You may wish to review the Laws of Thermochemistry and Endothermic and Exothermic Reactions before you begin.
Hydrogen peroxide decomposes according to the following thermochemical reaction:
H2O2(l) → H2O(l) + 1/2 O2(g); ΔH = -98.2 kJ
Calculate the change in enthalpy, ΔH, when 1.00 g of hydrogen peroxide decomposes.
The thermochemical equation tells us that ΔH for the decomposition of 1 mole of H2O2 is -98.2 kJ, so this relationship can be used as a conversion factor. Using the Periodic Table, the molecular mass of H2O2 is 34.0, which means that 1 mol H2O2 = 34.0 g H2O2.
Using these values:
ΔH = 1.00 g H2O2 x 1 mol H2O2 / 34.0 g H2O2 x -98.2 kJ / 1 mol H2O2
ΔH = -2.89 kJ
The change in enthalpy, ΔH, when 1.00 g of hydrogen peroxide decomposes = -2.89 kJ