A compound composed of only c and f contains 17.39 c by mass. what is its empirical formula

Since the Carbon C is 17.39% by mass hence the Fluorine F is 82.61% by mass. Divide each mass % by the respective molar masses, that is:

C = 17.39 / 12 = 1.45

F = 82.61 / 19 = 4.35

Divide the two by the smaller number, so divide by 1.45

C = 1.45 / 1.45 = 1

F = 4.35 / 1.45 = 3

So the empirical formula is:

CF3