Avogadro law and reactions in a gaseous phase [closed]








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When $\pu{100 mL}$ of hydrogen combines with $\pu{50 mL}$ of oxygen, why is $\pu{100 mL}$ of water produced and not $\pu{150 mL}$? As $100+50=150$


I understand that $\ce{2 H2 + O2 -> 2 H2O}$, when we balance the equation. But it is [Avogadro's Law](https://en.wikipedia.org/wiki/Avogadro%27s_law) that confuses me here.




The Avogadro law says by other words that


*At the same pressure and temperature, the number of gas molecules in the given volume doesn't depend on type of molecules.*


As there are created 2 molecules from 3, the final volume (at the same conditions as initial ones) shrinks to 2/3 of the initial one (assuming the ideal gas behaviour of water vapor with pressure low enough and temperature high enough to prevent vapor condensation).


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$100$ mL gas A + $50$ mL gas B gives $150$ mL total gas if there has not been any chemical reaction between the molecules of the gases, whatever the nature of the gases A and B. If there has been a chemical reaction between them, the volume is not conserved. Only the mass is conserved.


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