An equivalent statement uses thermodynamic activity instead of fugacity. The real gas pressure and fugacity are related through the dimensionless fugacity coefficient . The fugacity is closely related to the thermodynamic activity. Lewis introduced the thermodynamic concept of activity and coined the term "fugacity". Equilibrium constants are defined in terms of fugacity. By Henry's law, the fugacity of the solute is proportional to its concentration. It is called the fugacity coefficient (formula 36). The reaction is used to control the fugacity of oxygen in laboratory experiments. For an ideal gas, fugacity and pressure are equal and so . When dealing with gases, fugacity, "f", is used rather than activity. Fugacity, "f", is the product of partial pressure and fugacity coefficient. The word "fugacity" is derived from the Latin "fugere", to flee. Fugacity, activity, and chemical potential are very closely related to entropy. Fugacity is closely related to the chemical potential . The fugacity is most useful in mixtures.