Question

Hydrocarbons containing halogens are very useful molecules in synthetic chemistry. Calculate S° for the reaction product in the equation shown below if ΔS° for the reaction is -213.0 J K-1.
C2H4(g) + HI(g) --> C2H5I(l) (for HI(g) S°=206 J/mol/K)

Answer 1

I looked up in my text So for C2H4(g) (200.8 J/mol/K) since you didn't include that in your post. You should use the number in your text/notes and not the one I've used.
C2H4(g) + HI(g) --> C2H5I(l)
dSrxn = (n*So products) - (n*So reactants)
dSrxn = (n*So for C2H5I) - [(n*So for C2H4) + (n*So for HI)]
-213.0 = (1*So for C2H5I) - [(1*200.8) + (1*206)]
Solve for So for C2H5I. Post your work if you get stuck.