86 GENERAL CONDITIONS OF EQUILIBRIUM, chap.
where A is a constant, the value of which depends on the temperature.
If we had an equation of the form —
mP+nQ + oE+ . . . =:fS+f/T + hV+ . . .
which denotes a reaction where m molecules of a substance P, n molecules of substance Q, and o molecules of substance By etc., react to produce /molecules of substance S, g mole- cules of substance T, and h molecules of substance F, etc., the result would be —
m\Vi Cp + nhi Cq + oln C^+ . . . =fhiCa + g\nCr
+ A In C,. + . . .
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��where k is again a constant and C is the partial pressure of each of the substances indicated by the index.
Since at low pressures or in very dilute solution the partial pressure is almost proportional to the concentration, C in this case may be taken to mean simply the concentration. The " law of mass acticm " expressed in the above formula is called, after its two Norwegian discoverers, the Guldberg and Waage law (7). The law as originally stated referred to the concentrations of the reacting substances, but it was later shown to apply more strictly when C in the above formula indicates the partial pressure.
The law can be more simply derived by considering the action kinetically. Let us take the same example as befoi^, that represented by the equation —
NH4Cl:jtNH3 + HCl,
and let us consider the quantities in unit volume (1 c.c), i.e. the concentrations Cs, Ci, and C^ of the three substances present.
The number of decomposing molecules of ammonium chloride in unit volume is proportional to the total quantity of this substance (C3) present in the same volume, for each
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