Question

A student added 25.0 uL of a solution of magnesium chloride to a microcentrifuge tube along with a color changing indicator, and titrated with 1.00 x 10−2 M EDTA to the endpoint. The color change occurred at 21.7 uL. What was the [Mg₂+]?

Answer 1

Mg2+ + EDTA4- → MgEDTA2-
umoles EDTA used = 21.7 ul x 1x10^-2 um/ul = 21.7x10^-2 = 0.217
umoles EDTA

umoles Mg^2+ present = 0.217 umoles <-- see stoichiometry ratio of 1:1
[Mg^2+] = 0.217 umol/25ul = 0.00868 umol/ul = 0.00868 M or 8.7 mM