We are provided with the amount of energy released when one mole of carbon reacts. We mus first convert the given mass of carbon to moles and then compute the energy released for the given amount.
Moles = mass / atomic mass
Moles = 23.5 / 12
Moles = 1.96 moles
One mole releases 394 kJ/mol
1.96 moles will release:
394*1.96
= 772.24
The enthalpy change of the reaction will be -772.24 kJ
Answer:

Explanation:
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In this case, according to the given chemical reaction, it possible for us to set up the expression for the calculation of the enthalpy change as shown below:

Thus, given the values of the enthalpies of formation on the attached file, we obtain:
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Answer is: 1) formula mass.
Formula weight is given in atomic mass units (amu).
The unified atomic mass unit (amu) is a standard unit of atom mass.
Atomic mass of carbon atom is 12.01 amu.
Atomic mass of hydrogen atom is 1.01 amu.
Atomic mass of bromine atom is 79.9 amu.
In chemical compound C₃H₆Br₂ there are three carbon atoms, six hydrogen atoms and two bromine atoms.
M(C₃H₆Br₂) = 3 · 12.01 amu + 6 · 1.01 amu + 2 · 79.9 amu.
M(C₃H₆Br₂) = 201.89 amu; formula mass of one molecule.