Conversion factors in physical chemistry. - Journal of Chemical

Conversion Factors in Physical Chemistry PAUL PORTER SUTTON North Carolina State College, Roleigh, North Carolina

I

N PHYSICAL chemistry, even a t .the elementary as needed without any attempt to tie them up with level, it . 1s . often necessary to treat special instances of previously acquired data. It seems well, therefore, to the following general problem: A given quantity has emphasize that the conversion factors most commonly magnitude a when measured in the system of Units I ; called for can all be evaluated by comparison of corwhat would be its magnitude x when referred to 11, rectly chosen values of the molar gas constant. The another system of units? This type of problem can be following compilation makes this point clear: disposed of by the discovery of a conversion factor k so 1 cal. = 82.06/1.987 = 4.103 X that 1.987 cal./mol. X deg. = 82.06 10'cm.' X atm. x = ka

(1)

said factor being dependent on the systems I and I1 in such a way that 1unit of I

=

k units of I1

I

1.987 cal./mol. X deg. = jaules/mol. X deg.

(2)

Many of these factors are hewn to all who enter the study of physical chemistry but others are much less familiar, e. g., 1 cal. = 41.03 X atm., a relation frequently needed in problems centering around the exact Clapeyron equation. Despite their importance for numerical work, even in the field's most recent new text,' such relations are simply to the student - .presented PRUTTON. C. F., AND S. H. MARON, "Fundamental Principles Company, New York,

of Physical Chemistry/ The 1944.

cm.' X atm./mol. X dee.

82.06 -,a

x atm./mol. x deg.

=

cm.'/mol. X deg.

cm.'

1 cm.=X atm. = 1.987182.06 =

1 cal. = 8.315/1.987 = 4.185 1 joule = 1.987/8.315 = 2.389 X 10-'cal. 1 atm. = 8.315 X 107/82.06 = 1.013 x ioe dynes/cm.a 1 dyne/cma = 82.06/8.315 X 10' = 9.868 X 10-'atm.

The author's experience has shown that the average student can readily master the method illustrated in the above table and that bv its a ~ ~ l i c a t i oton s~ecific problems there is acquired an increased appreciation of dimensional reasoning as well as greater proficiency in computations.

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