Textbook Question M11

[Jake Matsumoto 3E]

Hi! Can someone explain M15 to me? I'm particularly confused why we can determine O2 is the limiting reactant when in the answer key, it only tests out the 5.77g of P4 in both the first and second reaction. If someone could explain this, that would be great. Thank you!

[Jaden 14B 3F]

The trick with this problem is that you have to separate it into two reactions, and think of them independently.
In the first part of the problem, O2 is in excess, compared to P4. You can calculate this by converting both amounts of starting reactant grams into moles of product.
After doing this, you will see that P4 limits the first reaction. Convert grams of P4 into moles of O2 to see how much is required for the reaction. Then, convert the 5.77 grams of O2 into moles to see how many moles of O2 you initially start with. Subtract the amount you need from the amount you have in order to determine the amount of O2 available for the second reaction.