Hey there!
We need to start this by finding the molar mass of (C₃H₅)₂S.
This molecule consists of 6 carbon atoms, 10 hydrogen atoms, and one sulfur atom. We need to add the molar masses of each atom up.
C: 6 x 12.01 = 72.06
H: 10 x 1.008 = 10.08
S: 1 x 32.07 = 32.07
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114.21 g/mol
Ok, now that we have established that the molar mass of (C₃H₅)₂S is 114.21 g/mol, we have 2.50 mol of (C₃H₅)₂S. If 1 mol has a mass of 114.21 g, then 2.50 mol will have a mass of 2.50 time 114.21.
2.50 x 114.21 = 285.525 grams
This should go to just three significant figures, so:
The mass of 2.50 mol of (C₃H₅)₂S is 286 grams.
Hope this helps!