11. (10 pts) a) Calculate the molar solubility of strontium sulfate, SrSO$_4$, at pH = 7 ($K_{sp}$ = 2.8 x 10$^{-7}$).
Hint: SO$_4^{2-}$ does not react with water at pH 7.
b) Will the molar solubility increase or decrease if SrSO$_4$ is in 0.1 M Na$_2$SO$_4$? Explain your answer.
c) Calculate the molar solubility of SrSO$_4$ in 0.1 M Na$_2$SO$_4$ without taking into account the ionic strength and activity of the ions.
d) Taking into account ionic strength and ion activity, will the solubility increase or decrease if SrSO$_4$ is in 0.1 M NaCl? Explain your answer.