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 21.3 uL. What was the [Mg2+]?

Answer 1

Mg^2+ + EDTA^4- -> MgETDA^2-

number µmoles of EDTA used = 21.3 µl x 1 x 10^-2 M = 21.3 µl x 1 x 10^-2 µm/µl = 21.3 x 10^-2 = 0.213 µmoles EDTA

µmoles Mg^+2 present = 0.213 µmoles because the stoichiometry ratio of the above equation is 1:1

[Mg^+2] = 0.213 µmol / 25 µl = 0.00852 µmol/µl = 0.00852 M = 8.52 x 10^-3 M

The answer is 8.52 x 10^-3 M