Answer
See the answer below.
Work Step by Step
$CaO(s)+SO_3(g)\rightarrow CaSO_4(s)$
Since the heat of the reaction above is given, to determine the heat of formation of calcium sulfate the heat of formation of its reactants, calcium oxide and sulfur trioxide, are required.
From appendix L:
$CaO(s): -635.09, $
$SO_3(g)-395.77 (kJ/mol)$
Hence:
$-402.7=\Delta_f H_{CaSO_4}--395.77--635.09$
$\Delta_f H_{CaSO_4}=-1433.56\ kJ/mol$
