Calculate the mass in grams of Iron (III) Chloride in a solution of 500 cm^3 which has a concentration of 0.200 mol/dm^3

First work out the moles of Iron (III) Chloride using the equation concentration = moles/volume, rearranging the eqaution in order to calculate moles gives moles = concentration x volume. Check the units!! Before calculating all the units must match, in this question the volume is given in cm³ so it must be converted into dm³ by dividing by 1000. moles = 0.2 x 500/1000 = 0.1 moles of Iron (III) Chloride Now we have to find the M_r of Iron (III) Chloride so we can use the equation Mass = M_r x moles The (III) denotes that the Iron has an oxidation state of +3 and so must be cancelled out by a negative charge of -3. Chloride ions have a charge of -1 as they are in group 7. So 3 Chloride ions are needed to cancel out the charges so the formula is FeCl₃  A_r of Iron = 55.8; A_r of Chlorine = 35.5 Therefore the M_r of FeCl₃ = 55.8 + 3 x 35.5 = 162.3 Finally substitute into the equation to find the mass. Mass = 162.3 x 0.1 = 16.2 g to 3 s.f.