"Washing Soda" (Sodium Carbonate) May Be Used to "Soften" Water

Multiple Choice

"Washing soda" (sodium carbonate) may be used to "soften" water by the removal of certain ions that would otherwise react with common soaps. When the "hardness" is due to calcium ion, the "softening" process may be represented as: Ca²⁺(aq) + CO₃^2-(aq) → CaCO₃(s)
What mass of sodium carbonate would be required to remove essentially all of the calcium ion from 750 L of solution containing 43 mg Ca²⁺ per liter?

A) 85 g
B) 67 g
C) 12 g
D) 48 g
E) 22 g