Note 324 — Chapter VIII · SIMULTANEOUS CHEMICAL AND PHYSICAL EQUILIBRIUM.—THE SOLUBILITY- OR ION-PRODUCT.
Reading edition
If n is the number of moles of solute dissolved in N moles of solute, the concentration of the solute may be expressed as n / (n + N), which is called its "mole fraction." This form of expressing concentrations is in many particulars preferable to the mole / liter form. For very dilute solutions (n is very small compared with N) the two forms become practically identical, but they are not so for more concentrated solutions, and the mole-fraction expression is then easier to treat rigorously.