^- ions that can exist in equilibrium with solid MgO at 25 °C is very low, making MgO practically insoluble in water.

The solubility of MgO can be calculated using the Ksp expression:

Ksp = [Mg²⁺][OH^-]^2

At equilibrium, the concentration of Mg²⁺ and OH^- ions will be equal, so we can substitute [Mg²⁺] = [OH^-] = x.

Ksp = x^3

x = [Mg²⁺] = [OH^-] = (Ksp)^1/3

Substituting the value of Ksp, we get:

x = (5.61 x 10^-12)^1/3 = 1.83 x 10^-4 M

Therefore, the solubility of magnesium oxide in water at 25 °C is approximately 1.83 x 10^-4 M.