## activity (relative activity), $$a$$

https://doi.org/10.1351/goldbook.A00115
Defined by the equation $a = e^{\frac{\mu - \mu^{o}}{R\,T}}$ where R is the gas constant, T the thermodynamic temperature, µ the chemical potential and µ0 the standard chemical potential the definition of which depends on the choice of the standard state.