Methods of Determining the Equivalents of the Elements Of Ascertaining their Molecular Weights Allotropy
The meaning of the word "equivalent" (symbol: eq or Eq), sometimes termed the molar equivalent is a unit of amount of substance used in chemistry and the biological sciences.
The equivalent is formally defined as the amount of a substance which will either:
1) react with or supply one mole of hydrogen ions (H+) in an acid–base reaction; or
2) react with or supply one mole of electrons in a redox reaction.
We shall now consider how the equivalent of an element may be determined. As already stated, some compound of the element is analysed, prefer ably one with hydrogen, oxygen, or chlorine, and the weight of the element which is in combination with, or which replaces 8 parts by weight of oxygen, is termed the equivalent of the element. But it is seldom that a direct method of estimating the equivalent can be practiced, for it is not always possible to obtain a compound of the element with hydrogen, or to deprive its oxide of oxygen, or its chloride of chlorine. In fact, each element has to be specially studied, and a method devised which will lead to the required information. It is, above all, necessary that the compounds dealt with shall be pure that is, that they shall not contain any other elements than those which it is desired to estimate, and that their composition shall be definite. For instance, if it were desired to find the equivalent of barium by estimating the proportion of chlorine in its chloride, it would be essential to obtain barium chloride free from the very similar elements calcium and strontium, and it would also be of the first importance to make sure that in weighing the chloride, the specimen should be free from water adhering to the powdered substance. 63
Methods of Determining the Equivalents of Elements
It is not always necessary to determine both constituents of the compound ; for example, the ratio of silver to chlorine can be found by dissolving a known weight of pure silver in nitric acid, and then adding to the solution some soluble chloride, such as hydrogen chloride ; silver chloride is then precipitated thus :
AgNO 3 . Aq. + NH 4 CLAq. AgCl + NH 4 NO 3 . Aq.
The silver chloride is collected on a filter, thoroughly washed, and after being dried, weighed. The Belgian chemist, Stas, working in this way, obtained from 108.579 grams of silver 144.207 grams of silver chloride. The relation between the atomic weight of oxygen, taken as the standard and placed equal to 16, and the formula weight of silver chloride was ascertained by heating to redness 138.789 grams of silver chlorate, 2AgClO 3 = 2AgCl + 3O 9 ; the weight of the residual silver chloride was 103.9795 grams, and that of the oxygen evolved taken as difference is 34.8095. The proportion
Oxygen Silver chloride Q Formula weight lost. Remaining. Of AgCl.
34. 8095 : 103.9795 :: 48 : i43-3 8l 7
gives the formula weight of silver chloride. The pro portion of silver it contains is found by the equation
144. 207 : 108.579 :: i43-3 8l 7 : T O7-95 8 3
Subtracting from 143.3817 the weight of the silver it contains, 107.9583, the remainder is the atomic weight of chlorine, which, for reasons already given, is identical with its equivalent, namely, 35.4234; and 107.96 is the equivalent of silver.
Knowing these facts, the atomic weight of, say, barium may be determined by dissolving a known weight of its chloride in water, and adding to the solution a solution of silver nitrate, so as to obtain a precipitate of silver chloride, which can be weighed, and from it the weight of the chlorine in the barium chloride deduced. Sub tracting this from the weight of the barium chloride taken, the remainder is the equivalent of barium. To determine whether or not this number is identical with its atomic weight, a determination of its specific heat must be made.
In some instances the process is a more direct one. To determine the equivalent of nickel, a weighed quantity of the metal has been heated in oxygen, and the gain in weight noted. Then, as this weight is to the weight of nickel taken, so is the equivalent of oxygen to that of nickel.
These examples will suffice to give a general idea of the processes used in determining atomic weights, though, as before stated, each element requires special treatment, and the selection of the best method is often a very difficult task. It is usual, moreover, to make determinations by several methods, if that be possible, so as to avoid any permanent source of error. Many observers, too, have made such determinations, and it is not always easy to eliminate a personal element from the results which they give. A committee of the German Chemical Society has recently published a table of atomic weights, reproduced below (with a few alterations and additions), in which the last digit of each number may in all probability be accepted as correct. A second column is added, containing the atomic volumes of the elements, so far as they are known. They represent the volumes in cubic centimeters occupied by the atomic weight of the element taken in grams, thus 197.2 grams of gold occupy 10.2 cubic centimeters. As the elements expand on rise of temperature, these results are not always comparative, but at present they are the best that can be obtained.
Table of Atomic Weights and Atomic Volumes
Atomic Atomic. Weight. Volume.
Aluminium . Al 27.1 10. I
Antimony. . Sb 120 17.9
Argon . . A 39.9 32.9
Arsenic . . As 75 13.3
Barium . . . Ba 137.1
Beryllium . . .Be 9.1 4.3
Bismuth . . Bi 208.5 2T.2
Boron . . . B n.o 4.1
Bromine . . Br 79-96 25.1
Cadmium . . . Cd 112 13.0
Caesium , . Cs 133
Calcium . . Ca 40 25.3
Carbon . . C 12.00 3.4
Cerium . . Ce 140 20.8
Chlorine . . . Cl 35-45
Chromium . . Cr 52.1 7.7
Cobalt . . .Co 59.0 6.7
Copper . . . Cu 63.6 7.1
Erbium . . . Er 166 ?
Fluorine . . F 19
Gadolinium . . Gd 156
Gallium . . . Ga 70 u.8
Germanium . . Ge 72
Gold . . . Au 197.2 10.2
Helium . . . He 4
Hydrogen. . . H 1.007
Indium . . .In 114 2 57
Iridium . . Ir I 93c 8.6
Iodine . . .1 126.85 25.7
Iron . . . Fe 56.0 6.6
Krypton . . . 81.5 37.8
Lanthanum . . La 138 22.9
ATOMIC WEIGHTS 67
Atomic Atomic
Weight. Volume.
Lead . . . Pb 206.9 18.2
Lithium . . .Li 7.03 11.9
Magnesium . . Mg 24.36 13.3
Manganese . . Mn 55.0 7.7
Mercury . . . Hg 200.3 T 4*8
Molybdenum . . Mo 96.0
Neodymium . . Nd 143.5
Neon . . . Ne 20
Nickel . . Ni 58.7 6.7
Niobium . . . Nb 94 14.5
Nitrogen . . N 14-04
Osmium . . Os 191 8.9
Oxygen . . . O 16.000 (standard).
Palladium . . . Pd 106 9.3
Phosphorus . . P 31.0 17.0
Platinum . . . Pt 195.2 9.1
Potassium. . . K 39 I 4 455
Praseodymium . . Pr 141
Rhodium . . . Rh 103.0 9.5
Rubidium . . . Rb 85.4 56.3
Ruthenium . . Ru 101.7 9.2
Samarium . . . Sm 150
Scandium . . Sc 44
Selenium . . . Se 79.1 18.5
Silicon . . .Si 28.4 11.4
Silver . . . Ag 107.93 10.3
Sodium . . . Na 23.05 23.7
Strontium . . Sr 87.6 34.5
Sulphur . . . S 32.06 15.7
Tantalum . . . Ta 183 17.0
Tellurium. . . Te 127.6 20.3
Thallium . . . Tl 204.1 17.2
Thorium . . . Th 232 29.8
Thulium . . . Tu 170?
Tin . . . . Sn 1 19.0 16.2
Atomic Atomic. Weight. Volume.
Titanium . . . Ti 48.1
Tungsten . . . W 184 9.6
Uranium . . U 240 13.0
Vanadium. . .V 51.2 9.3
Xenon . . .X 128 35.9
Ytterbium. . . Yb 173
Yttrium . . . Y 89
Zinc . . . Zn 65.4 9.5
Zirconium . . Zr 90.6 21.9
1. International Union of Pure and Applied Chemistry. "equivalent entity". Compendium of Chemical Terminology Internet edition.
2. International Union of Pure and Applied Chemistry (1998). Compendium of Analytical Nomenclature (definitive rules 1997, 3rd. ed.). Oxford: Blackwell Science. ISBN 0-86542-6155. section 6.3.
