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# Determining groundwater hardness

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Question 1:
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 the water hardness, a sample of groundwater is titrated with EDTA, a chelating agent; in the presence of the indicator eriochrome black T, symbolized 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 + In

A 50.00 mL sample of ground water is titrated with 0.0900 M EDTA. Assume that Ca2+ accounts for all of the hardness in groundwater. If 0.90 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 CaCO3 by mass?

_________________ M CaCO3 ______________ ppm CaCO3

Question 2:
A 50.0 mL sample containing Cd2+ and Mn2+ was treated with 65.4 mL of 0.0400 M EDTA. Titration of the excess unreacted EDTA required 17.8 mL of 0.0130 M Ca2+. The Cd2+ was displaced from EDTA by the addition of an excess of CN-. Titration of the newly freed EDTA required 26.9 mL of 0.0130 M Ca2+ . What were the molarities of Cd2+ and Mn2+ in the original solution?

______________ M Cd2+ ___________ M Mn2+