As a summary we can then make the following statement: The molecular weight of any gas may be determined by calculating the weight of 22.4 l. of the gas, measured under standard conditions.
Determination of molecular weights from density of gases. In an actual experiment it is easier to determine the density of a gas than the weight of a definite volume of it. The density of a gas is usually defined as its weight compared with that of an equal volume of air. Having determined the density of a gas, its weight compared with oxygen may be determined by multiplying its density by the ratio between the weights of air and oxygen. This ratio is 0.9046. To compare it with our standard for atomic weights we must further multiply it by 32, since the standard is 1/32 the weight of oxygen molecules. The steps then are these:
1. Determine the density of the gas (its weight compared with air).
2. Multiply by 0.9046 to make the comparison with oxygen molecules.
3. Multiply by 32 to make the comparison with the unit for atomic weights.
We have, then, the formula:
molecular weight = density × 0.9046 × 32;
or, still more briefly,
M. = D. × 28.9.
The value found by this method for the determination of molecular weights will of course agree with those found by calculating the weight of 22.4 l. of the gas, since both methods depend on the same principles.