To calculate the molar mass of water first you must look at the chemical formula for water.
1. Find the chemical formula for water.
2. Do an atom inventory of water.
One mole of water contains 2 moles of hydrogen atomes and 1 mole of oxygen atoms.
3. Find the molar mass of each element in water from the periodic table.
Hydrogen has a molar mass of 1.008 g/mol.
Oxygen has a molar mass of 15.994 g/mol
4. Multiply the number of atoms of each element by the molar mass of the element and add.
Hydrogen: 2 x 1.008 g/mol = 2.016 g/mol
Oxygen: 1 x 15.994 g/mol = 15.994 g/mol
2.016 + 15.994 = 18.01 g/mol