Molar mass of water (H₂O)
The molar mass of water (H₂O) is 18.015 g/mol, summed from the IUPAC standard atomic weights of H x 2, O. The reference substance for almost every solution calculation.
- Moles in the sample mass
- 0.555 092 98 mol
- Mass of that amount of substance
- 18.015
- Molecules in the sample mass
- 3.34285e+23
Show the working
- Mass of sample = 10
- Amount of substance = 1
- Molar mass = 1.008*2 + 15.999 = 18.015
- Moles in the sample mass = 10 / 18.015 = 0.555 093
- Mass of that amount of substance = 1 * 18.015 = 18.015
- Molecules in the sample mass = 0.555 093 * NA = 3.34285e+23
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- Molar mass = sum over elements of (atomic weight x number of atoms)
- For H₂O: 1.008 x 2 + 15.999 = 18.015 g/mol
- Moles = mass / molar mass; mass = moles x molar mass
- Molecules = moles x 6.02214076 x 10^23 (the Avogadro constant, exact by definition)
Molar mass in grams per mole is numerically equal to the molecular mass in daltons, so this figure doubles as the molecular weight.
Atomic weights are the IUPAC 2021 standard values. Several elements have a range rather than a single value because isotope ratios vary between sources; the conventional single value is used here.
The reference substance for almost every solution calculation.
Composition by mass
| Element | Atoms | Atomic weight | Contribution (g/mol) | Share of mass |
|---|---|---|---|---|
| Oxygen (O) | 1 | 15.999 | 15.999 | 88.809 % |
| Hydrogen (H) | 2 | 1.008 | 2.016 | 11.191 % |
Contributions add to the molar mass of 18.015 g/mol. Atomic weights are the IUPAC 2021 standard values.
Frequently asked
What is the molar mass of water?
18.015 g/mol, for the formula H₂O.
How do you convert grams of water to moles?
Divide the mass in grams by 18.015 g/mol. One gram of water is 0.0555093 mol.
How many grams are in one mole of water?
18.015 g, by definition of the molar mass.
Is molar mass the same as molecular weight?
Numerically yes, for the purposes of this calculation. Molecular weight is a dimensionless ratio and molar mass carries units of g/mol, but the digits are identical.
Where the numbers come from
- IUPAC 2021 standard atomic weights; Avogadro constant fixed by the 2019 SI.