FREEZING-POINT and DomNo-Potxr.
The direct and indirect osmotic-pressure meth ods just referred to are of the greatest practical importance to the chemist. For while the number of substances that can he vaporized (and hence whose molecular weights can be determined by the vapor-density method) is limited. practically all substances can be obtained in solution.
ell ENI ICA I. :METHODS:. It is often possible to determine the molecular weight of a substance by purely chemical methods. i.e. by studying its formation and chemical transformations. Con sider. for example, acetie acid. Analysis leads us to assign to it one of the following formulas: (rlTsnl, C,11,0.. C,11,0„ etc., corre,prinilint; to the moloeular weights. 30. 611, 90. 120. etc.
IS Cu M IsTRY.) The fact that acelie acid is readily formed from ordinary alcohol. whose molecule cannot possibly contain less than two carbon atoms (beeanse for every two carbon atoms it eontains a single atom of oxy gen) this fact renders it probable that the molecule of arctic arid, too. contains two carbon :atoms. In other words. it becomes probable that C..11,11, is the formula, and hence 60 the molecu lar xNeight. of acetic acid. That C112) cannot be the formula of acetic acid is shown by the fact that the molecule of silver ace tate, a compound made from acetic acid, cannot possibly contain less than two carbon atoms (because for every two carbon atoms it contains a atom of silver). To sum up, the mo
lecular weight of acetic :1c-id is prolmbly and at least GO. More exact infonnation chemical meth ods cannot furnish. The vapor-density Dietlithi leads to the definite conclusion that the molecu lar weight is 60 (the vapor-density of acetic acid is 30). It is because the molecular weights found by physical methods (i.e. on the basis of Avo gadro's rule) are invariably found to be in per fect agreement with chemical facts, that the molecular theory of the physicist can be, and is, inseparable from the atomic theory of the chem ist, the two theories widening each other's scope of usefulness, and together forming a powerful instrument for the study of nature.
See CHEMISTRY. and consult the literature of theoretical and physical chemistry recomniended in that article. See also Avot:Amio's RULE: ATOMIC BOILING-POINT; FREEZING POINT; SOLUTION; GASES. GENERAL PROPERTIES OF.