The vapor pressure of various substances can be determined using effusion. In this process, the material of interest is placed in an oven (referred to as a Knudsen cell) and the mass of material lost through effusion is determined. The mass loss (Δm) is given by Δm = Z_cAm Δt, where Z_cis the collisional flux, A is the area of the aperture through which effusion occurs, m is the particle mass, and Δt is the time interval over which the mass loss occurs. This technique is quite useful for determining the vapor pressure of nonvolatile materials. A 1.00-g sample of UF₆is placed in a Knudsen cell equipped with a 100.-μm-radius hole and heated to 18.2°C where the vapor pressure is 100. Torr.

a. The best scale in your lab has an accuracy of ±0.01 g. What is the minimum amount of time you must wait until the mass change of the cell can be determined by your balance?

b. How much UF₆ will remain in the Knudsen cell after 5.00 min of effusion?

Calculation of the collisional flux proceeds as follows:

A = Ï€r² = Ï€(1.00 × 10^-4 m)² = 3.14 × 10^-8 m²

M = 0.352 kg mol ^-1 / N_A­ = 5.85 × 10^-25 kg 

Transcribed Image Text:

= 1.00 x 104 m 1m r = 100. Um 10 um 133.3 Pa 100. tor (27(5.85 x 10-25 1.33 x 10* Pa 1 torr kg)(138 x 10-23 J K- Z. (2rmkT)"² x2914 K))"² 1/2 Z. = 1.10 x 1026 m² 1.22 x 10-22 kg ms gm