Page:Text-book of Electrochemistry.djvu/27

10 INTRODUCTION. CHAP.

its atomic weight expressed in grams (1 gram-atom of chlorine (Cl) is 35.45 grams).

Concentration. — In theoretical chemistry it is convenient to express the composition of a solution, not by the absolute weight of the dissolved substance, but by the number of dissolved gram-molecules. The concentration is then expressed by the number of gram-molecules per unit of volume (the litre), and a solution which contains 1 gram-molecule in the litre is said to be "1 normal" (1n) or simply "normal." A 0.1 normal solution of, for instance, hydrochloric acid, contains only 0.1 gram molecule, or 3.645 grams per litre: in a litre of normal sulphuric acid there are 98 grams of H₂SO₄. Use is frequently made of equivalent-normal solutions, that is solutions containing 1 gram-equivalent per litre. Thus an equivalent-normal solution of sulphuric acid contains 49 grams of sulphuric acid in the litre. In order to avoid confusion, this latter solution is denoted as $n\times {\frac {1}{2}}H_{2}S0_{4}$.

By "normal" is generally meant molecular-normal.^[1] Such a method of expressing the concentration has the disadvantage that it is not the same for one particular solution at all temperatures and pressures, since the volume of the solution changes slightly with variation of these factors. On this account the expression of concentration in percentage by weight may be preferable. However, the volume changes caused by variation of temperature and pressure, especially of aqueous solutions, are very inconsiderable. In practice the normality is determined at the ordinary temperature (+ 18° C.) and pressure (1 atmo.), and the value so obtained is used also for other temperatures and pressures. In more accurate work it is necessary to correct for the change of volume.

Temperature. — In scientific work all temperatures are registered in Celsius (or centigrade) degrees. In many cal- culations, particularly those used in the mechanical theory of heat and its applications, it is advisable to take as zero-

1. In analytical practice equivalent-normal solutions are used.