I'm not a chemist, but I need some insight into chemical thermodynamics.
In the book that I'm reading (Nevers, 2012), equation (4.28) relates the chemical potential of a pure species to it's Gibbs energy per mol:
$\qquad\mu_a^{(1)} = \bigg( \dfrac{\partial G}{\partial n_a} \bigg)_{T,P,n_b...} = g_a^{(1)}$
where the subscript stands for the species $a$, the superscript stands for it's phase, $\mu$ is the chemical potential and $g$ is the Gibbs energy per mol.
I understand the first equal sign. Though, I don't understand the second equal sign. What am I missing?