Map Sapling Learning macmillan learning Hardness in groundwater is due to the pr

Question

Map Sapling Learning macmillan learning Hardness in groundwater is due to the presence of metal ions, primarily Mg 2+ and Ca Hardness is 2+ generally reported as ppm CaCO3. 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 and turns blue 2+ when Ca s removed blue red Ca (In EDTA Ca EDTA 2 In A 50.00-mL sample of groundwater is titrated with 0.0900 M EDTA. Assume that Ca accounts for all of 2+ the hardness in the groundwater. If 11.40 mL of EDTA is required to titrate the 50.00-mL sample, what is the hardness of the groundwater in m and in parts per million of CaCO3 by mass? Number M CaCO Number ppm CaCO3

Explanation / Answer

find the moles of EDTA = molarity x volme

moles of EDTA = (0.090 M )x(0.0114 L) = 0.001026 moles of EDTA

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

moles Ca2+ = moles EDTA = 0.001026 moles

Molarity Ca2+ = moles of Ca2+ / volume of Ca2+

M = 0.001026 moles / 0.05 L = 0.02052 M Ca2+

molarity of CaCO3 = 0.02052 M (1 mole CaCO3 / 1 mole Ca2+) = 0.02052 M CaCO3

in ppm

(0.02052 moles CaCO3 / L) x (100 g CaCO3 / 1 mole CaCO3) x (1000 mg / 1 g)
= 2052 mg CaCO3 / L = 2052 ppm CaCO3

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