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 or mmol/L Ca2 .  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.

RED BLUE

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

A 50.00-mL sample of groundwater is titrated with 0.0450 M EDTA. Assume that Ca2 accounts for all of the hardness in the groundwater. If 11.30 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?

Explanation / Answer

First calculate moles of EDTA solution = M x L = 0.045 x 0.0113 = 5.1 x 10-4

Considering equivalent amount of reaction between EDTA and CaCO3, we have,

moles of CaCO3 = 5.1 x 10-4

a) Molarity of CaCO3 = 5.085 x 10-4 / 0.05 = 0.01

b) for ppm of CaCO3

thus, ppm = 0.01 x 100 (FW of CaCO3) x 1000 = 1000 ppm

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