|Chemical reactions which consumed
and produced gases were studied carefully by many chemists at the beginning
of the nineteenth century. In 1809, the French chemist Joseph-Louis Gay-Lussac
summarized the results of many experiments into what we now call Gay-Lussac's
law of combining volumes:
|When measured under the same conditions of temperature and pressure,
the volumes of gases which react together are in the ratio of small whole
|The measurements of volume made in 1809 were sufficiently accurate
to show that the volume relationships were in fact integers. For example,
one volume of hydrogen reacts with one volume of chlorine to produce one volume
of hydrogen chloride; two volumes of hydrogen react with one volume of oxygen
to produce two volumes of water vapor; and three volumes of hydrogen react
with one volume of nitrogen to produce three volumes of ammonia.
|The law of combining volumes was interpreted by the Italian chemist
Amedeo Avogadro in 1811, using what was then known as the Avogadro hypothesis.
We would now properly refer to it as Avogadro's law:
|Equal volumes of gases under the same conditions
of temperature and pressure contain equal numbers of molecules.
|Avogadro's interpretation was not accepted for some forty years, during
which confusion prevailed in distinguishing atoms from molecules. Elemental
gases were assumed to be monatomic. We now know that most of the common gaseous
elements actually exist as diatomic molecules: hydrogen, nitrogen, oxygen,
fluorine, and chlorine. Avogadro's interpretation cleared up many of these
discrepancies; it enabled explanation of the empirical results of Gay-Lussac
in terms of simple molecular reactions:
| H2(g) + Cl2(g)
--> 2 HCl(g)
2 H2(g) + O2(g) --> 2 H2O(g)
3 H2(g) + N2(g) --> 2 NH3(g)
|The Avogadro law is equivalent to the statement that volume is directly
proportional to number of atoms or molecules. Since the fundamental unit of
amount of substance, the mole, is equal to Avogadro's number of atoms or
molecules, the amount of substance n, in moles.
|In order to make proper calculations the following standards have
been instituted world wide.
|STP: Standard Temperature and Pressure ( 0oC
and 1 atmosphere )
1 mole of any gas at STP has a volume of 22.4 L
|But not many laboratories around the world operate at a temperature
of 0oC. A more conventional standard has been introduced
and is gradually being accepted.
|SATP: Standard Ambient Temperature and Pressure (
20oC and 1 atmosphere )
1 mole of any gas at SATP has a volume of 24.8 L