EDTA is a complex used to determine water hardness. The reaction between EDTA and CaCO₃ is in 1:1 ratio. To solve this problem, we must know the moles of CaCO₃ present (otherwise we can't solve the problem). Suppose you have 0.5 moles of CaCO₃. Since the ratio is 1:1, it would also need 0.5 moles EDTA. Then,
Volume = Moles/Molarity = 0.5 moles/0.05 M = 10 L of EDTA