SOLUTIONS 1 5 



contain an equal number of molecules. Under similar 

 conditions, the molecular weights of different substances have 

 therefore the same ratio as the weights of equal volumes of 

 their vapours. Hence if we fix arbitrarily the molecular 

 weight of any one substance, the molecular weight of all 

 other substances is thereby determined. The molecular weight 

 of hydrogen has been arbitrarily fixed as two, and hence the 

 molecular weight of any substance will be double its gaseous 

 density when compared with that of hydrogen. 



Gramme-Molecule. — A gramme-molecule is the molecular 

 weight of a body expressed in grammes. Occasionally 

 for brevity a gramme-molecule is spoken of as a " molecule." 

 Thus we may say that the molecular weight of oxygen is 

 16 grammes, meaning thereby that there are the same 

 number of molecules in 16 grammes of oxygen as there are 

 atoms in 1 gramme of hydrogen. 



Concentration. — The concentration of a solution is the 

 ratio between the quantity of the solute and the quantity of 

 the solvent. The concentration of a solution is expressed 

 in various ways. (a) The weight of solute dissolved in 

 100 grammes of the solvent, (b) The weight of solute 

 present in 100 grammes of the solution. (c) The weight 

 of solute dissolved in a litre of the solvent, (d) The weight of 

 solute in a litre of the solution. The most usual method is to 

 give the concentration as the weight of solute dissolved in 

 100 grammes or in one litre of the solvent. 



Molecular Concentration. — Many of the physical and 

 biological properties of a solution are proportional, not to 

 its mass or weight concentration, but to its molecular con- 

 centration, i.e. to the number of gramme-molecules of the 

 solute contained in a litre of the solution. Many physical 

 properties are quite independent of the nature of the solute, 

 depending only on its degree of molecular concentration. 



Normal Solution. — A normal solution is one which contains 

 one gramme-molecule of the solute per litre. A decinormal 

 solution contains one-tenth of a gramme-molecule of the 

 solute per litre, and a centinormal solution one-hundredth of 

 a gramme-molecule. A normal solution of urea, for example, 



