Write the balanced equation-
C6H6 + Cl2 --> C6H5Cl + HCl
1 mol C6H6 reacts with 1 mole Cl2 to yield 1 mole of C6H5Cl
Calculate the moles of each reactant-
36.8 g C6H6 / 78.1 g/mol = 0.471 mol C6H6
36.9 g Cl2 / 70.9 g/mol = 0.520 mol Cl2
0.471 mole C6H6 requires 0.471 mole Cl2. You have excess Cl2 so C6H6 is limiting.
0.471 mole C6H6 will yiels (at 100%) 0.471 mole C6H5Cl
0.471 mol x 112.5 g/mol = 53.0 g C6H5Cl
38.8 g actual / 53.0 theoretical = 0.732 (73.2% yield)