Thermodynamic activity
= Thermodynamic activity
{title2=$a_i=\exp[(\mu_i-\mu_i^\circ)/(RT)]$}
The thermodynamic activity is the dimensionless quantity defined by $\mu_i=\mu_i^\circ+RT\ln a_i$, where $R$ is the <molar gas constant>. For an ideal dilute solution with a specified <concentration> reference, $a_i=c_i/c^\circ$; departures from ideality are represented by an <activity coefficient> multiplying this ratio.