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n a Knudsen cell, the eusion orice is measured to be 0.50 mm2. If a sample...

n a Knudsen cell, the eusion orice is measured to be 0.50 mm2. If a sample of naphthalene is allowed to euse for 1.0 hr at a temperature of 40.3 oC, the cell loses 0.0236 g. From this data, calculate the vapor pressure of naphthalene at this temperature. p = ___ Torr

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Answer #1

A Knudsen cell is used to determine the sublimation pressure of naphthalene. The cell is designed such that the mean free path of the escaping molecules is greater than both the diameter and the thickness of the hole. Now, when the rate of escape of molecules is calculated, it is directly related to the pressure in the cell.

In the cell the gas molecules undergo frequent collisions with one another and the wall. This cell is separated from vacuum by a thin wall with a hole in it. Now, the molecule which collides with this hole, escapes into vacuum. This phenomenon is called Effusion.

The rate of Effusion is calculated as:

Z' = g/At where,

g - decrease in mass of naphthalene,

A - area of the orifice

t - time taken by naphthalene to decrease by g amount

The rest is calculated below:


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