Molar MassMolar mass of phosphoric acid (H3PO4)
The molar mass of phosphoric acid, H3PO4, is 97.99 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 97.99 grams.
97.99 g/mol
3 × 1.0080 + 30.974 + 4 × 15.999 = 97.994 · 8 atoms in the formula · also called orthophosphoric acid
About phosphoric acid
Phosphoric acid is a weak acid with three hydrogen atoms that can be replaced one at a time, so it forms three series of salts. It is used in fertilisers, in rust removers and, in small amounts, to give cola drinks their sharp taste.
Other names: orthophosphoric acid.
The working
The formula H3PO4 has 3 hydrogen atoms, 1 phosphorus atom and 4 oxygen atoms, 8 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Hydrogen (H) | 3 | 1.0080 | 3.024 | 3.09% |
| Phosphorus (P) | 1 | 30.974 | 30.974 | 31.61% |
| Oxygen (O) | 4 | 15.999 | 63.996 | 65.31% |
3 × 1.0080 + 30.974 + 4 × 15.999 = 97.994 g/mol
With the rounded atomic masses used in many school books (H = 1, P = 31, O = 16) the answer is 98 g/mol. The difference from 97.99 is too small to matter in most school sums.
Percentage composition
By mass, H3PO4 is 3.09% hydrogen, 31.61% phosphorus and 65.31% oxygen. The largest share comes from oxygen. In a 100 g sample there are 3.1 g of hydrogen, 31.6 g of phosphorus and 65.3 g of oxygen.
Worked examples
Grams to moles. How many moles are in 50 g of H3PO4? Divide the mass by the molar mass: 50 ÷ 97.99 = 0.5102 mol.
Moles to grams. What is the mass of 0.4 mol of H3PO4? Multiply: 0.4 × 97.99 = 39.2 g.
Counting particles. 50 g contains 0.5102 mol × 6.022 × 1023 = 3.073 × 1023 molecules, and 2.458 × 1024 atoms in all.
Making a solution. A 0.1 mol/L solution contains 0.1 × 97.99 = 9.799 g of H3PO4 in each litre. For 250 mL, weigh 2.45 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 9.799 g |
| 0.5 mol | 49 g |
| 1 mol | 97.99 g |
| 2 mol | 196 g |
| 5 mol | 490 g |
| Mass | Moles |
|---|---|
| 1 g | 0.0102 mol |
| 10 g | 0.102 mol |
| 100 g | 1.02 mol |
| 1 kg | 10.2 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of H3PO4?
The molar mass of phosphoric acid (H3PO4) is 97.99 g/mol, or 97.994 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of H3PO4?
100 ÷ 97.99 = 1.02 mol.
What is the mass of one molecule of H3PO4?
Divide the molar mass by the Avogadro constant: 97.99 ÷ 6.022 × 1023 = 1.627 × 10-22 g.
What is the percentage of oxygen in H3PO4?
Oxygen makes up 65.31% of the mass of H3PO4: 4 × 15.999 ÷ 97.994 × 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.