SIMPLE EQUATION FOR CONVERTING VOLUMES TO STANDARD CONDITIONS HOMER E. STAVELY Iowa State College, Ames, Iowa
SIMPLE calculation shows that, a t any temperature, the volume of one cc. of gas a t standard conditions will decrease by an amount equal to (1/760)(273/T) for each mm. decrease in pressure. Calculation of this difference for ordinary laboratory temperatures is as follows:
A
Tm9erolurc 'C 18 I9 20 21 22 23 24 25
Differercr 0.001233 0.001229 o.001225 0.001221 0.001217 0.001213 0.001209 o.oo1206
No such simple relation exists for the change in volume per degree a t a given pressure: this difference changes with temperature, and will be equal to (p/760) ( 2 7 3 ) [ 1 / T ( T I ) ] . The maximum variation for this differencefor ordinary laboratory conditions is shown in the following table.
+
18'C. 25-C.
9 = 728 nun, 6 = 740 ==. 0.00315 0,00321 0.00298 0.00304
Taking the change in volume pff mm. pressure as 0.0012, and the change per degree as 0.003, the empirical equation may be derived:
vo = 0.9192 - 10-'[1.2(?60 - P) - 3(24 - t ) ] , where Vois the volume of one cc. of gas a t standard conditions measured a t g mm. pressure and 1°C. The constant 0.9192 is obtained by calculating the volume a t 24'C. and 760 mm. This equation will give accurate results withii 0.0002 cc. in the range 18' to 25' and 728 mm. to 755 mm. Its value lies in the fact that it can often be solved mentally, or a t least with very little arithmetical labor. For different ranges of temperature and pressure, other constants may be calculated, and a similar equation set up which will give accurate results within a limited range.
