= Solution
The <Clausius gas> equation can be written
$$
P=\frac{NT}{V-Nb}=\frac{nT}{1-nb}=nT\left(1+nb+O(n^2)\right).
$$
Comparison with the <virial expansion> gives
$$
\boxed{B(T)=b}.
$$
For a <hard-sphere gas> of atomic radius $r_0$, the <pair potential> is infinite when the center separation is below $2r_0$ and zero otherwise. Thus
$$
B=\frac12\int_{r<2r_0}d^3r
=\frac12\frac{4\pi}{3}(2r_0)^3
=\boxed{\frac{16\pi r_0^3}{3}}.
$$
This is four times the volume of one atom and is the per-particle <excluded volume> appearing in the dilute <equation of state>.
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