Consider an amphoteric hydroxide, M(OH)2(s), where M is a generic metal. ksp=6x1

Question

Consider an amphoteric hydroxide, M(OH)2(s), where M is a generic metal.

ksp=6x10^-16

kf=0.04

Estimate the solubility of M(OH)2 in a solution buffered at pH = 7.0, 10.0, and 14.0.

Explanation / Answer

M(OH)2(s) ==> M^2+ + 2OH^- .......M(OH)2(s) + 2OH^- ==> M(OH)4^2- Ksp = (M^2+)(OH^-)^2 = 4E-16 Kf = [M(OH)4^2-)/(OH^-)^2 = 0.06 Solubility = (M2+) + [M(OH)4^2-] at pH 7, (H^+) = 1E-7 and OH^- = 1E-7 (Mg^2+) = Ksp/(1E-7)^2 = about 0.04 M [Mg(OH)2^2-] = 0.06*(1E-7)^2 = 6E-16; therefore, clearly the complex ion is not the predominant factor at pH = 7. You can follow through at pH = 10 (pOH = 4) and pH 14 (pOH = 0).[Note: don't confused with pOH = 0, that is (OH^-) = 1.0M These are ESTIMATES of the solubility.

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