29.5mg of an organic compound containing nitrogen was digested according to Kjeldahl's method and the evolved ammonia was absorbed in 20ml of 0.1M HCl solution. The excess of the acid required 15ml of 0.1M NaOH solution for complete neutralization. The percentage of nitrogen in the compound is:

Moles of HCl reacting with ammonia = moles of HCl absorbed - moles of NaOH solution required

Moles of HCl absorbed = 20 ml × 0.1 M = 2 millimoles
Moles of NaOH used = 15 ml × 0.1 M = 1.5 millimoles
Moles of HCl reacting with ammonia = 2 - 1.5 = 0.5 millimoles

Since the reaction between ammonia and HCl is 1:1, moles of ammonia = 0.5 millimoles

Mass of nitrogen = 0.5 millimoles × 14 mg/millimole = 7 mg

Percentage of nitrogen = (mass of nitrogen / mass of compound) × 100
Percentage of nitrogen = (7 mg / 29.5 mg) × 100 = 23.7%