Consider the one-component homogeneous hard-sphere fluid with molecule radius R . To obtain its equation of state in the form p = p ( ρ ρ , T ) (where p is the pressure, ρ ρ is the density of the fluid and T is the temperature) one can find the expression for the chemical potential μ μ and then use the Gibbs–Duhem equation to express p as a function of ρ ρ .