The molar mass of ammonium phosphate is 149.09 g/mol. Its chemical formula is (NH₄)₃PO₄, meaning each molecule contains three ammonium ions bonded to one phosphate ion. However, the term “ammonium phosphate” can refer to three different compounds, and knowing which one you need matters for getting the right answer.
How the Molar Mass Is Calculated
Triammonium phosphate, (NH₄)₃PO₄, contains 12 hydrogen atoms, 3 nitrogen atoms, 4 oxygen atoms, and 1 phosphorus atom. Using the standard atomic weights from IUPAC:
- Nitrogen (N): 14.007 g/mol × 3 = 42.021
- Hydrogen (H): 1.008 g/mol × 12 = 12.096
- Phosphorus (P): 30.974 g/mol × 1 = 30.974
- Oxygen (O): 15.999 g/mol × 4 = 63.996
Adding those together: 42.021 + 12.096 + 30.974 + 63.996 = 149.09 g/mol. This is the value listed in PubChem’s database for tribasic ammonium phosphate.
Three Compounds Share the Name
Chemistry courses and product labels use “ammonium phosphate” loosely, and the compound you’re working with depends on how many hydrogen atoms the phosphate ion retains. There are three forms, each with a different molar mass.
Triammonium phosphate, (NH₄)₃PO₄, is the “true” ammonium phosphate with all three hydrogens on the phosphate replaced by ammonium ions. Its molar mass is 149.09 g/mol. This is the compound most textbooks mean when they simply say “ammonium phosphate.”
Diammonium hydrogen phosphate, (NH₄)₂HPO₄, keeps one hydrogen on the phosphate and has two ammonium ions. Its molar mass is 132.06 g/mol. Often called DAP, this is the form you’ll encounter most in real-world applications because it’s the world’s most widely used phosphorus fertilizer, with a nutrient profile of 18% nitrogen and 46% phosphorus pentoxide.
Monoammonium dihydrogen phosphate, NH₄H₂PO₄, retains two hydrogens on the phosphate and pairs with a single ammonium ion. Its molar mass is 115.03 g/mol. Known as MAP, it’s also common in agriculture and fire retardants.
Which One Your Problem Means
If you’re solving a homework or exam problem that just says “ammonium phosphate” without further detail, assume triammonium phosphate at 149.09 g/mol. That’s the default in most general chemistry contexts. If the formula is written out in the problem, match it to the correct compound: (NH₄)₃PO₄ for tribasic, (NH₄)₂HPO₄ for dibasic, or NH₄H₂PO₄ for monobasic.
In agriculture or industrial settings, the compound is almost always DAP or MAP. Triammonium phosphate is less stable and less commercially important than the other two forms, which is why fertilizer labels and material safety sheets typically refer to the di- or mono- versions.
Quick Reference Table
- (NH₄)₃PO₄ (triammonium phosphate): 149.09 g/mol
- (NH₄)₂HPO₄ (diammonium phosphate, DAP): 132.06 g/mol
- NH₄H₂PO₄ (monoammonium phosphate, MAP): 115.03 g/mol
All three values use the 2024 IUPAC standard atomic weights: H = 1.008, N = 14.007, O = 15.999, and P = 30.974 g/mol.