Question

if you start with 20.0g of phosphorus 30.0g of oxygen gas, and 15.0g water, what is the maximum mass of phosphoric acid that can be formed?

Answer 1

This is a limiting reagent problem. There are other ways to do this but I like this method.
Basically you work three stoichiometry problems. I will assume you will use P for phosphorus (and not P4) and P2O5 for the oxide (and not P4L10) You will need to adjust the equation if yo use something other than what I use. I am rounding the atomic masses and molar masses; you should use better numbers and recalculate all.
4P + 5O2 + 6H2O ==> 4H3PO4
mols P = g/atomic mass P = 20.0/31 = 0.645

mols O2 = 30.0/32 = 0.937

mols H2O = 15.0/18 = 0.833

Now convert each of these mole amounts to mols H3PO4.
P is first.
0.645 mols P x (4 mols H3PO4/4 mols P) = 0.645 x 4/4 = 0.645 mols H3PO4.

Next is O2.
0.937 mols O2 x (4 mols H3PO4/5 mols O2) = 0.937 x 4/5 = 0.75 mols H3PO4.

Next is H2O.
0.833 mols H2O x (4 mols H3PO4/6 mols H2O) = 0.833 x 4/6 = 0.555 mols H3PO4.

In limiting reagent problems the correct for mols H3PO4 formed is the smallest value and that looks like 0.555 mol H3PO4.

Convert that number to grams. g = mols x molar mass.