The practical activity coefficient of electrolyte B is given by $${\gamma }_{\pm }=\frac{a_{\pm }}{\sqrt{{\nu }_{\text{+}}^{{\nu }_{\text{+}}}{\nu }_{\text{-}}^{{\nu }_{\text{-}}}}\frac{m_{\text{B}}}{m^{\text{+}}}}$$ where $a_{\pm }$ is the mean activity of B in solution, $m_{\text{B}}$ is the molality of B, $m^{+}=1\mathrm{mol}{\mathrm{kg}}^{-1}$, ${\nu }_{\text{+}}$ is the number of cations and ${\nu }_{\text{–}}$ the number of anions in the chosen group B which is taken as the electrolyte. $$\nu ={\nu }_{\text{+}}+{\nu }_{\text{-}}$$