1) Use the balanced chemical equation to find the molar ratios (proportions) of each product and reactant.
3N2H4(l)→4NH3(g)+N2(g)
=> molar ratios: 3 mol N2H4 : 4 mol NH3
2) Use the product to reactant molar ratio, and the quantity of reactant to determine the yield:
2.0 mol N2H4 * [4mol NH3] / [3mol N2H4] = 2*4/3 mol NH3 = 2.7 mol NH3
Answer: 2.7 mol