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water, which when vaporised, and measured under the same conditions of temperature and pressure, occupy 4 unit volumes. In other words, the number of molecules, in all cases * where gases and vapours are concerned, represent exactly the volumetric relations. In the cases quoted, it will be observed, the same ratio also subsists between the number of atoms of the reacting gases and the molecules of the compound, but this is not always the case, for example
Atomic equation, Hg + 2Cl = HgCl,. In this equation 3 atoms unite to produce i molecule, but the ratio between the volumes is not represented by the statement, I volume of mercury vapour and 2 volumes of chlorine produce 2 volumes of vapour of mercury chloride.
Molecular equation, Hg + Cl, = HgClz. By this we see that i molecule + (2 unit volumes) of mercury vapour and i molecule (2 unit volumes) of chlorine give i molecule (2 unit volumes) of vapour of mercury chloride. Again,
P + 3C1 = PCI; is an atomic equation, showing that i atom of phosphorus unites with 3 atoms of chlorine ; but it is not true that the ratio between the volumes is represented by the statement, i volume of phos. phorus vapour combines with 3 volumes of chlorine and gives 2 volumes of the vapour of phosphorus trichloride, as will be seen by comparison with the molecular formulæ
PA + 6C1, = 4PC13. This equation tells us that I molecule † (2 unit volumes) of phosphorus vapour combines with 6 molecules (12 unit volumes) of chlorine, producing 4 molecules (8 unit volumes) of phosphorus trichloride vapour.
Knowing the relative densities of gases compared with hydrogen, it is obviously possible, by ascertaining the actual weight in grammes of some definite volume of hydrogen, to calculate the actual weight of any given volume of any other gas.
Two units are in common use, namely
* See Dissociation, where apparent exceptions are explained. + The atomic volume of mercury vapour being equal to 2 unit volumes (p. 44). * The atomic volume of phosphorus is .5 of a unit volume (p. 44).
(1.) The weight of 1 litre of hydrogen, measured at a temperature of o° C., and under a pressure of 760 mm. of mercury.*
(2.) The volume occupied by i gramme of hydrogen, measured under the same conditions.
I. One litre of hydrogen, measured at the standard temperature and pressure, weighs .0896 grammes.t This number is known as the crith;I and by means of it the weight of 1 litre, and therefore any given volume, of any gas can be deduced : thus, the relative densities of oxygen, nitrogen, and chlorine are 16, 14, and 35.5 respectively, therefore i litre of these gases (measured always at the standard temperature and pressure) weighs 16 criths, 14 criths, and 35.5 criths respectively, or
I litre of oxygen weighs 16 X.0896= 1.4336 grammes.
1 . chlorine u 35.5 *.0896= 3.1808 » So also with reference to compound gases, where in each case the density is represented by the half of the molecular weight. Thus, the relative densities of hydrochloric acid, ammonia, and carbon dioxide are
Hcı’+35.5 = 18.25,
and the weights of i litre of these gases are therefore
I litre of hydrochloric acid=18.25 X.0896=1.6352 gramme. I , ammonia = 8.5 X.0896=0.7610 I carbon dioxide = 22.0 x.0896=1.9712 » II. The volume occupied by i gramme of hydrogen at the standard temperature and pressure is 11.165 litres. As the relative density of oxygen is 16, it obviously follows that 16 grammes
* This temperature and pressure is chosen as the standard at which volumes of gases are compared. See General Properties of Gases, chapter ix.
+ From time to time slightly different values have been given for this constant. The most recent determinations give the number .08988.
I From the Greek, signifying a barley-corn, and used symbolically to denote a little weight.
of this gas will also occupy 11.165 litres ; in other words, this number 11.165 represents the volume in litres of any gas, which will be occupied by the number of grammes corresponding to its relative density, thus
14 grammes of nitrogen . . occupy 11.165 litres.
chlorine . . , 11.165 ,
hydrochloric acid 11.165 , 22.0 , carbon dioxide. 2 11.165 , The number of grammes of a substance, equal to the number which represents its molecular weight, is spoken of as the grammemolecule. The molecular weight of hydrogen=2, therefore the gramme-molecule of hydrogen (that is, 2 grammes of hydrogen) will occupy 11.165 X 2= 22.33 litres. The molecular weight of oxygen=32, therefore 32 grammes of oxygen will occupy 22.33 litres ; in other words, 22.33 litres is the volume which will be occupied by the gramme-molecule of any gas.
By means of this important constant, 22.33, the volume of any, or all, of the gaseous products of a chemical change (when measured at the standard temperature and pressure) can be deduced directly from the equation representing the change, thus
Zn + H250,=ZnSO4+H, expresses the reaction taking place when zinc is dissolved in sulphuric acid. Just as in the former illustrations it carries the information that 65 grammes of zinc +98 grammes of sulphuric acid produce 161 grammes of zinc sulphate and 2 grammes of hydrogen. But 2 grammes of hydrogen occupy 22.33 litres, therefore by the solution of 65 grammes of zinc, the volume of hydrogen obtained will be 22. 33 litres.
So also in the following equation, which represents the formation of carbon dioxide from chalk (calcium carbonate) by the action upon it of hydrochloric acid
CaCO3 + 2HCl = CaCl2 + H.,0 + CO2 40+ 12 +48 2(1+35.5) 40+71 2+ 16 12+ 32
100 + 73 = 11 + 18 + 44 100 grammes of chalk, when acted upon by 73 grammes of hydrochloric acid, yield iii grammes of calcium chloride and 18 grammes of water, and 44 grammes of carbon dioxide.
Carbon dioxide is gaseous, therefore 44 grammes (the gramme
molecule) will occupy, at the standard temperature and pressure, 22.33 litres ; hence, by the decomposition of 100 grammes of chalk, 22.33 litres of carbon dioxide are produced.
This chapter may be concluded with one illustration of the methods employed in the exact determination of atomic weights which depends essentially upon the quantitative character of chemical reactions. By the three following processes the atomic weights of chlorine, potassium, and silver may be deduced.
1. By heating a known weight of potassium chlorate, the formula weight of potassium chloride is found
KCIO, = KCl + 30. 50 grammes of potassium chlorate when heated left a residue of potassium chloride weighing 30.395 grammes. 50 – 30.395 = 19.605 = grammes of oxygen evolved.
As potassium chlorate contains in its formula weight 3 atoms of oxygen (16 x 3 = 48), we get the expression, 19.605 : 30.395 = 48 : 74.40=formula weight of potassium chloride.
2. By dissolving a known weight of potassium chloride, and adding to it excess of silver nitrate, silver chloride is precipitated, which can be washed and dried and weighed, and from which the formula weight of silver chloride is obtained
KCI + AgNO3 = AgCl + KNO3. 10 grammes of potassium chloride were found to yield 19.225 grammes of silver chloride ; therefore, 10 : 19.225 = 74.40 : 143.03 = formula weight of silver chloride.
3. By the direct combination of silver and chlorine, by heating the metal in a stream of the gas, the ratio of chlorine to silver in silver chloride is found :
10 grammes of silver so treated yielded 13.285 grammes of silver chloride ; therefore,
13.285 : 10 = 143.03 : 107.66 = atomic weight of silver. Since the formula weight of silver chloride, AgCl = 143.03, .
therefore, 143.03 - 107.66 = 35-37 = atomic weight of chlorine. And since the formula weight of potassium chloride, KCI = 74.40,
therefore, 74.40 – 35.37 = 39.03 = atomic weight of potassium.
VALENCY OF THE ELEMENTS
WHEN chlorine unites with hydrogen, the combination takes place between one atom of chlorine (relative weight = 35.5) and one atom of hydrogen (relative weight = 1); but when oxygen combines with hydrogen, one atom of oxygen unites with two atoms of hydrogen. The compound ammonia consists of one atom of nitrogen, combined with three atoms of hydrogen ; while one atom of carbon, on the other hand, can unite with four atoms of hydrogen.
One atom of chlorine never combines with more than one atom of hydrogen ; its affinity for that element is satisfied, or saturated, by union with one atom.
The affinity of one atom of oxygen for hydrogen, however, is not satisfied by one atom of that element, but requires two atoms for its saturation ; while nitrogen requires three, and carbon four hydrogen atoms, in order to satisfy their respective affinities for this element.
This varying power of combining with hydrogen is seen in a number of other instances : thus, the elements fluorine, bromine, and iodine, resemble chlorine in being only able to unite with one atom of hydrogen. Sulphur, like oxygen, has its affinity for hydrogen saturated by two atoms of that element. Phosphorus and arsenic require three atoms of hydrogen in order to saturate their combining capacity, while silicon resembles carbon in combining with four hydrogen atoms. This combining capacity of an element is termed its valency. Elements like chlorine, Auorine, bromine, and iodine, whose atoms are only capable of uniting with one atom of hydrogen, are called monovalent (or sometimes monad) elements; while those whose atoms combine with two, three, or four hydrogen atoms, are distinguished as di-valent (or dyad), tri-valent (or triad), and tetra-valent (or tetrad) elements. All elements, however, are not capable of