Question

3. A water has a calcium (Ca?') activity of 104.5 and a sulfate (SO2?) activity of 100? Is this solution undersaturated or supersaturated with respect to gypsum. (Ksp = 1046)? Explain your answer (3 pts)

Answer 1

See we check the solubility of the solution by comparing the K_sp value for saturate solution with the K_sp value of the given solution.

K_sp^' = a_Ca2+ * a_so4(2-)

basically K_sp of the solution is given by the multiplication of the activities of the constituting ions

so here in this solution K_sp^' = 10^-4.5 * 10^-0.75 = 10^-5.25 =5.62 * 10^-6

and for saturated solution K_sp = 10^-4.6 = 2.51 * 10^-5

so we can see that K_sp^' < K_sp

in this case when the K_sp^' of the solution is lower than the K_sp of the saturated solution this indicates that the solution is undersaturated.

when K_sp^' > K_sp : the solution is supersaturated

and when K_sp^' = K_sp the solution is saturated