law of combining volumes (Gay-Lussac's law)

When gases react, they do so in a definite proportion by volume, if the volumes are measured at the same pressure and temperature. For example, in the reaction N2(g) + 3 H2(g) = 2 NH3(g), 3 liters of hydrogen will react with 1 liter of nitrogen to give 2 liters of ammonia if all volumes are measured at the same temperature and pressure.