Answer
0.14 J/K
Work Step by Step
For an ideal gas at constant temperature, recall that
$\Delta S=nR\ln \frac{V_{final}}{V_{initial}}$
Here, $n=1.6\,g\times\frac{1\,mol\,O_{2}}{32\,g}=0.050\,mol$
Then,
$\Delta S=(0.050\,mol)(8.314\,Jmol^{-1}K^{-1})(\ln\frac{3.5\,L}{2.5\,L})$
$=0.14\,J/K$
