# What is the heat of formation of water vapor?

If you look in the back of your textbook (the appendices), you should find that it is $\Delta {H}_{f}^{\circ} = - \text{241.8 kJ/mol}$. Make sure you look for $\text{H"_2"O} \left(g\right)$ under oxygen-based substances, not $\text{H"_2"O} \left(l\right)$.
$\text{H"_2(g) + 1/2"O"_2(g) -> "H"_2"O} \left(g\right)$