Page:Text-book of Electrochemistry.djvu/26

I. GRAM-EQUIVALENT. 9

Gram-equivalent. — In electrochemistry the equivalent weights of the various substances play an important part; and on this account we find the term "gram-equivalent" very often applied. By a gram-equivalent of zinc we mean 32.7 grams of this metal; a gram-equivalent of a substance whose equivalent weight is E, is E grams. The idea of an equivalent (and consequently also equivalent weight) can be applied not only to chemical elements and those substances which occur as ions, i.e. can be separated at the electrodes, but also to all compounds which can react chemically with these. By a gram-equivalent of carbon dioxide is meant that quantity which unites with a gram-equivalent of lime to form a gram-equivalent of calcium carbonate.

Gram-molecule. — Even more important in chemistry than the equivalent weight is the molecular weight. The methods for the determination of the molecular weight of dissolved substances, which plays a most important part in all branches of chemistry, will be described later; molecular weights are only relative values being referred to that of hydrogen as equal to 2 (or, more exactly, 2.016), or to that of oxygen as 32. Here we make use of the term "molecular weight" in the sense in which it is always applied in chemistry. Thus, for example, the molecular weight of hydrochloric acid is 36.46, and consequently 1 gram-molecule of this (HCl) is 36.46 grams, that is, the equivalent weight in grams; a gram-molecule of sulphuric acid is 98 grams, i,e. twice the gram-equivalent.

A gram-molecule of aluminium chloride (AlCl₃) is 133.5 grams, and one of ferric chloride (FeCl₃) is 162.3 grams; these, therefore, are three times the corresponding equivalent weights. Eecently the term "mol" has been introduced for gram-molecule.

Just as we speak of a gram-molecule, so may we also speak of a gram-ion. One gram-ion of chlorine signifies 35.45 grams of chlorine in the ionic condition (${\overline {CI}}$) ; a gram-ion of ${\overline {SO}}_{4}$ weighs 96 grams (96 being the sum of the atomic weights). In the same way a gram-atom of an element is