Grahams Law

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Grahams Law
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Grahams Law applies to effusion and diffusion.
Effusion refers to the escape of gas particles
through a pinhole in the side of a container into
a vacuum, such as a gas in a tank on a satellite
leaking into space.
Diffusion refers to the movement of gas particles
from an area of greater concentration to an area
of lesser concentration through another gas or
gases.
The rate of effusion or diffusion is
proportional to the velocity if the molecules
(the root mean square velocity).
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Grahams Law is derived from the equation for
root mean square velocity (urms (3RT/(MM))½.
The Grahams Law equation compares the rates of
diffusion (or of effusion) of two gases to each
other. The equation is
(
)
1/2
Ratea Rateb
(MM)b (MM)a

Note that the molar mass are inverted relative to
the rates, I.e. rate of diffusion of a is in the
numerator on the left side, but molar mass of a
is in the denominator on the right. This makes
sense because the lighter the particle is (the
smaller its mass is), faster (larger) its
velocity and rate of diffusion will be.
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For homework, print out and complete Grahams
Law Worksheet Practice Problems for this
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