Molar mass of H2O (Water)
The molar mass of H2O is 18.02 g/mol.
The most common solvent on Earth and the product of every acid–base neutralisation.
Step-by-step calculation
Add up the atomic masses of every atom in the formula:
2 × 1.008 + 1 × 15.999 = 18.02 g/mol
| Element | Atoms | Atomic mass (u) | Subtotal (g/mol) | % by mass |
|---|---|---|---|---|
| Hydrogen (H) | 2 | 1.008 | 2.016 | 11.19% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 88.81% |
| Molar mass of H2O | 18.02 g/mol | |||
Moles to grams for H2O
Multiply the number of moles by 18.02 g/mol to get the mass, and divide a mass by 18.02 g/mol to get the moles.
| Amount (mol) | Mass (g) | Mass (g) | Amount (mol) |
|---|---|---|---|
| 0.1 mol | 1.80 g | 1 g | 0.0555 mol |
| 0.25 mol | 4.50 g | 5 g | 0.2775 mol |
| 0.5 mol | 9.01 g | 10 g | 0.5551 mol |
| 1 mol | 18.02 g | 50 g | 2.7755 mol |
| 2 mol | 36.03 g | 100 g | 5.5509 mol |
| 5 mol | 90.08 g | 500 g | 27.7546 mol |
Experiments that use water
🧪 Acid–base titration
Find the end-point of HCl vs NaOH with phenolphthalein and plot the titration curve.
✨ Golden rain
Make lead(II) iodide, dissolve it hot, then watch glittering golden crystals form on cooling.
🐘 Elephant's toothpaste
Iodide-catalysed decomposition of 30% hydrogen peroxide produces a towering, steaming foam.
🫧 CO₂ and limewater
Marble + acid makes CO₂. Bubble it through limewater: milky, then clear again in excess.
Frequently asked questions
What is the molar mass of H2O?
The molar mass of H₂O (water) is 18.02 g/mol, so one mole of water weighs 18.02 grams.
How do you calculate the molar mass of H2O?
Multiply each element’s atomic mass by the number of atoms in the formula and add the results: 2 × 1.008 + 1 × 15.999 = 18.02 g/mol.
What percentage of H2O is oxygen?
Oxygen makes up 88.81% of the mass of H₂O.
How many grams are in one mole of H2O?
One mole of H₂O has a mass of 18.02 grams, and contains 6.022 × 10²³ formula units.