THE MEASUREMENT OF GASES. MOLECULAR HYPOTHESIS 47 



273 



^ = 95 c.c. This correction enables us to compare all volumes 



\jL 



of gases as if they had been measured at the same standard tem- 

 perature, namely at 0. 



Corrections for Pressure and Temperature Combined. 



Since the volume changes, due to alterations in pressure and 

 in temperature, are independent of one another, the corrections 

 may be made either separately or together. The latter is more 

 convenient. Thus, a sample of gas occupies 190 c.c. at 17 and 

 750 mm., what will be the volume under standard conditions, 

 namely and 760 mm. pressure? 



970 yen 



New volume = 190 X X = 176.6 c.c. 



Correction for the Tension of Aqueous Vapor. When 

 a gas is measured over mercury, the latter gives off practically 

 no vapor at room temperature, and the foregoing are the only 

 corrections required. If, however, the sample of gas is stand- 

 ing over water, then the volume is not that of the gas, but that 

 of the gas plus a certain amount of water vapor. The latter 

 must be subtracted. To do this we have to remember that, 

 in a gaseous mixture, each one of the several gases (or vapors) 

 exercises the same pressure as if it were present alone (Dalton's 

 law of partial pressures). Now the pressure of the water vapor, 

 in a gas standing over water, at each temperature is known (see 

 p. 62 and Appendix IV). It varies from 13.5 mm. at 16 to 

 23.5 mm. at 25. When, therefore, the gas is measured over 

 water, the pressure of the water vapor is subtracted from the 

 barometric reading, before the above-mentioned corrections are 

 applied. 



For example, a specimen of a gas, standing over water, occupies 

 175 c.c. at 19 and 752 mm., what is the volume of the same gas 

 at and 760 mm. when the gas is free from water? At 19 



