I don't know how to do this

How many moles of water can be made from 4 moles of oxygen gas and 16 moles of hydrogen gas? What is the limiting reagent?

Write the balanced equation.
2H2 + O2 ==> 2H2O
2 mols H2 + 1 mol O2 will produce 2 mol H2O.

So 16 mols H2 will require 8 mols O2. BUT we don't have 8 mols O2 SO O2 must be the limiting reagent.

Now, let's work from the oxygen side.
1 mol O2 requires 2 mols H2.
So 4 mols O2 will require 8 mols H2. Do we have that much? yes, we do. How much H2O will be produced. 1 mol O2 will produce how much H2O? The equations tells us 2 mols. Therefore, 4 mols O2 will produce ????H2O.
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