Find the empirical formula for a compound which contains 32.8% chromium and 67.2% chlorine

67.2% Cl = 67.2g Cl x 1 molCl / 35.45g Cl = 1.89 / 0.63 = 3 Cl

32.8% Cr = 32.8g Cr x 1 molCr/51.99g Cr = 0.63 / 0.63 = 1 Cr

The empirical formula is CrCl3

First convert the mass in grams to moles, then divide both by the smallest number.