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# Enthalpy change of combustion watch

1. Hi, just started OCR Salters B and I'm finding it tough/weird already (partly due to being used to Edexcel science). Anyhow:

I've gotten stuck on how to calculate the 'Enthalpy change of combustion' for Methanol - I just have no idea what I'm meant to do as I'm supposed to give an answer in kJ/mol...

Formula of fuel: CH3OH
Mass of 1 mole of fuel (is therefore): 32g
Mass of fuel burned off: 1.07g
Therefore number of moles used (1.07 / 32): 0.03

Energy transferred by this number of moles of fuel: 8230.42J
(98.45 x 20 x 4.18) *We measured the mass in g of water + 20 was the temp change*
Energy transferred by 1 mole: 274347.33J (1 / 0.03 * 8230.42)
Enthalpy change of combustion: ? kJ/mol

What do I do/where have I gone wrong?!

Thanks
2. I reckon this is where:

The units used for the mass of the water (and the temperature change) has to cancel out the units used in the specific heat capacity of water to leave alone a unit of energy.

If I recall correctly the specific heat capacity of water is 4.18...KJ/Kg so the mass of the water should be expressed in Kg

Edit: no, what you have done is not wrong. 4.18 KJ/Kg is the same as 4.18 J/g so I guess you just have to divide by 1000 to give energy transferred in KJ.

You should just divide your answer by 1000 to give it in KJ/mol

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