What mass of ethanol #C_2H_5OH(l)# must be burned to supply 500.0 kJ of heat?
Based on 100% ethanol (zero water content) 16.95 g.
The enthalpy of combustion of ethanol is widely quoted in the public domain and is 1360 kJ/mol.
Therefore in order to supply 500 kJ of heat you would need to burn (500/1360) = 0.368 moles of ethanol.
The molecular weight of ethanol is 46.07 g/mol, so the mass required would be 0.368 x 46.07 = 16.95 g.
This is based on 100% ethanol - in practise ethanol will have a certain water content, and this of course does not burn or generate heat. If you have a water content of, say, 2%, then the mass of ethanol required would need to be slightly increased to compensate for this.