A compound with a formula mass of 45.08 g/mol is found to be 85.64% Carbon and the remainder Hydrogen. Find the molecular formula:

Molecular formula=C3H6
Explanation
As with all of these problems, we assume 100 * g of unknown compound. And this, we determine the elemental composition by the given percentages.
Miles of carbon= 85.64*g/12.011*g*mol-1=14.25*mol
Clearly there are 3 mols of hydrogen per mol of carbon. And thus the empirical formula is CH2.
And molecular formula=n*(12.011+2*1.00794)*g*mol-1
And this n=3, and molecular formula=C3H6.