A compound contains 3.2% of oxygen. The minimum mol. wt. of the compound is

The compound must contain at least one oxygen atom 

So, a minimum of 1 g atom of oxygen will be present in 1 g molecule, i.e., 1 mole of the compound.

If M is the mol.wt. of the compound then since 16 is the atomic mass of oxygen so minimum of 16 g of oxygen will be present in M g of the compound

$\mathrm{Thus},\mathrm{\%}\mathrm{of}\mathrm{oxygen}=\frac{16}{\mathrm{M}}\times 100$

$\begin{array}{l}\mathrm{or}3.2=\frac{16\times 100}{\mathrm{M}}\\ \mathrm{or}\mathrm{M}=500\end{array}$