Page:Text-book of Electrochemistry.djvu/42

 CHAPTER III.

The Laws of Avogadro and van't Hoff.

Boyle's Law. — When a gas is contained in a vessel the volume of which can vary, as, for instance, in a cylinder with a movable piston, then, if the volume v be changed by moving the piston, the pressure^ changes in inverse proportion ; if the volume be changed to half what it originally was, the pressure is doubled. This law is expressed by the general formula : —

pv = constant.

Boyle proved this for pressures greater than 1 atmo., and Mariotte afterwards proved it for lower pressures.

Gay-Lussac's (Charles's) Law.— The above law is only applicable when the temperature of the gas remains constant. If the temperature rises, the product pv increases, as Gay- Lussac found, by ^^f 3 of its value at 0° for each Celsius degree. In other words, the product pv is proportional to the absolute temperature T —

pv = constant x T.

Avogadro's Law. — Avogadro showed that the constant in this formula was the same for all gases if a gram-molecule of the gas be taken. In the usual form of the equation —

2J0 = BT

It = 84688 when 2? is measured in grams per square centimetre and V in cubic centimetres.

This is found by considering 1 gmm-molecule of oxygen,

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