I weighed out 0.2416 g disodium EDTA and diluted it with about 250 mL of H2O and stuck it in a plastic bottle to use for titration. My question is, to determine the molarity of my stock EDTA standard sol'n, is it just (0.2416g / 292.25 g) / 0.250 L = 0.00331 M? Or do I have to use the VOLUME of EDTA solution used to titrate to the endpoint? (which in my case is 27.54 mL, 27.80 mL, and 27.26 mL respectively). I'm really leaning towards the first equation I wrote, but I'm not 100% sure. Also, how would I go about calculating the moles of metal ions that I have in my water sample (50mL). EDTA has a 1:1 stoich ratio with them so would I just (M EDTA x V EDTA) = (M ions x V ions)? So then it would be something like (M EDTA x titration volume) = (M ions x 0.050 L)? So, once I figure out the moles of metal ions in solution, to figure out the total hardness of the water sample do I just do (moles of metal ions x MW of CaCO3 mg) / (0.050L) to get it in mg/L (ppm)? Thanks in advance for your help.