A 0.166 g sample of an organic compound was digested with cone. $\mathrm{H}_2 \mathrm{SO}_4$ and then distilled with NaOH . The ammonia gas evolved was passed through 50.0 mL of $0.5 \mathrm{~N} \mathrm{H}_2 \mathrm{SO}_4$. The used acid required 30.0 mL of 0.25 N NaOH for complete neutralization. The mass percentage of nitrogen in the organic compound is $\_\_\_\_$ .