Chemistry Question on equation?

Question

I need to figure out the mass of the final product, H3PO4.
They give me the starting amounts and each balanced equation:
20.0g P
30.0g O2
15.0g H2O

4P + 5O2 ->P4H10

P4O10 + 6H20 -> 4H3PO4

Answer 1

This is sneaky because one of your reagents is going to be limiting. You should try and find out which. Here's how:
From the overall equations, for each mole of P, you wind up with one mole of phoshoric acid. Since you start with 20 g of P, or about 0.53 moles, this limits your production to that much H3PO4.
To provide hydrogen for each mole of phosphoric acid, you need 1.5 moles of water. Thus, for your 0.53 moles of acid, you need about 0.77 moles of water. You have that, so thats good.
For each mole of phosphoric acid, you need 4-gram atoms of oxygen (I use this for some can come from the water). For 0.53 moles of acid, you need 2.12 g-atoms of O. You can verify that you have sufficient O available for this chore, so your yield will be 0.53 moles. Multiply by the mol wt of phosphoric acid to get your answer.