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 0.0250 M EDTA. Assume that Ca2 accounts for all of the hardness in the groundwater. If 11.70 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 CaCO3 = ?

ppm CaCO3 = ?

Explanation / Answer

mini moles of EDTA=0.2925

this means 0.2925 moles of ca2+ or caco3 is present in 50 ml

so Molarity=5.85*10^(-3)M

moles/L of Caco3 present=5.85*10^(-3)*100.08

=0.585 g/L

=585 mg/L

1 ppm=1mg/L

so ppm CaCO3=585

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