If the heat of combustion for a specific compound is −1290.0 kJ/mol and its molar mass is 46.73 g/mol, how many grams of this compound must you burn to release 661.60 kJ of heat?

Answer 1

Hi there Basia! The heat of combustion is just the amount of heat let off when you burn a compound. We are looking for the mass of compound needed to produce 661.60 kJ of heat. Since we want our answer in grams, we should convert the heat of combustion from kJ/mol to kJ/g using the compound's molar mass:

-1290.0 kJ/mol × (1 mol)/(46.73 g) = -27.6054 kJ/g

Generally when heat is released in a reaction (the reaction is "exothermic"), we write it as a negative value, as was done in giving -1290.0 kJ/mol for the heat of combustion. When heat is an input into the reaction it's written as a positive value. In this case, we are looking to release 661.60 kJ of heat, so:

-661.60 kJ = -27.6054 kJ/g × M g

where M is our unknown mass of the compound. Divide by -27.6054 kJ/g to get:

M = -661.60 kJ × (1 g)/(-27.6054 kJ) = 23.97 g

So, you'll need to burn 23.97 grams of the mystery compound to release 661.60 kJ of heat!

Answer 2

661.60 kJ (1 mol / -1290.0 kJ) (46.73 g/ mol) = 24.00 g

See video for detailed explanation! :)