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Calculating the amount of released energy.

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mjc123:
In a, what you have shown is the burning of 2 moles of H2. Therefore the enthalpy of combustion is 241.8 kJ/mol, and E = 120.9 MJ.
The answer for b looks correct, but it has far too many sig figs.

Enthalpy:
I don't see the enthalpy of formation of Syntin anywhere in the enthalpy of combustion. It should be there. +133kJ/mol (Syntin is nicely endothermic).

(The enthalpy H is an energy. It's the one used in such computations. "Energy" isn't accurate enough. But for high school, this difference is normally ignored.)

500 moles H2 produce 500 moles H2O. -241.8kJ/mol is the enthalpy of formation for gaseous water at 298K. You compute here the lower calorific power of the fuel. Condensed water releases more heat, which defines the higher calorific power. And at a different temperature, the result would differ too.

Syntin is a rocket fuel, hence burned with oxygen. At that price, I don't imagine torching it in a stove. But in a rocket, the hot combustion would produce much CO, so computing with CO2 is quite artificial.

We can guess your symbols Q, n, m... but they are no universal convention.

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