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

EDTA is a chelator that will bind to metal cations such as Mg2+. Each molecule of EDTA interacts with 1 Mg2+ ion.

If you used 21.3 uL of 0.01 M EDTA, you have 21.3 * 0.01 = 0.213 umol of EDTA, which means 0.213 umol of Mg2+ was bound. Since you started off with 25 uL of solution, this means the concentration of Mg2+ was 0.213 umol/25 uL = 0.00852 M.

NOTE: I didn't bother converting the uL to L, which is why I use umol instead of mol. A umol/uL is the same as a mol/L, because you've just multiplied both units by a factor of 1000.