Butane gas reacts with oxygen gas to give carbon dioxide gas and water vapor (gas). If you mix butane and oxygen in the correct stoichiometric ratio, and if the total pressure of the mixture is 392 mmHg, what is the pressure (in mmHg) of water vapor after the reaction has completed (temperature and volume do not change).

118.776 mmHg

Explanation:

The equation of the reaction is;

C4H10(g) + 13/2 O2(g) > 4CO2(g) + 5H20(g)

Now the mole ratio  according to the balanced reaction equation is;

1 : 6.5 : 4 : 5

Hence, the total number of moles present = 1 + 6.5 + 4 + 5 = 16.5 moles

Mole fraction of water vapour = 5/16.5 = 0.303

We also know that;

Partial pressure= mole fraction * total pressure

Partial pressure of H20(g) = 0.303 *  392 mmHg = 118.776 mmHg

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your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.