Stoichiometry. You start with some amount of nitrous acid. You add some base. If neutralization went to completion, you are able to easily calculate amount of the acid left, and amount of conjugate base (product of the reaction) - this is just a limiting reagent question. You know amounts, you know the final volume - just plug and chug.
chemical calculators - buffer calculator, concentration calculator
www.titrations.info - all about titration methods