To find the mass percent of fluorine in the compound, you must first determine the mass in g of fluorine in the compound. We do this by subtracting the mass of carbon from the total mass of the sample. From this we get 171 - 90 = 81g of fluorine in CF4 sample. Now to get the mass percent, we divide the fluorine mass by the sample mass to get the percent mass. 81/171 = .4737 which multiplied by 100 to get percentage give 47.37%.
