gravimetric stoichiomerty..... eh? HELP calculate the mass of iron(III) oxide (rust) produced by the reaction of 500g of iron with oxygen from the air so i have no idea what do do here, my teacher sucks monkeys balls. so i thought to do the mass of iron(III) oxide divided by the 500g but i know thats wrong. PLEASE HELP ME! I actually have no idea of what im suppose to do.