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