Question #cafad

The idea here is that the enthalpy change of combustion for this compound in given in kilojoules per mole, which means that you have a pretty good idea of what the enthalpy change of reaction will be when one mole undergoes combustion.

More specifically, you know that the combustion of one mole of your compound gives off, hence the minus sign, #"1160.0 kJ"# of heat.

So, you know how much heat is given off per mole, which means that you can determine how many moles must undergo combustion in order to release #"427.20 KJ"# of heat

#427.20color(red)(cancel(color(black)("kJ"))) * "1 mole"/(1160.0color(red)(cancel(color(black)("kJ")))) = "0.3683 moles"#

Since you know the compound's molar mass, which tells you what the mass of one mole of the substance is, you can determine how many grams would contain that many moles

#0.3863color(red)(cancel(color(black)("moles"))) * "84.45 g"/(1color(red)(cancel(color(black)("mole")))) = color(green)("32.62 g")#

Therefore, when #"32.62 g"# of the compound undergoes combustion, #"427.20 kJ"# of heat are being released, which is equivalent to saying tha tthe enthalpy change of reaction in this case will be

#DeltaH_"rxn" = -"427.20 kJ"#

Remember, the minus sign is just there to symbolize heat lost.