Molecular mass determination according to Dumas

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About the method according to Dumas can be similar to the method by Victor Meyer the molar mass of a vaporizable liquid be determined. Today, however, the method only has a didactic significance for science teaching, e.g. B. chemistry classes , physics or chemistry studies .

Basics

Using the general gas equation for ideal gases , the molar mass can be calculated from the mass and the volume of the evaporated liquid.

If this equation is solved for the molar mass M , then:

Here m is the mass of the substance, R the general gas constant , T the temperature of the gas volume in Kelvin , p the pressure of the gas volume and V the volume of the gas.

With the temperature T in Kelvin , the pressure p in Pascal and the volume V in m 3 . The mass m can be given in g or kg, accordingly the unit for the molar mass M is g / mol or kg / mol.

Procedure

A glass flask , which has an extended glass tip , is weighed and then placed in a heating bath, e.g. B. water , slightly heated. The flask is then removed from the heating bath and the glass tip is dipped into the liquid to be examined. As it cools, the liquid is drawn into the flask. Now the flask is immersed in the heating bath again until the liquid has completely evaporated. Then the glass tip is melted shut. After removing from the heating bath, the flask is cooled to room temperature, dried and weighed. The flask is then immersed in water with the fused glass tip and the tip is broken off, with the flask now being almost completely filled with water due to the negative pressure (the vaporized substance is again condensed). The exact volume of water drawn into the flask is determined from the density of the water by reweighing the flask.

The following calculation steps are necessary for the evaluation:

  • The volume of the flask and thus the volume V of the gaseous substance corresponds to the volume of the water in the flask.
  • The mass of the empty piston results from the difference between the piston weighed at the beginning and the mass of the air in the piston, which is calculated from the density of the air (taking pressure and temperature into account) and the volume.
  • The mass m of the substance results from the difference between the piston after melting and the empty piston.

The air pressure that prevailed during the melting process is taken as the pressure p . The temperature T is that of the heating bath in Kelvin.

Experimental challenges and variants of the process

One experimental challenge is to determine the optimal point in time for sealing the piston: This should take place exactly when no more steam is escaping from the piston. The escape of the steam should be visible as light streaks in the air at the outlet opening . The piston may only be closed after this streaking has stopped. As long as there is still liquid in the flask, the temperature in it is not higher than the boiling point of the sample substance. The disappearance of all liquid can be difficult to see if the piston wall is well wettable and no drops form. When all the liquid has evaporated, the steam still has to be warmed to the temperature of the bath. As long as the steam is heated further, some of it will escape from the flask and it should not be closed, otherwise the mass is too high. But as soon as no more steam escapes, the steam mixes with the ambient air at the opening. If you wait too long, the mass measured is too low because of the dilution with air. Inexperienced experimenters can therefore obtain molecular weights with an error of 10%. In order to determine the correct sealing time, an improvement in the method has therefore been proposed, in which the actual temperature in the evaporation flask is measured with the aid of a thermocouple. Molecular weights were thus obtained in a student laboratory that deviated from the true value by less than 4% in 80% of the tests.

Melting off the piston enables complete closure with the lowest possible piston weight, but has a cost disadvantage, since a new piston must be used for each attempt. Instead, pistons with Teflon taps can be used; however, these are heavier, which makes the exact mass determination somewhat less precise due to the difference formation. A simpler and inexpensive, but also relatively imprecise method uses Erlenmeyer flasks , which are sealed with aluminum foil, in which a small hole is punched. An acceptable result can thus be achieved for cyclohexane . To determine the volumes of the Erlenmeyer flasks, a water filling is measured with a measuring cylinder. This is faster than weighing, but also less accurate.

Historical

The Avogadro's law , on which the determination is based on Dumas was of Amedeo Avogadro been postulated 1811th In his dissertation in 1832, Jean-Baptiste Dumas reported on his new method for determining vapor density, with which he had determined the vapor densities of mercury , phosphorus and sulfur . The two reviewers of Von Dumas' work, Louis Joseph Gay-Lussac and Louis Jacques Thénard , explain in their report that, according to the vapor densities obtained from Dumas, hydrogen sulfide contains only one sixth of the amount of sulfur in sulfur vapor and hydrogen phosphide only a quarter of the amount of phosphorus in phosphorus vapor. The latter corresponds to what is expected for white phosphorus P 4 from today's perspective . At the temperature used by Dumas, the sulfur in the gas phase consisted mainly of S 6 molecules, while the sulfur crystals at room temperature are made up of ring-shaped S 8 molecules. The different molecular sizes, even in the case of elements, initially led to difficulties that were gradually clarified, especially by Stanislao Cannizzaro . Nevertheless, Dumas' method quickly established itself as the standard method for determining molar mass.

literature

  • Wolfgang Gottwald, Werner Puff: Physico-chemical internship. VCH, Weinheim 1987, ISBN 3-527-26498-1 , p. 31 ff.
  • H. Böhland: Chemische Schulexperimente Volume 5: General, physical and analytical chemistry. Verlag Harri Deutsch Thun, Frankfurt 1979, ISBN 3-87144-358-1 , p. 33 ff.

Web links

Individual evidence

  1. ↑ a b Julie J. Kaya, J. Arthur Campbell: Molecular weights from Dumas bulb experiments . In: ACS (Ed.): Journal of Chemical Education . tape 44 , no. 7 July 1967, ISSN  1938-1328 , pp. 394-395 , doi : 10.1021 / ed044p394 .
  2. ^ A b c d Douglas J. Grider, Joseph D. Tobiason, Fred L. Tobiason: Molecular weight determination by an improved temperature monitored vapor density method  . In: ACS (Ed.): Journal of Chemical Education . tape 65 , no. 7 July 1988, ISSN  1938-1328 , pp. 641-643 , doi : 10.1021 / ed065p641 .
  3. ↑ Ed DePierro and Fred Garafalo: Some Insights Regarding a Popular Introductory Gas Law experiment . In: Journal of Chemical Education . tape 82 , no. 8 , August 1, 2005, ISSN  1938-1328 , p. 1194-1196 , doi : 10.1021 / ed082p1194 .
  4. ↑ Lorenzo Romano Amadeo Carlo Avogadro: Essai d'une manière de déterminer les masses relatives des molécules élémentaires des corps, et les proportions selon lesquelles elles entrent dans les combinaisons . In: Journal de physique, de chimie, d'histoire naturelle et des arts . tape 73 , 1811, pp. 58–76 ( digitized in the Google book search).
  5. ↑ Jean-Baptiste Dumas : Sur la Densité de la Vapeur du Phosphore . In: Joseph Louis Gay-Lussac, François Arago (ed.): Annales de chimie et de physique . tape 49 . Crochard, Paris 1832, p. 210–214 ( online at Gallica Bibliothèque nationale de France [accessed March 1, 2016]).
  6. ^ Jean-Baptiste Dumas: Dissertation on the densité de la vapeur de quelques corps simples . Thèse présentée à la faculté des sciences de Paris. Thuau, Paris July 1832, OCLC 493226550 ( online [accessed February 27, 2016]).
  7. ^ Jean-Baptiste Dumas: Dissertation on the densité de la vapeur de quelques corps simples . In: Joseph Louis Gay-Lussac, François Arago (ed.): Annales de chimie et de physique . tape 50 . Crochard, Paris 1832, p. 170–181 ( online at Gallica Bibliothèque nationale de France [accessed February 27, 2016]).
  8. ^ Louis Joseph Gay-Lussac , Louis Jacques Thénard : Rapport sur un Mémoire de M. Dumas, ayant pour titre: Sur la Densité de La Vapeur de quelques corps simples . In: Louis Joseph Gay-Lussac, François Arago (ed.): Annales de chimie et de physique . tape 50 . Crochard, Paris 1832, p. 178–181 ( online at Gallica - Bibliothèque nationale de France [accessed March 4, 2016]).
  9. ↑ Horst Remane : Jean-Baptiste André Dumas. Retrieved February 27, 2016 .