= Solution
At fixed <temperature>, <volume> and the numbers of all other species, the <chemical potential> is $\mu_\alpha=(\partial F/\partial N_\alpha)_{T,V,N_{\alpha'\ne\alpha}}$. For the combined noninteracting mixture, add the free <energies> from (a) and (b). Since the <thermal de Broglie wavelengths> are independent of <particle> number at fixed <temperature>, differentiation gives
$$
\boxed{\mu_{\rm CO}=k_BT\ln(n_{\rm CO}\lambda_{\rm CO}^3),\qquad
\mu_{\rm O_2}=k_BT\ln(n_{\rm O_2}\lambda_{\rm O_2}^3),}
$$
$$
\boxed{\mu_{\rm CO_2}=k_BT\ln(n_c\lambda_c^3)-\frac\epsilon2.}
$$
Each logarithm has a dimensionless argument, and each <chemical potential> has units of <energy> per <molecule>. The binding offset must be kept consistently when comparing species.
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