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 sum using the listed atomic weights 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
Calculate Another Molar Mass →
Looking for its bonding structure? See the H₂O Lewis Structure →
| 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. Using the listed, rounded standard atomic weights gives approximately 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 total using these atomic weights is approximately 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 approximately 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
Convert grams ↔ moles of H₂O in the full calculator →
Need a full reaction calculation? Open the Stoichiometry Calculator →
Try Another Formula
Enter any formula to get its molar mass, elemental composition, and the full step-by-step calculation.
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.
- Empirical Formula CalculatorTurn percent composition into a formula.
- Chemical Equation BalancerBalance equations with coefficients and atom checks.
Frequently Asked Questions
What is the molar mass of H₂O?
Approximately 18.015 g/mol. Using the listed, rounded standard atomic weights, the calculation 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.
Atomic weight reference
Calculations use the rounded standard atomic weights shown in the breakdown, not exact isotope masses. Natural isotope composition can vary, and some standard atomic weights are reported as intervals. See the CIAAW standard atomic weights for the reference values. Use your course’s table when its rounding differs.