Molar MassMolar mass of phosphorus pentoxide (P2O5)
The molar mass of phosphorus pentoxide, P2O5, is 141.94 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 141.94 grams.
141.94 g/mol
2 × 30.974 + 5 × 15.999 = 141.943 · 7 atoms in the formula · also called phosphorus(V) oxide
About phosphorus pentoxide
P2O5 is the empirical formula of phosphorus(V) oxide; the actual molecule is P4O10, with twice the molar mass. It is a powerful drying agent, and fertiliser strength is quoted in terms of it.
Other names: phosphorus(V) oxide.
The working
The formula P2O5 has 2 phosphorus atoms and 5 oxygen atoms, 7 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Phosphorus (P) | 2 | 30.974 | 61.948 | 43.64% |
| Oxygen (O) | 5 | 15.999 | 79.995 | 56.36% |
2 × 30.974 + 5 × 15.999 = 141.943 g/mol
With the rounded atomic masses used in many school books (P = 31, O = 16) the answer is 142 g/mol. The difference from 141.94 is too small to matter in most school sums.
Percentage composition
By mass, P2O5 is 43.64% phosphorus and 56.36% oxygen. The largest share comes from oxygen. In a 100 g sample there are 43.6 g of phosphorus and 56.4 g of oxygen.
Worked examples
Grams to moles. How many moles are in 250 g of P2O5? Divide the mass by the molar mass: 250 ÷ 141.94 = 1.761 mol.
Moles to grams. What is the mass of 0.2 mol of P2O5? Multiply: 0.2 × 141.94 = 28.39 g.
Counting particles. 250 g contains 1.761 mol × 6.022 × 1023 = 1.061 × 1024 molecules, and 7.425 × 1024 atoms in all.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 14.19 g |
| 0.5 mol | 70.97 g |
| 1 mol | 141.9 g |
| 2 mol | 283.9 g |
| 5 mol | 709.7 g |
| Mass | Moles |
|---|---|
| 1 g | 0.007045 mol |
| 10 g | 0.07045 mol |
| 100 g | 0.7045 mol |
| 1 kg | 7.045 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of P2O5?
The molar mass of phosphorus pentoxide (P2O5) is 141.94 g/mol, or 141.943 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of P2O5?
100 ÷ 141.94 = 0.7045 mol.
What is the mass of one molecule of P2O5?
Divide the molar mass by the Avogadro constant: 141.94 ÷ 6.022 × 1023 = 2.357 × 10-22 g.
What is the percentage of oxygen in P2O5?
Oxygen makes up 56.36% of the mass of P2O5: 5 × 15.999 ÷ 141.943 × 100.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.