How much oxygen be required to produce 3.9 mol of water

1.95mol

Explanation:

First let us generate the equation for the reaction:

2H2 + O2 - > 2H2O

From the equation,

1mole of O2 reacted to produce 2moles of H2O.

Therefore, Xmol of O2 will react to produce 3.9 mol of H2O i. e

Xmol of O2 = 3.9/2 = 1.95mol