Answer
$H^o = -196.6kJ$
Work Step by Step
$H^o_f : SO_2(g) = -296.9 kJ/mol$
$H^o_f : O_2(g) = 0 kJ/mol$
$H^o_f : SO_3(g) = -395.2 kJ/mol$
$2SO_2(g) + O_2(g) --> 2SO_3(g)$
$H^o = [2*(-395.2)] - [2*(-296.9) + 0]$
$H^o = [-790.4] - [-593.8]$
$H^o = -196.6kJ$
