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 or mmol/L Ca^2+. 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. Ca(ln)^2+ + EDTA right arrow Ca(EDTA)^2+ + In A 50.00-mL sample of groundwater is titrated with 0.0100 M EDTA. Assume that Ca^2+ accounts for all of the hardness in the groundwater. If 11.30 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

from equation

M1V1 = M2V2

(11.3*0.01) = (50*M2)

M2 = molarity of Ca2+ = 0.00226 M

Mass of Ca2+ = 0.00226*(50/1000)*40.078 = 0.00453 grams

PPM = 0.00453/50 *10^6

    = 90.6

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