One gram of solid calcium sulfate [CaSO( s )] is added to 1.0 L of pure water. S

Question

One gram of solid calcium sulfate [CaSO(s)] is added to 1.0 L of pure water. Some of the solid then dissolves according to the following reaction, which has an equilibrium constant of K=10:

                        CaSO4(s) <--> Ca2+ + SO4 2-

(a)What are the concentration (mol/L) and activity of CaSO(s) before any dissolution occurs?

(b)What is the activity of CaSO(s) after enough dissolution has occurred so that the reaction reaches equilibrium, assuming that not all of the solid dissolves?

(d)Estimate the ionic strength of the solution based on the result of part (c). Then, compute and with the Extended Debye-Hückel equation and make a new estimate of the concentration of each ion in the equilibrated solution. Iterate between calculating the concentrations of Ca and and the corresponding ionic strength and activity coefficients until each calculation converges. What fraction of the original solid is dissolved at equilibrium?

Explanation / Answer

CaSO4(s) <--> Ca2+ + SO4 2-

Concentration of CaSO4 = 1/136.14 *1/1

              = 0.00734 M

after enough dissolution.

K = [Ca+2][SO4^2-]/[CaSO4]

10 = x^2/(0.00734-x)

x = 0.000733 M

concentration of Ca^2+ = 0.00733 M

concentration of SO4^2- = 0.00733 M

concen of CaSO4 after dissolution = 0.00734-0.00733 = 1*10^-5 M

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