Ammonia dissolves in water to form an alkaline solution. Nitric acid, HNO3, is an acid. When they react, the ammonia neutralises the acid and they form the salt ammonium nitrate, NH4NO3. The equation is NH3 + HNO3 → NH4NO3.
Ammonium nitrate is the only product. Unlike the reaction of an acid with a metal hydroxide, no water is formed, because ammonia has no hydroxide ions to join with the hydrogen ions from the acid. Instead, the ammonia takes a hydrogen ion from the acid to become the ammonium ion, NH4+.
Ammonium nitrate is a very useful fertiliser. It is soluble in water, so plant roots can absorb it. It is also rich in nitrogen, because it contains nitrogen in both the ammonium ion and the nitrate ion. Its relative formula mass is 80 and the two nitrogen atoms have a combined mass of 28. The percentage of nitrogen is found by dividing 28 by 80 and multiplying by 100, which gives about 35% nitrogen by mass.