We are given the amount of the reactants to be used in the reaction. We use these as the starting values for the calculations. We calculate as follows:
50 g S ( 1 mol / 32.066 g ) = 1.56 mol S 105 g F2 ( 1 mol / 38.00 g ) = 2.76 mol F2
The limiting reactant would be F2 since it will be consumed completely in the reaction. The excess reactant would be S.
Mass of excess = 1.56 mol - 2.76(1/3) = 0.64 mol S excess