Answer
$S(s) + \frac{3}{2}O_2(g) --> SO_3(g)$
$\Delta H ^o _f (SO_3(g))= -395.2 kJ/mol$
Work Step by Step
1. Write the unbalanced equation:
$S(s) + O_2(g) --> SO_3(g)$
2. Balance it:
$S(s) + \frac{3}{2}O_2(g) --> SO_3(g)$
3. The $\Delta H^o$ values can be founded in the book's final pages.
$\Delta H ^o _f (SO_3(g))= -395.2 kJ/mol$
