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 CaCOs 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 when Ca2 is removed. red blue CalInEDTA Ca(EDTA)In A 50.00-mL sample of groundwater is titrated with 0.0200 M EDTA. Assume that Ca2 accounts for all of the hardness in the groundwater. If 10.50 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 CaCO ppm CaCO, Tools x 102

Explanation / Answer

Part a

moles of EDTA = Molarity of EDTA x volume of EDTA

= (0.0200 mol/L)(10.50 mL x 1L/1000 mL )

= 0.00021 moles

From the stoichiometry of the reaction

Molar ratio of  EDTA : Ca2+ = 1:1

moles of Ca2+ = moles of EDTA = 0.00021 moles  

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

= (0.00021 moles) / (50.00 mL x 1L/1000mL)

= 0.0042 M Ca2+

Molarity of CaCO3 = 0.0042 M

Part b

Mass concentration of CaCO3 = molarity x molecular weight

= 0.0042 mol/L x 100 g/mol

= 0.42 g/L

ppm CaCO3 = 0.42 g/L x 1000mg/g

= 420 mg/L

= 420 ppm

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