Consider the reaction below.

Question

C3H8 + 5 O2  3 CO2 + 4 H2O
How many litres of CO2 will be produced when 10.0 L of O2 are completely
reacted with C3H8? Assume that the volumes of the reagents and products are
measured at constant pressure and temperature.

Answer 1

With all gases in the equation one may use a shortcut in which volumes are considered as mols.
Convert 10 L O2 to ? L CO2.
10 L O2 x (3 mols CO2/5 mols O2) = 10 x 3/5 = ? L CO2.