Lesson 7-7

Graham's Law

     Temperature is defined as - "the average kinetic energy of the particles of a substance."  Therefore, if two gases are at the same temperature, their particles must have the same average kinetic energy:

at equal temperatures - K.E. _{gas 1} = K.E. _{gas 2}

     The formula for Kinetic energy is:

K.E. = 1/2MV²

     Let us consider two gases, and see what this says about them.

Table 7-7a Comparison of two gas samples at equal temperature.
GAS	He	C3H8
Molar Mass	4.00 g	44.1 g
Temperature	T1 = T2
Kinetic Energy	K.E.1 = K.E.2
Therefore:	1/2m1V12 = 1/2m2V22
	m1V12 = m2V22
	V12     m2 ---- = ---- V22   m1
So:

     And so we get Graham's Law, which states - "The relative rates at which two gases under indentical conditions of temperature and pressure will diffuse vary inversely as the square roots of the molecular masses of the gases."  It shows us that, at equal pressure and temperature, less massive gases will diffuse more rapidly than more massive gases.

Example 1.  What is the ratio of the velocity of helium atoms to the velocity of radon atoms when both gases are at the same temperature?

Please forward all questions, comments and criticisms to Gregory L. Curran.
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Last Modified February 07, 2008