The molecular weight of Water [H₂O] is 18.0154.

To calculate molecular weight of any compound, the first step is to know the constituent atoms and their number in that particular compound. Then calculate the total weight of each atom by multiplying its atomic weight by its number. The sum of total weight of all constituent atoms will be the molecular weight of the compound. Note that the value of atomic weight may differ slightly from different sources.

CALCULATION PROCEDURE: Water [H₂O] Molecular Weight Calculation

Step 1: Find out the chemical formula and determine constituent atoms and their number in a Water molecule.
From the chemical formula, you will know different atoms and their number in a Water molecule. Chemical formula of Water is H₂O. From the chemical formula of Water, you can find that one molecule of Water has two Hydrogen (H) atoms and one Oxygen (O) atom.

Step 2: Find out atomic weights of each atom (from periodic table).

Atomic weight of Hydrogen (H) : 1.008 (Ref: Lanl-1)
Atomic weight of Oxygen (O) : 15.9994 (Ref: Jlab-ele008)

Step 3: Calculate the total weight of each atom present in a Water molecule by multiplying its atomic weight by its number.

Number of Hydrogen atoms in Water: 2
Atomic weight of Hydrogen: 1.008
Total weight of Hydrogen atoms in Water: 1.008 x 2 = 2.016

Number of Oxygen atoms in Water: 1
Atomic weight of Oxygen: 15.9994
Total weight of Oxygen atoms in Water: 15.9994 x 1 = 15.9994

Step 4: Calculate the molecular weight of Water by adding up the total weight of all atoms.

Molecular weight of Water: 2.016 (Hydrogen) + 15.9994 (Oxygen) = 18.0154

So the molecular weight of Water is 18.0154.

Water [H₂O] Molecular Weight Calculation

REFERENCES:

• Lanl-1: https://periodic.lanl.gov/1.shtml 
• Pubchem-Hydrogen : https://pubchem.ncbi.nlm.nih.gov/element/Hydrogen 
• Jlab-ele006; https://education.jlab.org/itselemental/ele006.html
• Pubchem-6: https://pubchem.ncbi.nlm.nih.gov/element/6
• Jlab-ele008: https://education.jlab.org/itselemental/ele008.html
• Pubchem-8: https://pubchem.ncbi.nlm.nih.gov/element/8

See also:

• Apiose (C5H10O5) Molecular Weight Calculation