Writing down the entire equation with electrons included will help solve the problem:
#FeCl_3(aq) + 3e^(-) rarr Fe(s) + 3Cl^(-) (aq) #
There are three electrons on the left side because the iron in iron trichloride has an oxidation state of #+3#, and it gets reduced to an oxidation of zero in its elemental form. The three electrons balance the opposing charges on both sides. Notice how for every mole of #FeCl_3# is reduced, three moles of electrons are required.
Besides the #FeCl_3# to electron mole ratio, you need to know the molar mass of #FeCl_3# which is 162.195g g/mol. (You can do this this using the periodic table.)
Now, using a T-chart, you can solve the problem.
#100g FeCl_3 * ((1 mol FeCl_3) / (162.195g))* ((3 mol e^-)/ (1 mol FeCl_3))#
Multiplying it all out, you will receive an answer of #1.8496#. Checking the units, you will notice that grams #FeCl_3# cancel out in the first conversion, and moles #FeCl_3# cancel out in the second conversion, leaving just moles electrons.
