25.00-mL aliquots of the solution from Problem 1 are titrated with EDTA to the F

Question

25.00-mL aliquots of the solution from Problem 1 are titrated with EDTA to the Ferrochrome Black T end point. A blank containing a small measured amount of Mg^2+ requires 2.60 mL of the EDTA to reach the end point. An aliquot to which the same amount of Mg^2+ is added requires 28.55 mL of the EDTA to reach the end point. How many milliliters of EDTA are needed to titrate the Ca^2+ ion in the aliquot? How many moles of EDTA are there in the volume obtained in Part a? What is the molarity of the EDTA solution?

Explanation / Answer

How many millimeters of EDTA are needed to titrate the Ca2+ ion in this aliquot?

= 28.55 - 2.60 = 25.95 mL

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moles of solute = molarity of solute x L of solution

Since EDTA and Ca2+ react in a 1:1 mole ratio,
moles EDTA = moles Ca2+
moles Ca2+ in a 25.00-mL aliquot = M Ca2+ x L Ca2+ = (0.02045)(0.02500) = 0.0005113 moles Ca2+ = moles EDTA
Molarity of EDTA = moles EDTA / L EDTA = 0.0005113 / 0.02756 = 0.01855 M

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