What's wrong with my heat of combustion calculations? (1 Viewer)

[.ben]

Hi, below are my calculations for the heat of combustion of ethanol. Can you tell me what's wrong wtih them? i get 1922.8 which is way off what the book says which is 1350. can you help me? thanks

 

[Riviet]

I think it's the m in the equation. This m is not the mass of the fuel burnt, rather it is the volume or mass of water burnt in ml. According to Excel, which is where I presume you got the data from, it says to use 100ml of water. However, I worked it out with that value and got a way off answer as well.

 

[.ben]

o so m is not mass of alkanol burnt??!?!

 

[pLuvia]

Nope, the m is the mass of the water you used, I think 100ml = 100grams?

 

[Riviet]

That's right. 1g of substance is equivalent to 1ml of it.

Edit: this strictly applies for water only, as pointed out by Tennille.

 

[.ben]

O! that was the mistake, thanks guys.

edit: ok i did it again with the mass of water at 0.1 L and i got 417.9999999 kJ/mol whcih is way off 1350??

 

[tennille]

That's right. 1g of substance is equivalent to 1ml of it.

But that only applies for water, which is the case for this question.

 

[pLuvia]

O! that was the mistake, thanks guys.

edit: ok i did it again with the mass of water at 0.1 L and i got 417.9999999 kJ/mol whcih is way off 1350??

Considering most of the heat energy was lost to the surroundings, beaker, gauze etc. You would expect the heat of combustion to be much lower than the actual result

 

[.ben]

ok, but that much? wow

 

[Just Some Guy]

Shouldn't m be in in grams, not litres?

 

[.ben]

Really i've always thought it to be litres???

 

[pLuvia]

It depends what the HOC unit is. I.e. if the specific capacity of water is 4.18JK^-1L^-1x10³ or 4.18JK^-1g^-1

 

[.ben]

Ok then