If 6 moles of hydrazine react with an excess of oxygen, how many moles of water will be produced?

12 moles

Explanation:

According to the law of conservation of mass, mass can neither be created nor be destroyed. Thus the mass of products has to be equal to the mass of reactants. The number of atoms of each element has to be same on reactant and product side. Thus chemical equations are balanced.

According to stoichiometry :

1 mole of hydrazine produces 2 moles of water

Thus 6 moles of hydrazine produce = moles of water

Thus 12 moles of water are produced.