0.310 moles
First, look up the atomic weights of the elements involved.
Atomic weight carbon = 12.0107
Atomic weight hydrogen = 1.00794
Atomic weight sulfur = 32.065
Molar mass (C3H5)2S = 6 * 12.0107 + 10 * 1.00794 + 32.065
= 114.2086 g/mol
Moles (C3H5)2S = 35.4 g / 114.2086 g/mol = 0.309959145 mol
Since there's just one sulfur atom per (C3H5)2S molecule, the number of moles of sulfur will match the number of moles of (C3H5)2S which is 0.310 when rounded to 3 significant digits.
