In part A, ratio of volume of hydrogen gas to that of nitrogen is nearly equal to 3 (actually in each of the three cases, the ratio is 2.94, 3.01 and 2.97. respectively). It indicates that the three molecules of hydrogen combine with one molecule of nitrogen to form one molecule of ammonia gas.
In part B, the ratio of the volume of hydrogen to that of oxygen is about 2 (actually in three cases, the ratio is 1.97, 2.00, 1.8 respectively. It indicates that two molecules of hydrogen combine with one molecule of oxygen to form one molecule of water.
From above, we find that the two gases combine chemically to yield another gas at the same temperature and pressure and the volumes of these gases bear simple ratio of small integers. This is the ** Lussac' law. We can also understand the law on the basis of atomic picture and Avogadro'* hypothesis.
