How do i solve this plz help or else i will fail

Calculate - analysis of compound composed of iron and oxygen yields 174.86 of fe and 75.14g of o. what is the empirical formula for this compound?

How many grams of oxygen gas are contained in a 15 l sample at 1.02 atm and 28°c? show your work.

How many moles of lead are in 1.50 x 10^12 atoms of lead? could you explain the answer as well and not just give it to me i am refreshing for finals and i need to know how to do it