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

most likely he is cheating on you

c. because step up transformer steps up the input voltage i.e increases the input voltage  

the number of turns are greater on the secondary side of step up transformer  

transformer does not handle dc current