1- Wood had no definite formula as the exact makeup varys between wood types. However it will mostly be comprised of Carbon, Hydrogen, Oxygen and Nitrogen as well as other elements.
2- If the stove is burning methane the formula would be CH4 + 2O2 -> 2H2O + CO2 other fuels will have similar reactions..
3- Air is approx 20% O2 and 80% N2 however it will contain traces of other gasses such as CO2 helium, neon NOx
4- For all intents and purposes water is H20 it may contain tiny amounts of minerals as well as dissolved gasses but its best off not worrying about them
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