H₂O Molar Mass
Water
The molar mass of H₂O (water) is approximately 18.015 g/mol, commonly rounded to 18.02 g/mol. Each molecule carries two hydrogen atoms and one oxygen atom, so the exact sum is (2 × 1.008) + 15.999 = 18.015 g/mol — about 11.19% hydrogen and 88.81% oxygen by mass.
Molar mass of H₂O
18.015 g/mol
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| Formula | H₂O |
|---|---|
| Molar Mass | 18.015 g/mol |
| Molecular Weight | 18.015 |
| Hydrogen Atoms | 2 |
| Oxygen Atoms | 1 |
| Total Atoms | 3 |
How to Calculate the Molar Mass of H₂O
The molar mass of H₂O is the sum of two hydrogen atomic weights and one oxygen atomic weight. The IUPAC standard atomic weights give exactly 18.015 g/mol — commonly rounded to 18.02 g/mol for classroom use.
H₂O
- Hydrogen: 2 × 1.008 = 2.016
- Oxygen: 1 × 15.999 = 15.999
2.016 + 15.999 = 18.015 g/mol
≈ 18.015 g/mol
Step 1 Read the chemical formula
H₂O is the chemical formula for water. Read it left to right: hydrogen first, then oxygen. Subscripts sit after the element they modify; a symbol with no subscript counts one.
Step 2 Count the atoms
H₂O contains 2 hydrogen atoms and 1 oxygen atom. The subscript ₂ applies only to hydrogen — oxygen keeps its implied count of 1 — so the molecule has two H and one O.
Step 3 Find the atomic masses
Look up each element's IUPAC standard atomic weight on the periodic table: H = 1.008 and O = 15.999. Most atomic weights are not whole numbers — each one averages an element's naturally occurring isotopes.
Step 4 Multiply by the atom counts
Each element contributes its atomic weight once per atom: H: 2 × 1.008 = 2.016; O: 1 × 15.999 = 15.999.
Step 5 Add the contributions
2.016 + 15.999 = 18.015 g/mol. The IUPAC total is 18.015 g/mol, commonly rounded to 18.02 g/mol for classroom use. One mole of H₂O — 6.022 × 10²³ molecules, 3 atoms each — has exactly this mass in grams.
H₂O Formula Breakdown
H₂O
- H = Hydrogen × 2
- O = Oxygen × 1
- Total atoms = 3
The subscript ₂ applies only to hydrogen — oxygen keeps its implied subscript of 1. Coefficients written in front of a formula (the 2 in 2H₂O) count separate molecules and do not change the molecule itself.
Elemental Composition of H₂O
Each element's share of the total mass of one mole of H₂O. One mole — about 18.015 g — contains 2.016 g of hydrogen and 15.999 g of oxygen, and every sample scales by the same ratio. Stated as percent composition, H₂O is 11.19% hydrogen and 88.81% oxygen by mass.
| Element | Atoms | Atomic Weight | Mass Contribution | Mass % |
|---|---|---|---|---|
| Hydrogen (H) | 2 | 1.008 | 2.016 | 11.19% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 88.81% |
| Total | 3 | 18.015 | 100% |
What Is the Molecular Weight of H₂O?
For H₂O the two terms describe the same number from two directions. The molar mass is the mass of one mole of water molecules: 18.015 g/mol. The molecular mass is the mass of one single H₂O molecule: about 18.015 u (unified atomic mass units), also written 18.015 Da. The values are numerically identical because the mole is defined that way — 1 g/mol corresponds to 1 u per molecule.
You will also see "molecular weight of water" and "relative molecular mass of H₂O", which name the same quantity. Homework and lab manuals often round to 18.02 g/mol or even 18 g/mol; the IUPAC value is still 18.015. Sources may print 18.015, 18.02, or 18.016 — all match for classroom work.
| Molar mass | 18.015 g/mol — the mass of one mole of H₂O molecules |
|---|---|
| Molecular mass | ≈ 18.015 u (Da) — the mass of one single H₂O molecule |
How Many Grams Are in One Mole of H₂O?
One mole of H₂O has a mass of approximately 18.015 grams. That is the whole conversion — the molar mass is the conversion factor between moles and grams. Going the other way, divide by the same factor: 36.030 g of H₂O is 36.030 ÷ 18.015 = 2 mol. Type an amount below to convert either way, using 18.015 g/mol.
1 mol of H₂O has a mass of 18.015 g
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Related Chemistry
- H₂O Lewis StructureTwo O–H single bonds, two lone pairs on oxygen, bent ~104.5° geometry.
- Molar Mass CalculatorMolar mass, molecular weight, and composition for any formula.
- Stoichiometry CalculatorConvert between moles, mass, and reaction amounts.
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Frequently Asked Questions
What is the molar mass of H₂O?
Approximately 18.015 g/mol. Using IUPAC standard atomic weights, the exact value is 2 × 1.008 (hydrogen) + 1 × 15.999 (oxygen) = 18.015 g/mol. Classroom work often rounds this to 18.02 g/mol.
How do you calculate the molar mass of H₂O?
Add two hydrogen atomic masses to one oxygen atomic mass: (2 × 1.008) + 15.999 = 18.015 g/mol. In general: read the subscripts in the formula, multiply each element's atomic weight by its atom count, then add the contributions.
What is the molecular mass of H₂O?
Approximately 18.015 u (Da) per molecule. Molecular mass and molar mass are numerically identical for water — one is the mass of a single molecule in unified atomic mass units, the other is the mass of one mole of molecules in grams.
What is the molecular weight of water?
About 18.015 — molecular weight is numerically identical to the molar mass of H₂O. One water molecule has a molecular mass of roughly 18.015 u; one mole of water weighs 18.015 g. Sources that print 18.02 or 18.016 are using the same quantity rounded or drawn from a slightly different atomic-weight set.
How many grams are in one mole of H₂O?
About 18.015 grams. One mole contains 6.022 × 10²³ water molecules, and the molar mass converts that amount directly to mass: 1 mol × 18.015 g/mol = 18.015 g.
What is the molar mass of 2H₂O?
The molar mass of H₂O itself remains 18.015 g/mol. The leading 2 in 2H₂O is a coefficient, not part of the formula — it counts two separate water molecules (or two moles of water), for a total mass of 2 × 18.015 = 36.030 g. Do not confuse that coefficient with the subscript ₂ inside H₂O, which means two hydrogen atoms per molecule.
How many atoms are in H₂O?
Three in total: two hydrogen atoms and one oxygen atom in every water molecule.