A gaseous compound has the following composition by mass: C, 25.0%; H, 2.1%; F, 39.6%; O, 33.3%. Its molecular weight is 48 g/mol

Answer 1

And you want to know what? The formula? empirical formula? molecular formula?
Take a 100 g sample which will give you
25.0 g C
2.1 g H
39.6 g F
33.3 g O

Now convert each to moles, then find the molar ratios of the elements to each other with the smallest being 1.00. The easy way to do that is to divide the smallest number by itself (assuring you that this number will be 1.000), then divide the other numbers by the same small number. Round to whole numbers and that will be the empirical formula.
To deduce the molecular formula, add up the empirical formula mass and
48/(empirical formula mass) = ?? and you round this to a whole number. It will be 1.00 or 2.00 or 3.00 (when rounded). This indicates the number of empirical formula units it takes to make a molecular formula; i.e,
(CxHyOzFw)n
Post your work if you get stuck.