given the equation, N2H4 + N2O4 -> N2 + H2O, how many moles of N2)=O4 are needed to react with 3.5 moles of N2H4

Explanation:

I think you mean

2N2H4 + N2O4 ==> 3 N2 + 4H2O

The equation says that for Every mol of N2O4, you need 2 moles of N2H4.

1/2 = x/3,5       Multiply both sides by 3.5

3.5*1 / 2 = x

1.75 = x

Therefore you need 1.75 moles N2O4 are needed for 3.5 moles of N2H4