"HNO2 aq" signifies an aqueous solution of nitrous acid, HNO2. Nitrous acid is a weak acid, which only exists in the solution or as nitrite salts. Its molar mass is 47.013 grams per mole, and its density is approximately 1 gram per milliliter.
Nitrous acid is prepared from solutions of nitrite salts by adding a stronger acid. It is unstable and decomposes into nitrogen dioxide, NO2, nitric oxide, NO, and water: 2 HNO2 ? NO2 + NO + H2O. Nitrogen dioxide then reacts with water to produce nitric acid, HNO3, and nitrous oxide: 2 NO2 + H2O ? HNO3 + HNO2. The net effect of the previous two reactions is the decomposition of nitrous acid into nitric acid, nitric oxide and water: 3 HNO2 ? HNO3 + 2 NO + H2O.