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 0.0600 M EDTA. Assume that Ca^2+ accounts for all of the hardness in the groundwater. If 14.50 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

Molarity of EDTA (M1) = 0.06 M

Volume of EDTA (V1) = 14.50 mL

Volume of sample (V2) = 50 mL

Molarity of Ca2+ ions = M2 can be calculated using

M1V1 = M2V2

M2 = M1V1 / V2

= 0.06 M X 14.50 mL/ 50 mL

= 0.0174 M

CaCO3 (s)---> Ca2+(aq) + CO32-(aq)

Hence Molarity of CaCO3 = Molarity of Ca2+ ions = 0.0174 M

We know, 1 M = 1 mol L-1

Molarity of CaCO3 = 0.0174 mol L-1

Molar mass of CaCO3 = 100.0869 g mol-1

Amount of CaCO3 = 0.0174 X 100.0869 g L-1

= 1.7415 g L-1

1g = 1000 mg

Amount of CaCO3 = 1.7415 X 1000 mg L-1

= 1741.5 mg L-1

We know, 1 ppm = 1 mg L-1

Amount of CaCO3 = 1741.5 ppm

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