Inorganic chemistry for advanced students / by Sir Henry Roscoe and Arthur Harden.

  • Henry Enfield Roscoe
Date:
1910
Catalogue details

Licence: In copyright

Credit: Inorganic chemistry for advanced students / by Sir Henry Roscoe and Arthur Harden. Source: Wellcome Collection.

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    EXPERIMENTAL DETERMINATION OF THE MOLECULAR WEIGHTS OF GASES AND VOLATILE LIQUIDS AND SOLIDS In order to determine the molecular weight, it is necessary, as we have just seen, to ascertain the density of the substance in the state of gas. It is therefore possible to determine the molecular weight of any substance which can be converted into a gas without undergoing decomposition. The method employed is in principle always the same ; the weight of a given volume of the gas or vapour is compared with the weight of an equal volume of air or some other gas of known density, measured under the same conditions of temperature and pressure. Practically somewhat different methods have to be used when the substance is a gas under ordinary conditions, and when it is a liquid or a solid. 1. Permanent Gases For this purpose it is only necessary to determine the specific gravity of the gas, air being usually taken as the practical unit of comparison. A large glass balloon, capable of holding from i to io litres of the gas, is employed, and is first of all freed from air as far as is possible by the vacuum pump and weighed. In weighing a body of such large volume, it is essential to make allowance for the buoyancy of the air, since the body to be weighed appears to be lighter than it really is by an amount equal to the weight of the air which it dis- places. This is best accomplished (Fig. 2) by suspending a vessel of similar size and shape to the other arm of the balance, by
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