Which of the following is the most correct ion-product Ksp expression for the Na₂S (s) compound in water?
1) Ksp = [Na⁺]^2[S2-]
2) Ksp = [Na⁺]^2[S2-]^2
3) Ksp = [Na⁺]^2[S2-]^3
4) Ksp = [Na⁺]^3[S2-]^2