Water hardness is often expressed or defined as the content of the ions of Calcium and magnesium that make it "hard". To know the total hardness of water, EDTA titration is often used expressed as the mol of EDTA per liter of Solution.
mol of solute = (12.45 mL)(0.01111 mol/L)*(1L/1000mL)=1.3832 x 10-4.
Therefore, water hardness = 1.3832x10-4 mol/0.025 L = 5.53278x10^-3 M