Question

Hardness in groundwater is due to the presence of metal ions, primarily Mg 2 + and Ca 2 + . Hardness is generally reported as ppm CaCO 3 . 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 Ca 2 + and turns blue when Ca 2 + is removed.

Ca(In)2+

+

EDTA

⟶

Ca(EDTA)2+

+

In

A 50.00 mL sample of groundwater is titrated with 0.0550 M EDTA. If 10.60 mL of EDTA is required to titrate the 50.00 mLsample, 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.

hardness in molarity:M CaCO3

hardness in parts per million:ppm CaCO3

Answer 1