To find the percentage of nitrogen in ammonium nitrate (NH₄NO₃), you compare the total mass of nitrogen in the formula to the molar mass of the whole compound.
Step 1: Determine the molar mass of ammonium nitrate
Ammonium nitrate has the formula NH₄NO₃, containing 2 nitrogen (N) atoms, 4 hydrogen (H) atoms, and 3 oxygen (O) atoms. Using standard atomic masses (N = 14.01 g/mol, H = 1.01 g/mol, O = 16.00 g/mol):
- Nitrogen: 2 × 14.01 = 28.02 g/mol
- Hydrogen: 4 × 1.01 = 4.04 g/mol
- Oxygen: 3 × 16.00 = 48.00 g/mol
Total molar mass = 28.02 + 4.04 + 48.00 = 80.06 g/mol
Step 2: Calculate the mass fraction of nitrogen
Percentage of nitrogen = (mass of nitrogen ÷ total molar mass) × 100
Percentage of nitrogen = (28.02 ÷ 80.06) × 100 ≈ 35.0%
Result
Ammonium nitrate is approximately 35% nitrogen by mass. This is one reason it's widely used as a nitrogen fertilizer — it has one of the highest nitrogen contents among common straight nitrogen fertilizers, and unlike some other sources, it supplies nitrogen in two forms (ammonium, NH₄⁺, and nitrate, NO₃⁻), both of which are usable by plants.
General formula for any compound
% element = (number of atoms of element × its atomic mass ÷ molar mass of compound) × 100
This same method works for finding the percentage composition of any element in any compound — just substitute the correct atomic masses and molecular formula.
Note: Commercial fertilizer labels may show slightly different nitrogen percentages (often around 33–34%) due to added coatings, fillers, or moisture that dilute the pure compound.