This gas must be carbon dioxide. Probably you added excess NaOH, which leaves a solution of NaOH and NaNO3 (and NH3). After evaporation, NaOH and
NaNO3 remain, but the NaOH absorbs CO2 from the air and at the time when all is dry, all of it will have been converted to Na2CO3. The acid NaHSO4 is
strong enough to release CO2 from Na2CO3. |