Calculating help please :D (1 Viewer)

[Dubble-u25]

1g of petrol produces 45.8g of energy. Given that the ▲H of combustion of ethanol is -1360kJ/mol, calculate the mass of ethanol required to produce the same amount of energy as 1g of petrol.

I have no idea what to do :$

 

[nightweaver066]

Grams of energy?

 

[Dubble-u25]

I have no idea, that's what the question said... i guess it could be a typo?

 

[Kimyia]

I thought I recognised that question...I had that in my chem prelim exam. Yeah, it was a typo. It was meant to be kJ of energy, not grams.
So you know 1g of petrol gives out 45.8kJ of energy and you want to same for ethanol but all your given is 1 mol of ethanol = -1360kJ/mol. So what do you do? Work out the number of moles of ethanol you need so 45.8/1360 = 0.03368 moles. Now you know moles = mass/molar mass. So 0.03368 = mass/12+12+6+16 -> molar mass of ethanol. Rearrange to find mass of ethanol which is 1.55g
Fun question eh?

 

[Dubble-u25]

Thanks plus repped
No wonder I couldn't do it and lol it was in my friend's prelim paper too...

 

[Kimyia]

Thanks plus repped
No wonder I couldn't do it and lol it was in my friend's prelim paper too...

No problemo. And thanks for the rep. Yeah, I remember people freaking out in the exam at the grams typo. How did your friend go in the paper?

 

[Dubble-u25]

She did well considering she had a terrible teacher for chem. I don't think that anybody noticed the typo, even the teacher.

 

[Kimyia]

She did well considering she had a terrible teacher for chem. I don't think that anybody noticed the typo, even the teacher.

Our teacher didnt notice until marking but it was a guessing game as to whether we should use kJ or J in the exam I'm glad your friend did well.