Hardness in groundwater is due to the presence of metal ions, primarily Mg2 and

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 here 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.

A 50.00-mL sample of groundwater is titrated with 0.0500 M EDTA. Assume that Ca2 accounts for all of the hardness in the groundwater. If 11.60 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?

M of CaCO3 ? and ppm of CaCO3

Explanation / Answer

The reaction between Ca2+ and EDTA is as follows:

Ca^+2 + EDTA ----> Ca(EDTA)^2+

First calculate the mmole oF EDTA:

mmoles EDTA = Molarity EDTA x mL EDTA = (0.0500)(11.60)

= 0.580 mmoles EDTA

Since EDTA reacts with Ca2+ in a 1:1 mole ratio, then

mmoles Ca2+ = mmoles EDTA = 0.580
Molarity Ca2+ = mmoles Ca2+ / mL Ca2+ = 0.580 / 50.00

= 0.0116 M Ca2+
x (1 mole CaCO3 / 1 mole Ca2+) = 0.0116 M CaCO3

(0.0116 moles CaCO3 / L) x (100 g CaCO3 / 1 mole CaCO3) x (1000 mg / 1 g) x
= 1160 mg CaCO3 / L

= 1160 ppm CaCO3

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