To find the mass percent of chlorine (Cl) in CCl3F, we need to determine the mass of chlorine in the compound and divide it by the total mass of the compound.

The molar mass of CCl3F is:

12.01 g/mol (1 C atom) + (3 x 35.45 g/mol for 3 Cl atoms) + 18.99 g/mol (1 F atom) = 187.40 g/mol

Now, let's calculate the mass of chlorine in CCl3F:

Mass of Chlorine (Cl) in CCl3F = 3 x 35.45 g/mol (for 3 Cl atoms) = 106.35 g/mol

Next, we calculate the mass percent of chlorine:

Mass Percent of Chlorine = (Mass of Chlorine / Total Mass) x 100

Mass Percent of Chlorine = (106.35 g/mol / 187.40 g/mol) x 100

Mass Percent of Chlorine ≈ 56.72%

Therefore, the mass percent of chlorine in CCl3F is approximately 56.72%.