The mole

A sample of hard water contains 240 mg of calcium ions per dm³. Soap reacts with calcium ions according to the equation: $\text{Ca}^{2+} + 2\text{S}^- \to \text{CaS}_2$. 1.0 g of soap contains 3.0 mmol of $\text{S}^-$ ions. What is the minimum mass of soap needed to remove all the calcium ions in 500 cm³ of this water?