Physicochemical Problems

Abstract

When considering any chemical process it is convenient to start with the Law of Mass Action, which states that the rate at which a substance reacts is proportional to its active mass. For gases and vapours the active mass is determined by the partial pressure which, from Avogadro’s hypothesis (equal volumes of all gases contain an equal number of molecules when at the same temperature and pressure), is directly related to the concentration expressed as parts by volume. For example the partial pressure of any anaesthetic in a mixture which is 4% by volume is 0.04 times atmospheric pressure. For substances in solution the active mass is determined by the molecular concentration, if the solution behaves ideally. If the empirical formula, C _a H _b N _c O _d etc. is known, the molecular weight can be calculated and the concentration is usually expressed in moles/litre, i.e. in molarity. Sometimes the concentration may be expressed as moles/kg solvent, i.e. in molality, or as the ratio of the number of molecules of solute to the total number of solute and solvent molecules present, i.e. as the mole fraction. If the solute and solvent do not behave ideally, and this applies particularly to salts in water, it is necessary to multiply the molarity by an activity coefficient to obtain the activity,which corresponds to the active mass.