Answer : The number of moles of water formed can be, 128 moles

Solution : Given,

Moles of hydrogen gas = 138 moles

Moles of oxygen gas = 64 moles

First we have to calculate the limiting and excess reagent.

The balanced chemical reaction will be,

2H_2+O_2rightarrow 2H_2O

From the balanced reaction we conclude that,

As, 2 moles of hydrogen gas react with 1 mole of oxygen  gas

So, 138 moles of hydrogen gas react with frac{138}{2}=69 moles of oxygen  gas

But the given moles of oxygen gas is 64 moles.

That means oxygen gas is a limiting reagent and hydrogen gas is an excess reagent.

Now we have to calculate the moles of water.

From the reaction we conclude that,

As, 2 moles of oxygen react to give 1 mole of water

So, 64 moles of oxygen react to give 64times 2=128 moles of water

Therefore, the number of moles of water formed can be, 128 moles