To find the solubility of calcium hydroxide Ca OH 2 in water at 25C, we can use the solubility product constant Ksp expression. The balanced chemical equation for the dissolution of Ca OH 2 in water is:Ca OH 2 s Ca aq + 2OH aq Let's denote the solubility of Ca OH 2 as s mol/L. When Ca OH 2 dissolves, it produces one Ca ion and two OH ions. Therefore, the concentration of Ca ions will be s mol/L, and the concentration of OH ions will be 2s mol/L.Now, we can write the Ksp expression:Ksp = [Ca] * [OH]^2Given that Ksp for Ca OH 2 is 5.5 10^-6, we can substitute the concentrations of the ions in terms of s:5.5 10^-6 = s * 2s ^2Now, we can solve for s:5.5 10^-6 = 4s^3s^3 = 5.5 10^-6 / 4s^3 = 1.375 10^-6s = 1.375 10^-6 ^1/3 s 0.0111 mol/LSo, the solubility of calcium hydroxide Ca OH 2 in water at 25C is approximately 0.0111 mol/L.