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activity coefficient (γ) The ratio of chemical activity (i.e. the effective concentration, a) of a component in a solution, to the actual mole fraction (X) present in solution: (γ = a/X). Values for activities are determined experimentally in a number of ways, including measuring the ratio of the vapour pressure (p) of a known concentration of the substance in solution to the vapour pressure (p*) of the pure substance: a = p/p*. In an ideal solution the activity coefficient = 1, and the activity of the component is equal to its mole fraction. In general, the greater the amount of dissolved material, the lower the activity coefficients of each of the species present.