How many moles of HNO_3 will be produced when .51 mol of N_2O_5 reacts in the equation N_2O_5 + H_2O -> 2HNO_3?

It says that $1$ $\text{mole}$ of dinitogen pentoxide reacts with 1 mole of water to produce $2$ $\text{moles}$ of nitric acid.
Now you start with a $\text{0.5 mole}$ quantity of ${N}_{2} {O}_{5}$. I think it is pretty clear how much nitric acid you are going to get.