Answer
$$2.75 \mathrm{\ mol}
$$
Work Step by Step
In isothermal process $T_{i}=T_{f},$ which implies $\ln \left(T_{f} / T_{i}\right)=0 .$
Therefore, Eq. $20-4$ leads us to
$$
\Delta S=n R \ln \left(\frac{V_{f}}{V_{i}}\right) \Rightarrow n=\frac{22.0}{(8.31) \ln (3.4 / 1.3)}=2.75 \mathrm{mol}
$$
