NaOH+HNO3---> NaNO3+H2O

Question

calculate the number of moles of every ion present in solution after the reaction has occurred.

Answer 1

Th secret here is to know which compounds are ionic (thus they dissociate) and which are not (and remain molecular).

Answer 2

It is not possible unless you know the amount of reactants you started with.

Answer 3

YES 15g NaOH dissolves in 150 mL of 0.250 M nitric acid.

Answer 4

Here is a sample stoichiometry problem I have posted on Jiskha. Remember that moles = M x L.
http://www.jiskha.com/science/chemistry/stoichiometry.html