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. A 50.00-mL sample of groundwater is titrated with 0.0550 M EDTA. Assume that Ca2 accounts for all of the hardness in the groundwater. If 14.70 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?

Explanation / Answer

from 14.7*0.055 = M2*50

M2= 14.7*0.055/50=0.01617M

b) 0.01617 M meand =0.01617 moles/ L= 0.01617*100 ( Molecular weight of CaCO3)/ L =1.617g/L

1.617g/1000 gm of solution = 1.617*10-3 g/g= 1617 ppm

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