### HW #7.47 part B

Posted:

**Mon Jan 19, 2015 11:00 am**Hi! The only part that I do not understand of this question is why we multiply by 5471KJ/mol. Where did this come from and why do we multiply by it to get the KJ/year? Thanks !

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=76&t=5041

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Posted: **Mon Jan 19, 2015 11:00 am**

Hi! The only part that I do not understand of this question is why we multiply by 5471KJ/mol. Where did this come from and why do we multiply by it to get the KJ/year? Thanks !

Posted: **Mon Jan 19, 2015 12:57 pm**

Hi,

In the question it states that the enthalpy of combustion of gasoline can be approximated by that of octane. Since we are trying to find the yearly energy we need a value of energy when we are computing the yearly energy. In problem 7.45 it states that the enthalpy of combustion for octane is -5471kJ. You can use this value, because the problem said to approximate the combustion of gas with that of octane.

Hope this helps.

In the question it states that the enthalpy of combustion of gasoline can be approximated by that of octane. Since we are trying to find the yearly energy we need a value of energy when we are computing the yearly energy. In problem 7.45 it states that the enthalpy of combustion for octane is -5471kJ. You can use this value, because the problem said to approximate the combustion of gas with that of octane.

Hope this helps.

Posted: **Thu Jan 22, 2015 7:52 pm**

For these kinds of problems, one way to check if you have the right answer is to see what units your answer is and see if it makes sense. In the context of this question, the units kJ/year would be most appropriate so you would try and get your final answer with that unit.