If the decomposition of 1.00 mole of a substance that has a molar mass
of 40.0 g/mol absorbs 60.0 kJ of energy, how much energy would be
absorbed by the decomposition of 20.0 g. of the substance?
a. 15.0 kJ
b. 1200 kJ
c. 60.0 kJ
d. 30.0 kJ

Hi teatea!!

If the decomposition of 1.00 mole of a substance that has a molar mass
of 40.0 g/mol absorbs 60.0 kJ, this means that the Heat of formation
(or Enthalpy of reaction) for this substance is 60 kJ/mol.
1 mol of this substance weights 40g and we have 20g such is 0.5 moles
of the substance. Then by simple calculation we conclude that the
energy absorbed by the decomposition of 20.0 g. of the substance will
be 30 kJ.

For references about this topic see the following pages:
"Enthalpies of Reactions":
http://www.science.uwaterloo.ca/~cchieh/cact/c120/heatreac.html

"Thermochemistry":
http://server.chem.ufl.edu/~itl/2045_s99/lectures/lec_8.html

"Heat of Formation and Hess’s Law":
http://www.westminster.net/faculty/allan/Ch07/03_Notes_Calculations.doc


Hope this helps, if you need a clarification, please post a request
for it.

Regards.
livioflores-ga