Molecular Weight Calculator
How It Works
Molecular weight (MW), also called molar mass, is the sum of the atomic weights of all atoms in a molecule. It is expressed in grams per mole (g/mol) and is essential for converting between mass and moles in chemistry calculations.
This calculator uses IUPAC standard atomic weights: Carbon 12.011, Hydrogen 1.008, Oxygen 15.999, Nitrogen 14.007, Sulfur 32.06, and Chlorine 35.45. Enter the number of each atom in your molecule to get the molecular weight.
For example, glucose (C6H12O6) has MW = 6(12.011) + 12(1.008) + 6(15.999) = 180.156 g/mol. Water (H2O) = 2(1.008) + 15.999 = 18.015 g/mol. One mole of any substance contains Avogadro's number (6.022 x 10^23) of molecules.
Frequently Asked Questions
What is the molecular weight of water?
Water (H2O) has a molecular weight of 18.015 g/mol, calculated as 2(1.008) + 15.999. This means 18.015 grams of water contains 6.022 x 10^23 water molecules.
What is the difference between molecular weight and formula weight?
Molecular weight applies to molecules (covalent compounds). Formula weight applies to ionic compounds (like NaCl) that do not form discrete molecules. The calculation method is the same — sum the atomic weights.
How do I calculate moles from grams?
Divide mass in grams by molecular weight: moles = mass / MW. For example, 36 grams of water = 36 / 18.015 = 2.0 moles.
Why are atomic weights not whole numbers?
Atomic weights are weighted averages of all naturally occurring isotopes. Carbon is 12.011 because natural carbon is 98.9% C-12 and 1.1% C-13. The decimal reflects this isotopic mixture.