if you had 118.7 gm of Sn; how many grams of oxygen would need to convert al the Sn into SnO^2
convert g of Sn to moles.
You will need that many moles of O2.
Then find the mass for that many moles of O2.
You will need that many moles of O2.
Then find the mass for that many moles of O2.