Gas Volumes in Reactions

For a gas made in a reaction: the mole ratio gives its moles, and at room conditions $V = n \times 25\ \mathrm{L/mol}$.

2H₂O(l) → 2H₂(g) + O₂(g): $0.1\ \mathrm{mol}$ of water gives $2.5\ \mathrm{L}$ of H₂ and $1.25\ \mathrm{L}$ of O₂, $3.75\ \mathrm{L}$ in all.

Twice the moles, twice the volume

Count only the gases, marked (g). Equal moles of gas take equal volumes, so gas volumes follow the mole ratio.