The idea here is that when kept under constant pressure and temperature, the volume of gas is directly proportional to how many moles of gas it contains
This means that for a chemical reaction that involves gases kept under the same conditions for pressure and temperature, the mole ratios given by the stoichiometric coefficients become equivalent to volume ratios.
#color(blue)(|bar(ul(color(white)(a/a)n_1/n_2 = V_1/V_2color(white)(a/a)|))) ->#the mole ratio is equivalent to a volume ratio
So, the balanced chemical equation that describes the formation of hydrogen chloride from hydrogen gas and chlorine gas looks like this
#"H"_ (2(g)) + "Cl"_ (2(g)) -> color(red)(2)"HCl"_ ((g))#
Since all the gases are kept under the same conditions for pressure and temperature, this is equivalent to saying that
Since you must produce
#"volume of Cl"_(2(g)) = color(green)(|bar(ul(color(white)(a/a)color(black)("1 L")color(white)(a/a)|)))#