Question

If the endpoint is reached when 50.00 mL of a tap water sample is titrated with 12.00 mL of 5.00 x 10^-3 M EDTA solution, what is the hardness of the water sample in ppm?

Answer 1

The reactio of EDTa with metal ions is in 1:1 stoichiometric ratio

Moles of EDTA used = Molarity X volume = 5 X 10^-3 X 12 =60 X 10^-3 millimoles

concentration of metal ion in water = moles / Voume of water =60 X 10^-3 millimoles / 50 =1.2 X 10^-3 M

so concentration = 1.2 X 10^-3 moles / Litres = 1.2 X 10^-3 X 100 grams / Litres = 0.12 g / Litres

The molecular weight of CaCO3 = 100 and the hardness is represented in terms of CaCO3 concentration

ppm = parts per million = micrograms / Litres = 0.12 X 1000 micrograms / Litres = 120 ppm