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) Ca(In)^2+ + EDTA ------> Ca(EDTA)^2+ + In (blue)

A 50.00-mL sample of groundwater is titrated with 0.0600 M EDTA. Assume that Ca2 accounts for all of the hardness in the groundwater. If 12.70 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?

(Hint: Start by finding the number of moles of EDTA. Then use the reaction stoichiometry to find the number of moles of Ca2 .)

Explanation / Answer

12.70 mL of 0.0600 M EDTA = 12.70 x 0.0600/ 1000 moles = 0.000762 moles

from the reaction stoichiometry, 1 mole of EDTA reacts with 1 mole of Ca2+

therefore no. Of moles of Ca in 50 mL sample = 0.000762

hence molarity = 0.000762 x 1000/ 50 = 0.01524 M

molar mass of CaCO3 = 100 g/ mol

therefore ppm of CaCO3 = 0.01524 x 100 x 106/ 1000 = 1524 ppm

Related Questions

Navigate

• Browse (All)
• Browse H
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.