NOTES AND PROBLEMS 47million atoms of hydrogen and 2 million atoms of chlorine. There-
fore the molecules of hydrogen and of chlorine must each con-
tain at least 2 atoms.
Since 2 volumes of ammonia yield only 1 volume of nitrogen,
the same reasoning shows that the molecule of nitrogen must
contain at least 2 atoms.
Molecular Weights. The atomic weight of oxygen has been
settled by convention as 16, and since there are at least 2 atoms of
oxygen in a molecule, the molecular weight of oxygen must be at
least 32. Let us neglect the qualification " at least " and take the
molecular weight of oxygen as 32 without qualification, making
this our standard from which to reckon all other molecular and
atomic weights.
On this basis the molecular weight of any gaseous substance is
easily calculated if we know the weight of a measured volume
of that gas under any stated conditions. From this determination
the weight of 1 liter of gas under standard conditions can be com-
puted. Then, the weight of 1 liter of the gas is to the weight of
1 liter of oxygen as the molecular weight of the gas is to 32, the
molecular weight of oxygen.
Gram Molecular Weight; Mole. The molecular weight is an
abstract number although the molecular weights are related to each
other in the same ratio as the actual weights of the molecules.
The gram molecular weight is, however, a concrete quantity, being
the molecular weight number in grams. It is a quantity that is so
frequently spoken of that the name has been contracted to mole.
A mole of oxygen is 32 grams of oxygen.
Gram Molecular Volume or Molal Volume. One mole, or 32
grams of oxygen, under standard conditions occupies a volume of
22.4 liters. According to Avogadro's principle, the same volume
would contain exactly 1 mole of any other gas under standard con-
ditions; this volume, therefore, assumes a great importance, and it
is called the gram molecular volume or for brevity molal volume.
To find the molecular weight of any gaseous substance it is
necessary therefore only to find the weight in grams of 22.4 liters
of the gas under standard conditions. In actual practice it is not
convenient to weigh containers which hold 22.4 liters. It is only
necessary to know the weight of any volume and the conditions of
the experiment. From these data the molecular weight can be
calculated.