How many grams of oxygen is required to oxidize 450 g of chromium?

Given:
m (Cr) = 450 g
Find: m (O2) -?
Decision:
1) Write the reaction equation:
3O2 + 4Cr => 2Cr2O3;
2) Calculate the molar masses of Cr and O2:
M (Cr) = Mr (Cr) = Ar = 52 g / mol;
M (O2) = Mr (O2) = Ar (O) * N (O) = 16 * 2 = 32 g / mol;
3) Calculate the amount of substance Cr:
n (Cr) = m (Cr) / M (Cr) = 450/52 = 8.7 mol;
4) Determine the amount of substance O2:
n (O2) = n (Cr) * 3/4 = 8.7 * 3/4 = 6.5 mol;
5) Calculate the mass of O2:
m (O2) = n (O2) * M (O2) = 6.5 * 32 = 208 g.
Answer: The mass of O2 is 208 g.