Question

Hardness in groundwater is due to the presence of metal ions, primarily Mg2+ and Ca2+ . Hardness is generally reported as ppm CaCO3 . To measure water hardness, a sample of groundwater is titrated with EDTA, a chelating agent, in the presence of the indicator Eriochrome Black T, symbolized as In. Eriochrome Black T, a weaker chelating agent than EDTA, is red in the presence of Ca2+ and turns blue when Ca2+ is removed.

red blue
Ca(In)2+ + EDTA ⟶ Ca(EDTA)2+ + In
A 50.00 mL sample of groundwater is titrated with 0.0500 M EDTA . If 14.40 mL of EDTA is required to titrate the 50.00 mL sample, what is the hardness of the groundwater in molarity and in parts per million of CaCO3 by mass? Assume that Ca2+ accounts for all of the hardness in the groundwater.

Answer 1

millimoles EDTA = mL x M = 14.40 mL x 0.0500 M = 0.72
ratio of EDTA to Ca^2+ = 1:1; therefore, 0.72 millimoles CaCO3 in the sample.
grams CaCO3 = moles x molar mass = 0.00072 x 100.1 = 0.0721 or 72.1 mg CaCO3.
That's 72.1 mg CaCO3 in a 50 mL sample. How much in a liter? That's
72.1 mg CaCO3 x (1,000 mL/50 mL) = 1442 mg/L = 1442 ppm.
I'll leave the molarity to you. Check my work. You can find hardness calculators on-line. Why not plug the numbers from the problem into one of those calculators and see what you get?