Molar MassMolar mass of water (H2O)
The molar mass of water, H2O, is 18.02 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 18.02 grams.
18.02 g/mol
2 × 1.0080 + 15.999 = 18.015 · 3 atoms in the formula · also called dihydrogen monoxide
About water
Water is the most familiar compound of all. Because its molar mass is about 18 grams, one mole of water is about 18 millilitres, a little more than a tablespoon. Nearly nine tenths of its mass is oxygen.
Other names: dihydrogen monoxide.
The working
The formula H2O has 2 hydrogen atoms and 1 oxygen atom, 3 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) | 2 | 1.0080 | 2.016 | 11.19% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 88.81% |
2 × 1.0080 + 15.999 = 18.015 g/mol
With the rounded atomic masses used in many school books (H = 1, O = 16) the answer is 18 g/mol. The difference from 18.02 is too small to matter in most school sums.
Percentage composition
By mass, H2O is 11.19% hydrogen and 88.81% oxygen. The largest share comes from oxygen. In a 100 g sample there are 11.2 g of hydrogen and 88.8 g of oxygen.
Notice that hydrogen has more atoms in the formula (2) than oxygen (1), yet gives a smaller share of the mass. Percentage by mass depends on how heavy each atom is, not only on how many there are.
Worked examples
Grams to moles. How many moles are in 25 g of H2O? Divide the mass by the molar mass: 25 ÷ 18.02 = 1.388 mol.
Moles to grams. What is the mass of 0.1 mol of H2O? Multiply: 0.1 × 18.02 = 1.802 g.
Counting particles. 25 g contains 1.388 mol × 6.022 × 1023 = 8.357 × 1023 molecules, and 2.507 × 1024 atoms in all.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 1.802 g |
| 0.5 mol | 9.008 g |
| 1 mol | 18.02 g |
| 2 mol | 36.03 g |
| 5 mol | 90.08 g |
| Mass | Moles |
|---|---|
| 1 g | 0.05551 mol |
| 10 g | 0.5551 mol |
| 100 g | 5.551 mol |
| 1 kg | 55.51 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of H2O?
The molar mass of water (H2O) is 18.02 g/mol, or 18.015 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of H2O?
100 ÷ 18.02 = 5.551 mol.
What is the mass of one molecule of H2O?
Divide the molar mass by the Avogadro constant: 18.02 ÷ 6.022 × 1023 = 2.991 × 10-23 g.
What is the percentage of oxygen in H2O?
Oxygen makes up 88.81% of the mass of H2O: 1 × 15.999 ÷ 18.015 × 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.