638 TABLES 690-696.— KINETIC THEORY OF GASES * 



TABLE 690— PRESSURE AND NUMBER OF MOLECULES 



1. Units of Pressure 



An = normal atmosphere 



= ?60mmHg at 0°C and 45° latitude 

 = 1.01325 X 10° microbars 

 1 dyne cm" 2 = 1 microbar = 0.75 micron 

 1 micron = 10" 3 mmHg = 1.333 microbars 

 = 1a* 

 Pmm = pressure in mmHg 



Pli = pressure in microns = 10" 3 P mm 

 Ptib = pressure in microbars = 1.333 X 10~ 3 Pmm 



2. Number of molecules per unit volume 



For ideal gas, 



PV= RoT 

 Where V = volume per gram-molecular weight 

 P = pressure 



T = absolute temperature in degrees Absolute (°K) 

 = degrees Centigrade -f- 273.16 



For ideal gas at 0°C and A n — 1, 



V= V.= 22,414.6 cm 3 

 Hence R„ = 62.364 mm liter, deg" x K g mole" 1 

 = 8.3146 X 10 7 erg deg" 1 K g mole" 1 

 p — density of gas/g/cm 3 

 = 1.2027 X 10" 8 MPpb/T- 1 g cm" 8 

 = 1.6035 X 10- 5 MP r /r 1 gcm- > 

 Where M — molecular weight in grams 

 n = number of molecules per cm 3 

 = 7.244 X 10 1B Pfib/T 



= 9.656 X 10 18 Pmm/T 



3. The number of molecules per cm 8 for different temperatures and pressures 



7(°K) P^ Pmm tl T' P^ Pmm H 



273.16 1.0133X10 9 760 2.687XlO tB 298.16 1.333X10 3 1 3.240X10 10 



298.16 " " 2.462X10 10 273.16 1.000 7.50X10" 4 2.653X10" 



273.16 1.333 XlO 3 1 3.536X10 18 298.16 " " 2.430X10" 



* Prepared by Saul Dushman, General Electric Co. The formulae and calculations in this section are 

 based on a more comprehensive discussion in chapter 1 of his "Scientific Foundations of Vacuum Tech- 

 nique" (John Wiley & Sons, New York, 1949). 



TABLE 691.— MEAN FREE PATHS, L, MOLECULAR DIAMETERS, 5, AND 

 RELATED DATA FOR WATER AND MERCURY VAPORS* 



* For reference, see footnote 219, p. 636. 



** Pmm = vapor pressure at r°C. t N* = number of molecules/cm 2 for monomolecular layer. 



In the case of H 2 0, for which the values of L (path length) and 5 (diameter) for a series of temper- 

 atures are given in the table, the Sutherland relation was used with C = 650 and Tjus = 926X1 0~ s cgs 

 units. 



In the case of Hg the values of v (viscosity) used are based on 1 = 219. 4°C. Values at other tern- 



of Su 



peratures were derived by means of Sutherland's relations, with C — 942.2 



SMITHSONIAN PHYSICAL TABLES 



