[rawka]

So i am going over a pre exam that i took and i am trying to figure out how the answer is 44.1kJ.. here is a photo  of the question. if someone could help with breaking down how to get this answer id really appreciate the help.

[rawka]

i also found this on youtube but im still lost as to how it works.

https://www.youtube.com/watch?v=jJFkom86-xs

[sjb]

What is the energy evolved when one mole decomposes?

[rawka]

im just trying to figure out how this question works. like how i would get the answer of 44.1kj based on the info provided.

[Corribus]

Enthalpies of formation are given in kJ per mole. The question is asking you how much energy is evolved/absorbed when a certain fraction of a mole decomposes.

It's like if you go to the grocery store and a pound of cherries costs 5 dollars. How much money does it cost to buy half a pound of cherries?

Also, write out a balanced equation. This will always help.

[rawka]

Ok i get what you are saying but how does that play into this? Like im still confused ...

[Corribus]

The first thing to do is identify what the decomposition reaction is. Writing a balanced equation is the first step in just about any general chemistry problem. You can't do anything else until you know what the reaction in question is.

Most key parameters for reactions are given on a per mole basis. The question however wants you to report a total amount of energy absorbed for a certain number of moles of starting material. This is essentially just a unit conversion.

xxx kJ/mol * yyy mol = zzz kJ

[Borek]

Additional hint: Hess's law.

[rawka]

569.4(kJ/mol) - 452.0(kJ/mol) = 117.4(kJ/mol)
117.4(kJ/mol) * 0.35(mol) = 41.09kJ


instructor marked the correct answer as 44.1kJ....
What's wrong with this equation?