# Question #b60b5

Nov 9, 2014

Using the data given, the heat of combustion would be 308 kJ/mol.

To calculate the heat of combustion, you must determine the amount of heat energy released and divide by the number of moles of ethanol combusted.

Heat:
The heat of combustion is used to raise the temperature of 800 g of water from 23 C to 31 C, or a difference of 8 C. The heat capacity of water is 4.184 J/g/C, so the total heat energy released is

$4.184 \frac{J}{g - C} \times 800 g \times 8 C = 26800 J = 26.8 k J$

The molar mass of ethanol is 46.07 g/mol. Therefore, the mole of ethanol combusted is

$\frac{4 g}{46.07 \frac{g}{m o l}} = 0.0868 m o l$

Finally, the heat of combustion of ethanol is calculated to be

$\frac{26.8 k J}{0.0868 m o l} = 308 \frac{k J}{m o l}$

HOWEVER! The actual heat of combustion of ethanol is 1370.7 kJ/mol so the data given in the problem are not correct. Most likely, the person who made up the problem forgot to convert from kcal/mol to kJ/mol because the heat of combustion in these units is 328 kcal/mol.