I have tried to search for it but i am still not understanding how to work this out. I have to work out the average total hardness of water (expressed as CaCO3/L) and find the percent standard deviation. EDTA 0.005 M solution was used to titrate 50 ml aliquots of tap water using eriochrome black T as an indicator. My average volume of EDTA solution used was 9.76 mL. Howdo i work out hardness.