The activity coefficient \(f_{\rm{B}}\) of a substance B in a liquid or solid mixture containing mole fractions \(x_{\rm{B}}\), \(x_{\rm{C}}\), ... of the substances B, C, ... is a dimensionless quantity defined in terms of the chemical potential \(\mu _{\rm{B}}\) of B in the mixture by: \[R T \ln (x_{\ce{B}} f_{\ce{B}}) = \mu_{\ce{B}}({\ce{cd}},T,P,x) - \mu_{\ce{B}}^{\ast}({\ce{cd}},T,p)\] where \(x\) denotes the set of mole fractions \(x_{\rm{B}}\), \(x_{\rm{C}}\), ... . The activity coefficient \(\gamma _{\rm{B}}\) of a solute B in a solution (especially a dilute liquid solution) containing molalities \(m_{\rm{B}}\), \(m_{\rm{C}}\), ... of solutes B, C, ... in a solvent A is a dimensionless quantity defined in terms of the chemical potential \(\mu _{\rm{B}}\) by: \[R\ T\ \ln \left(\frac{m_{\rm{B}}\ \gamma _{\rm{B}}}{m^{\ominus}}\right)=\mu _{\rm{B}}- \left\{\mu _{\rm{B}}- R\ T\ \ln \left(\frac{m_{\rm{B}}}{m^{\ominus }}\right)\right\}^{\infty }\]