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 ground water 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.Ca(ln)2 + + EDTA rightarrow Ca(EDTA)2+ + In A 50.00-mL sample of groundwater is titrated with 0.0350 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 CaCOj by mass?

Explanation / Answer

mmoles EDTA = Molarity EDTA * mL EDTA = (0.0350)(10.20) = .357 mmoles EDTA

EDTA reacts with Ca2+ in a 1:1 mole ratio, then

mmoles Ca2+ = mmoles EDTA = 0.357

Molarity Ca2+ = mmoles Ca2+ / mL Ca2+ = 0.357 / 50.00 = .00714 M Ca2+
x (1 mole CaCO3 / 1 mole Ca2+) = .00714 M CaCO3

(.00714 moles CaCO3 / L) x (100 g CaCO3 / 1 mole CaCO3) x (1000 mg / 1 g) x
= 714 mg CaCO3 / L = 714 ppm CaCO3

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