[Pirouette]

This one question has been driving me insane, and I cannot for the life of me figure out how I am getting the answer wrong every time. It's from my revision guide:

(1) Calculate the enthalpy of combustion of ethanol from the data below:

Mass of ethanol used = 1g
Mass of water = 100cm³
Temperature of water at start = 22°C
Temperature of water at end = 90°C

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As you can see it's a pretty easy question, but when I try to work it out, I always get it wrong. I know I should be using 'q=mcΔT' but I can't get it right. The book says the correct answer is 1313.8 kJ/mol.

I know it's an easy question. Help me, anyone? Rep if you do!

[Sinuhe]

Using the formula: q = (100 g * 4.19 J/g*K * (90-22) K) = 28.492 kJ (we're heating 100 g of water - the ethanol is burnt in an isolated compartment, so you don't include its mass here)
This is the energy released for 1 g of EtOH. But 1 g of EtOH is equivalent to 0.0217 mol, so the heat released per mole is 28492 J / 0.0217 mol = 1313 kJ/mol. (I don't think the number of significant figures in the answer is warranted though!)

[Pirouette]

Originally Posted by Sinuhe

Using the formula: q = (100 g * 4.19 J/g*K * (90-22) K) = 28.492 kJ (we're heating 100 g of water - the ethanol is burnt in an isolated compartment, so you don't include its mass here)
This is the energy released for 1 g of EtOH. But 1 g of EtOH is equivalent to 0.0217 mol, so the heat released per mole is 28492 J / 0.0217 mol = 1313 kJ/mol. (I don't think the number of significant figures in the answer is warranted though!)

Ah, I knew it was something not too difficult! Thanks very much, you get my rep for today!

[Sinuhe]

No problem, glad to be of help.