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Chemistry Hess's Law

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billyboy:
a. Write out the balanced chemical equation for the complete combustion of propene, C3H6 (g).

b. Given the following:

C3H6 (g) + H2 (g) --> C3H8 (g) ∆H ̊R = -124.3 kJ/mol

C3H8 (g) + 5O2 (g) ---> 3CO2 (g) + 4H2O(l) ∆H ̊R = -228.5 kJ/mol

H2 (g) + 1/2O2 (g) ---> H2O(l) ∆H ̊R = -285.5 kJ/mol


Calculate the heat of combustion for propene in kJ/mol.


I did the balancing and I got two different answers: 2C3H6 + 9O2 ---> 6CO2 + 6H2O and C3H6 + 9/2 O2 ---> 3CO2 + 3H2O

My issue comes in when it comes down to forming the equation with the others because every time I look at it the question seems impossible to me.

I also know that the final answer should be -67.3kj/mol which can be done by reversing the last chemical equation but the overall answer would not make sense.

Borek:
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