text: In a liquid mixture, the i-th compound's value for this quantity can be computed by differentiating the excess Gibbs energy with respect to x_i.
full_question: In a liquid mixture, the i-th compound's value for this quantity can be computed by differentiating the excess Gibbs energy with respect to x_i. Another model computes this quantity by differentiating the virial expansion of the excess Gibbs energy with respect to molality. In addition to using the Margules and Pitzer models, you can compute this quantity with the Debye-Huckel equation. For 10 points, name this dimensionless quantity that models a liquid's deviation from being an ideal solution.
first_sentence: In a liquid mixture, the i-th compound's value for this quantity can be computed by differentiating the excess Gibbs energy with respect to x_i.