Hardness in groundwater is due to the presence of metal ions, primarily Mg^2+ an

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 here 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. A 50.00-mL sample of groundwater is titrated with 0.0200 M EDTA. Assume that Ca^2+ accounts for all of the hardness in the groundwater. If 11.90 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 CaCO^3 by mass?

Explanation / Answer

Using molarity equation

(a) M1V1 (EDTA) =M2V2 (Ca2+)

0.0200 x 11.90 = M2 x 50.00 (50 mL water sample requires 11.90 mL of 0.0200 M EDTA solution)

or M2 = 0.00476 M

Since 1 mole CaCO3 contains 1 mole Ca2+, Thus Molarity of solution= 0.0048 M of CaCO3 equivalents

(b) Strength = Molarity x molecular weight of CaCO3 = 0.00476 mol/L x 100 g/mol = 0.476 g/L = 476 mg/L = 476 ppm

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