The "empirical formula"empirical formula is the simplest, whole number ratio defining constituent atoms in a species. The "molecular formula"molecular formula is a whole number multiple of the "empirical formula"empirical formula, and of course the multiple MIGHT be ONE.
And thus...."molecular formula"=nxx{"empirical formula"}molecular formula=n×{empirical formula}
And we take the relevant masses...
[(2xx12.011+4xx1.00794)*g*mol^-1]xxn-=underbrace(42.08*g*mol^-1)_"given molecular mass"
And so we solve for n by the quotient....
n=(42.08*g*mol^-1)/[1xx12.011+2xx1.00794*g*mol^-1]=3
And so our molecular formula is 3xxCH_2-=C_3H_6...this is propylene, or cyclopropane...
