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Question

Write the equilibrium expression for dissolution of CaSO4(s) and determine the solubility of calcium sulfate at 25℃ in g/L H2O. (Ksp=9.1x10-6)

Answer 1

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CaSO4 → Ca{2+} + SO4{2-}

Ksp = [Ca{2+}] [SO4{2-}] = 9.1x10^-6

[Ca{2+}] = √ (9.1x10^-6) = 0.0030166 mol/L

To produce that concentration of Ca{2+} ions, 0.0030166 mole of CaSO4 must dissolve in a liter of solution.

(0.0030166 mol/L CaSO4) x (136.1406 g CaSO4/mol) = 0.41068 = 0.41 g/L CaSO4