Molar mass of water
The molar mass of water (H₂O) is 18.015 g/mol: the sum of the atomic weights 2 × 1.0080 + 15.999. School problems round it to 18 g/mol (H = 1, O = 16).
Breakdown by element
M(H2O) = 2 × 1.0080 + 15.999 = 18.015 g/mol
The result is rounded to the precision of the least precise atomic weight. Atomic weights from IUPAC (CIAAW). Agrees with PubChem: 18.015 g/mol. PubChem CID 962
What is water?
Water covers about 71% of the Earth’s surface and occurs in nature in all three states: ice, liquid water and water vapour.
The H₂O molecule is polar, so water dissolves many salts, acids and gases. Ice is less dense than liquid water (0.917 g/cm³ at 0 °C), which is why it floats.
Properties
- State at 25 °C
- Liquid
- Density
- 1.000 g/cm³ 4 °C
- Melting point
- 0 °C
- Boiling point
- 99.97 °C
Sources: PubChem (HSDB, NIOSH), Wikipedia (CRC Handbook), NIST Chemistry WebBook; each value agrees in two of them.
Molar mass problems with water
For school problems (H = 1, O = 16): M(H₂O) = 18 g/mol
Problem 1
How many moles are in 54 g of H₂O?
n = m ÷ M = 54 g ÷ 18 g/mol
Answer: 3 mol
Problem 2
What is the mass of 1.5 mol of H₂O?
m = n × M = 1.5 mol × 18 g/mol
Answer: 27 g
Problem 3
What is the mass percentage of H in H₂O?
w(H) = 2 × M(H) ÷ M(H₂O) = 2 × 1 ÷ 18 ≈ 0.111
Answer: 11.1%
Problem 4
How many molecules are in 0.1 mol of H₂O?
N = n × NA = 0.1 mol × 6.02 × 10²³ mol⁻¹
Answer: 6.02 × 10²²
Common substances
What is molar mass?
Molar mass M is the mass of one mole of a substance, about 6.022 × 10²³ of its particles. It is measured in grams per mole (g/mol).
To find it, add up the atomic weights of all the atoms in the formula: multiply each element’s atomic weight by its subscript, the number of its atoms. In g/mol, the molar mass equals the relative molecular mass Mr.
For water: M(H₂O) = 2 × 1.0080 + 15.999 = 18.015 g/mol.