A compound contains 3.2%  by mass of oxygen. The minimum molecular weight of the compound is ____?

For molecular mass to be minimum one gram atom of O (16g oxygen) should be present.

 3.2 g oxygen is present in 100 gm compound

16 g oxygen will be present in $\frac{100\times 16}{3.2}=500$

Minimum molecular mass of compound = 500 $gmo{l}^{-1}$