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
