The mineral vanadinite has the formula pb5 (vo4) 3cl. what mass percent of chlorine does it contain?

The molar mass of vanadinite is 1416.27 g/mol. Looking at the molecular formula we can see that there is 1 mole of Cl for every mole of vanadinite. Therefore:

mass percent = [ (1 mole Cl * 35.45 g/mol) / (1 mole Pb5 (VO4) 3Cl * 1416.27 g/mol) ] * 100%

mass percent = 2.50%