Page:Outlines of Physical Chemistry - 1899.djvu/32

 m : m - d : d m : 2 = d : —

��12 OUTLINES OF PHYSICAL CHEMISTRY

Knowing the density of any gas with respect to air we get:

m, molecular weight of the gas

m, molecular weight of hydro- gen

d, density of the gas (with re- spect to air)

d, density of hydrogen (with

respect to air) = —^

Remark : If the density of the gas had been taken relatively to hydrogen, then d = 1, and we should have m = 2 d, that is, the molecular weight of a substance is equal to twice its vapour density relatively to hydrogen (taken as unity).

Correction of Molecular Weights

There must necessarily be a simple relation between the chemical equivalent, 1 the atomic weight and the molecular weight of an element. We can correct a molecular weight experimentally found by comparing it with the chemical equivalent, which is much more accu- rately known. For example: the density of chlorine is gram of hydrogen unites with exactly 35*87 grams of chlorine— this latter number is the equivalent of chlorine. As molecular weight of chlorine we adopt that multiple of 85*37 which approximates most closely to 70*44, the experimentally found value, that is, 2 x 85*37 = 70*74.

Molecular Weights of some Elements known in the

Gaseous State

M Temperature

Hydrogen ... 2 at 0°

Nitrogen. 2802 „ 0°

1 In the case of elements of uneven atomicity the equivalent is fre- quently equal to the atomic weight. For elements of even atomicity it is generally the half of the atomic weight. These relations, however, are not absolutely constant. We shall see later what is meant by the atomicity of an element.

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