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Activity Coefficients

Represent the deviation of a species' effective concentration from its molar concentration in solution, accounting for interactions.
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The statement of the theorem

For a species ii in a solution, the activity aia_i is defined by the product of its activity coefficient γi\gamma_i and its effective concentration CiC_i: ai=γiCi a_i = \gamma_i C_i where CiC_i is the molar concentration. The activity coefficient γi\gamma_i quantifies the deviation from ideal behavior, such that for an ideal solution, γi=1\gamma_i = 1. The chemical potential μi\mu_i is then given by: μi=μi∘+RTln⁡ai=μi∘+RTln⁡(γiCi) \mu_i = \mu_i^\circ + R T \ln a_i = \mu_i^\circ + R T \ln (\gamma_i C_i)