So,
The molecular mass of the empirical formula of the compound SO2, can be found multiplying the molar mass of each element by the number of atoms and then sum. Let me explain this here below:
[tex]\begin{gathered} S\colon\frac{32g}{\text{mol}}\cdot1 \\ O\colon\frac{16g}{\text{mol}}\cdot2 \\ mSO_2=\frac{64g}{mol} \end{gathered}[/tex]We're given that the molecular mass of the molecular formula is 256g/mol. So, we could find the ratio between the mass of both formulas:
[tex]\frac{256}{64}=4[/tex]This means that the mass of the molecular formula is 4 times greater than the mass of the empirical formula.
Thus, the molecular formula will be:
[tex]S_4O_8[/tex]