The Gas Laws

Robert Boyle was beginning his examination of the behavior of air. In addition to his numerous observations on the properties of air and speculations of the nature of chemical change, Boyle was the first person to use the term ” chemist ” to describe his activities.

Boyle used a simple apparatus to examine the relationship between pressure and volume.
In this apparatus, a small volume of air was trapped in a glass tube by mercury. A flexible tube connected this tube to a small reservoir of mercury that could be raised or lowered relative to the trapped gas volume.

The difference in height of the interior and exterior mercury levels is, of course, the pressure relative to the pressure of the atmosphere.

Boyle was able to demonstrate that,

V x P = constant

or that the volume and pressure are inversely proportional.

This relationship is called Boyle’s Law.

While Boyle observed that there was an additional relationship between temperature and volume he did not examine it in detail and it remained for the Frenchman, Jacques Charles (1746 – 1823) to report in 1787 that,

V = kT (k = constant)

or that volume is directly proportional to temperature.
This relationship is Charle’s Law.

Somewhat later on in 1848, the English scientist William Thompson, who was later raised to the Peerage as Lord Kelvin, recognized that a graph of volume vs. temperature for any gas resulted in a straight line that intersected the temperature axis at -273.15°C.
This temperature later became known as absolute zero and was used as the basis for an absolute temperature scale.

Temperature units corresponding to the Centigrade scale in this measure are called Kelvins.

Following in the French tradition, Joseph Lewis Gay-Lussac carried out a number of experiments on the properties and reactions of gases, resulting in his law of combining volumes that formed the basis for modern concepts of stoiochiometry.

Gay-Lussac’s ideas directly influenced the Italian, Amedeo Avogadro to propose that equal volumes of gases at the same temperature and pressures contained the same number of molecules.

Unlike the other ideas presented here, Avogadro’s ideas were more theoretical and (at the time) speculative, and were not accepted by the scientific community that was just then struggling with the implications of Dalton’s atomic theory.

Another way of stating Avogadro’s idea mathematically is to write V = kn (k = constant) where n is the number of molecules.

We now express n in terms of the unit “mole” which contains one Avogadro number of molecules, atoms or particles.

Avogadro’s number itself was not first measured until a student of Rutherford collected the helium formed by alpha particles from a sample of radium.

Knowing the pressure of the helium gas and the number of alpha decays over time of the sample, the student was able to obtain an estimate of Avogadro’s number.
Increasingly sophisticated experiments over the years have now set this number as 6.023 x 1023.

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