Find the delta H for the following reaction...

...if 2.61g of dimethyl ether, CH3OCH3 is burned at a constant pressure and gives off 82.5kJ of heat.
CH3OCH3(l) + 3O2(g) -> 2CO2(g)+3H2O(l)

the answer is -1456 kJ/mol rxn, but how?

"gives off" 82.5kJ of heat,...
means that their small scale experiment had a
dH = - 82.5 kJ

use molar mass to find moles:
(2.61g of CH3OCH3) (1 mole CH3OCH3 / 46.07 grams) = 0.05665 moles of CH3OCH3

find dH per mole
(- 82.5 kJ) / (( 0.05665 moles of CH3OCH3 ) = -1456 kJ/mole