Answer
$n=3.5 mol$
Work Step by Step
We know that:
$\Delta S=S_f-S_i=nR\ln \frac{v_f}{v_i}+nC_V\ln \frac{T_f}{T_i}$
As $T_i=T_f$ so $\ln(1)=0$ and thus the second term becomes zero and we obtain:
$\Delta S=nR\ln \frac{v_f}{v_i}$
This can be rearranged as:
$n=\frac{\Delta S}{R\ln (\frac{v_f}{v_i})}$
We plug in the known values to obtain:
$n=3.5 mol$
