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Chemistry. — - ''On the osmotic pressure of solutions o f non-electro- 

 lytes, in connection ivith the deviations from the laws of ideal 

 gases." By J. J. van Laar. (Coniiininicated by Prof. H. W. 

 Bakhuis Roozeboom.) 



Communicated in the meeting of Ai)ril 27, 1906). 



1. Fiy H. N. Morse and J. C. W. Frazer ^) very accurate 

 experiments were recently made on the determination of the osmotic 

 pressure of dilute sugar solutions in water. The solutions had a 

 concentration up to I -normal, and as c is then about V34 [the 

 association factor of the water is viz. at 18° C. about 1,65, so that 

 in 1 L. of water about 55,6 : 1,65 = 34 Gr.mol. of water (simple 

 and complex molecules) are present], the difference between the 

 exact expression — log (1 — x) and the approximate value x [formula 

 (2)] is not yet appreciable. It is however not so with the difference 

 between the molecular volume of the solution v := (1 — x) t\ -\- xt\ 

 {v supposed to be a linear function oïx, about which more presently) 

 and the molecular volume of the solvent i\, when i\ (the molecular 

 volume of the dissolved sugar) cannot be put equal to i^j. We shall see 

 that this difference for 1-normal solutiojis amounts to 19%, so that by 

 means of the experiments we can very well ascertain, if we have to 

 make use of v or oi' i\. And these have really taught us, that the osmotic 

 pressures measured agree (and even with vevy great accuracy) Avitli 

 the calculated values, o?ilg when r^^ is put in the numerator, and 

 not V. This harmonizes therefore perfectly with what I have repeatedly 

 asserted since 1894 ^). (What I have called above v^ for the sake of 

 symmetry, was formerly always indicated by v^). Not tiie molecular 

 volume therefore of the whole solution, but the molecular volume 

 of the solvent in the solution. And this deprives those of their last 

 support, who in spite of all evidence (for not the dissolved substance, 

 but the solvent brings about that pressure) persist in trying to explain 

 the osmotic pressure by a pressure of the molecules of the dissolved 

 substance comparable with the gas pressure. If such a thing could 

 be thought of, v should be taken into consideration and not v^, for 

 the molecules of the dissolved substance move in the whole volume v 

 and not in the volume Vj, which is perfectly fictitious with regard 



1) Amer. Ghem. - Jouin. 34, 1905, p. 1—99. See also t.be extensive abstract 

 N-' 274 in the Phys. Chem. Gentralblatt III (1906). 



2) See inter alia my previous paper on this subject in These Proceedings, May 

 27, 1905, p. 49. (Some remarks on Dr. Ph. Kohnstamm's last papers). 



