Final answer:
The solubility of CaSO4(s) in 0.400 M Na2SO4(aq) at 25°C can be determined by comparing the concentration of sulfate ion in the solution to the Ksp value of CaSO4. If the concentration of sulfate ion is lower than 9.86 x 10^-5 M, no CaSO4 will precipitate.
Step-by-step explanation:
The solubility of CaSO4(s) in 0.400 M Na2SO4(aq) at 25°C can be calculated using the solubility product constant (Ksp) and the concentration of the sodium sulfate solution. The solubility of CaSO4 can be found by comparing the reaction quotient (Q) to the Ksp value. If the concentration of the sulfate ion (SO4^2-) in the sodium sulfate solution is less than 9.86 x 10^-5 M, no CaSO4 will precipitate.