I have previously posted this question, but now I am lost

Question

question:
A calorimeter is filled with 10.0 millimole (mmole) of methane gas and an excess of oxygen. When burned, the ignition wire releases 107.2 J of heat. The heat capacity of the calorimeter (including the bomb and water) is 4.319 kJ oC-1.
The initial temperature is 24.75 degrees celsius and the final temperature is 26.85 degrees celsius

Calculate ΔHcomb for methane and determine the thermochemical reaction for the combustion of methane

Answer 1

I thought I did this for you a couple days ago and found you made a typo on the moles (instead of millimoles) CH4. What's giving you trouble?

Answer 2

Oh so answer would be 907 J/mol?
also for the thermochemical equation you said it would be CH4 + 2O2 ==> CO2 + 2H2O with ΔH=- 890 kJ/mol but I don't know how to calculate that value

Answer 3

I think I remember that dH I found on the web was 890 kJ/mol for dHcomb. I used that to calculate how much CH4 I had to start with and that's when I discovered you probably made a typo with that 10.0 mols you posted then. This problem is
q for combustion is 4.319 kJ/mol x dT = 4.319 x (2.1) = 9.0699 kJ/10 mmols.
Then subtract the wire from that.
9.0699 kJ-0.1072 kJ = 8.96327 kJ/10 mmols or
8.9627/0.0100 = 896.27 kJ/mol CH4 which I would round to 896.3 kJ/mol.
Then delta H = -896.3 kJ/mol
I think you should have four places here but if your prof is picky about significant figures, that 10.0 actually only allows three and the answer should be rounded to 896. So if the problem was 10.00 millimols you have four places; if 10.0 you have only three allowed.

Answer 4

Ok I understand it now! Thank you so much!