A 100ml aliquot of a water sample containing Ca(II) and Mg(II) is titrated with 22,74mL of EDTA 0.00998 mol/L at PH 10. Another aliquot of 100ml of the same sample is treated with NaOH to precipitate Mg(OH)2 and is then titrated with PH 13 with 15.86 ML of the same EDTA solution. Calculate the total hardness (mg/ML CaCO3). CaCO3 = 100.09 g/mol.

Answer 1

I may be missing something here but I don't know why the second step of pptng the Mg(OH)2 is necessary since EDTA at pH of 10 titrates both Mg and Ca.
millimols Ca + Mg = mL x M EDTA, then mmols x molar mass CaCO3 = mg CaCO3/100 mL. Convert to mg/L.