The expression which represents the chemical potential of the $\mathrm{i}^{\text {th }}$ sp species $\left(\mu_{\mathrm{i}}\right)$ in a mixture $(\mathrm{i} \neq \mathrm{j})$ is
The source book printed no answer; this one was worked out and checked — see the explanation.
Explanation
With S, V and n_j held constant the natural function is U (here written E): μ_i = (∂U/∂n_i)_{S,V,n_j}. The H, A and G derivatives give μ_i only at their own natural variables (S,P), (T,V), (T,P).