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Charles' law states that, at a constant pressure, the volume of a mixed amount of gas is directly proportional to its absolute temperature:
= k 
= 
Why must the temperature be absolute? If temperature is measured on a Celsius (non absolute) scale, T can be negative. If we plug negative values of T into the equation, we get back negative volumes, which cannot exist. In order to ensure that only values of V≥ 0 occur, we have to use an absolute temperature scale where T≥ 0 . The standard absolute scale is the Kelvin (K) scale. The temperature in Kelvin can be calculated via T _{k} = T _{C} + 273.15 . A plot of the temperature in Kelvin vs. volume gives :
Avogadro's law states that the volume of a gas at constant temperature and pressure is directly proportional to the number of moles of gas present. It's mathematical representation follows:
fracVn = k 
1 mole (mol) of gas is defined as the amount of gas containing Avogadro's number of molecules. Avogadro's number ( N _{A} ) is
N _{A} = 6.022×10^{23} 
The numbers 22.4 L, 6.022×10^{23} , and the conditions of STP should be near and dear to your heart. Memorize them if you haven't already.
Charles', Avogadro's, and Boyle's laws are all special cases of the ideal gas law:
PV = nRT 
Units  Value of R  

0.08206  

8.314  

8.314  

1.987  

62.36 
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