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 CaC03 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 rightarrow Ca(EDTA)2+ + In A 50.00-mL sample of groundwater is titrated with 0.0100 M EDTA. Assume that Ca2+ accounts for all of the hardness in the groundwater. If 10.20 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

no of moles of EDTA used = 0.085*10.1 = 0.858 millimoles

1 mole of EDTA requires ! mole of Ca2+ so

no of moles of Ca2+ = 0.858 milllimoles

[Ca2+] = 0.858millimoles / 0.05 L = 17.16 millimoles/L

[CaCO3] = 17.16*10-3 M

or

=17.16*10-3*100g*1000 ppm

= 1716ppm of CaCO3

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