The molecular mass of sulfur is 256 u.
As you might have guessed this is a boiling point elevation problem. The formula for boiling point elevation is
##T_b = K_bm##
We need to look up the values of ##T_b^o## and ##K_b## for CS. They are
##T_b^o## = 46.2 C and ##K_b## = 2.37 Ckg/mol
##T_b = T_b^o T_b## = (49.48 46.2) C = 3.28 C
We can use this information to calculate the ##m## of the solution.
##m = (T_b)/K_b = (3.28 C)/(2.37 Ckgmol)## = 1.38 molkg
We have 100.0 g or 0.1000 kg of CS. So we can calculate the moles of sulfur.
Molality = ##moles of [solute](http://socratic.org/chemistry/solutions-and-their-behavior/solute)/kilograms of [solvent](http://socratic.org/chemistry/solutions-and-their-behavior/solvent)##
Moles of solute = molality kilograms of solvent = 1.38 molkg 0.1000 kg = 0.138 mol
So 35.5 g of sulfur = 0.138 mol
Molar mass of sulfur = ##(35.5 g)/(0.138 mol)## = 257 g/mol
The experimental molecular mass of sulfur is 257 u (The actual molecular mass of S is 256 u).