How do you calculate the empirical formula of phosgenite?
Phosgenite is a mineral. The empirical formula (which in this case is also the molecular formula) is shown. I don't know what you meant by "calculate" - you can look up the formula for the compound: http://www.geologypage.com/2014/06/phosgenite.html
Experimentally the formula would be determined by different reactions to observe the amounts of lead, carbonate and chloride in the molecule. These would then be combined with the known oxidation states and likely ions of the elements to derive the empirical formula. A comparison of the empirical formula to the molecular weight will derive the molecular weight.