The molecular speeds of gaseous molecules are analogous to those of rifle bullets; why is then odour of the gaseous molecule not detected so fast?

No, doubt molecules in the gaseous state move almost with the speed of rifle bullets. But these molecules donot follow a straight path. Since they undergo collisions with one another at a very fast rate, the path followed is zig-zag. Thus, the odour cannot be detected at the same speed at which the molecules move.

Carbon dioxide is heavier than oxygen and nitrogen but is does not form the lower layer of the atmosphere. Explain.

The diffusion of the gases is quite independent of the gravitational pull by the earth. The molecules of carbon dioxide remain distributed throughout the atmosphere. Therefore, carbon dioxide does not form the lower layer of the atmosphere.

Out of dry air and wet air, which is heavier?

Dry air mainly contains gases like nitrogen and oxygen along with small proportions of the other gases. The wet air on the other hand, also contains a small amount of water vapours. Now, vapour density of water vapours is less than that of nitrogen and oxygen because molar mass of H2O (18) is less than that of N2 (28) and O2 (32). This means that heavier molecules or nitrogen and oxygen have been replaced by lighter molecules of water vapours when the dry air changes to we air. Therefore, dry air is heavier than wet air.

Can we apply Dalton’s law of partial pressure to a mixture of carbon monoxide and oxygen?

No, the law cannot be applied because these gases readily combine to form carbon dioxide. The law can be applied only to the non reacting gases.

Why a bottle of liquid ammonia is cooled before opening the seal?

If the seal is opened without cooling, the liquid ammonia suddenly vaporises and gushes out of the bottle with force. This may lead to serious accidents.