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. 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 Ca is removed. red blue Ca(ln)"+ + EDTA Ca(EDTA)"++In A 50.00-mL sample of groundwater is titrated with 0.0100 M EDTA. Assume that Ca2 accounts for all of 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 molarity and in parts per million of CaCOs by mass? Number M CaCo, Number ppm CaCO,

Explanation / Answer

Solution-Uing formula

mmoles EDTA = Molarity EDTA x mL EDTA =

(0.0100)(11.40) = 0.114 mmoles EDTA

Here as we cab see EDTA reacts with Ca2+ in a 1:1 mole ratio, So

mmoles Ca2+ = mmoles EDTA =  0.114

Now,

Molarity Ca2+ = mmoles Ca2+ / mL Ca2+

=  0.114 / 50.00 = 0.00228 M Ca2+
x (1 mole CaCO3 / 1 mole Ca2+) = 0.00228 M CaCO3

(0.00228 moles CaCO3 / L) x (100 g CaCO3 / 1 mole CaCO3) x (1000 mg / 1 g) x
= 228 mg CaCO3 / L = 228 ppm CaCO3

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