Molecular Weight Calculator
Calculate molecular weight and molar mass of chemical compounds
Enter a chemical formula with proper element symbols and subscripts
Molecular Weight
18.016 Da
Molecular Formula
Same as empirical
Element Composition
| Element | Count | Mass (g/mol) | Percentage |
|---|---|---|---|
| H (Hydrogen) | 2 | 2.016 | 11.19% |
| O (Oxygen) | 1 | 16.000 | 88.81% |
Mass to Moles
1 g = 0.055506 mol
100 g = 5.5506 mol
Moles to Mass
1 mol = 18.016 g
0.1 mol = 1.802 g
Additional Properties
How it works
Molecular weight (molar mass) is the mass of one mole of a substance, found by adding up the atomic weights of every atom in its formula. It converts between grams and moles, the link between what you weigh out and what reacts.
Molar mass
Molar mass = Σ (atomic weight × atom count) moles = grams ÷ molar mass
- atomic weight
- from the periodic table (g/mol)
- atom count
- number of each element in the formula
Worked example
- Water, H₂O
- H ≈ 1.008, O ≈ 16.00
- 2 × 1.008 + 1 × 16.00
Molar mass of water ≈ 18.02 g/mol.
Good to know
- “Molecular weight” and “molar mass” are used interchangeably; the value is the same number in g/mol.
- One mole is 6.022 × 10²³ particles (Avogadro's number).
- Knowing molar mass lets you convert a weighed sample to moles for reactions and concentrations.
Related Calculators
Frequently Asked Questions
How is molecular weight calculated?
Sum the atomic masses of every atom in the formula. For water (H₂O): 2 × 1.008 + 15.999 ≈ 18.02 g/mol. Subscripts multiply the preceding element, and the atomic masses come from the periodic table.
What's the difference between molecular weight and molar mass?
They're numerically equal but conceptually different: molecular weight is the relative mass of one molecule in atomic mass units (amu), while molar mass is the mass of one mole of the substance in g/mol. In practice the terms are used interchangeably.
How do I handle parentheses and hydrates in formulas?
Multiply everything inside parentheses by the subscript that follows: Ca(OH)₂ contains one Ca, two O, and two H. Hydrates like CuSO₄·5H₂O add the water's mass times its coefficient — five waters add about 90.08 g/mol.
How do I convert between grams and moles?
Moles = mass ÷ molar mass, and mass = moles × molar mass. With NaCl (58.44 g/mol), 117 g is almost exactly 2 moles. This conversion underlies nearly every stoichiometry and solution-preparation calculation.
Why aren't atomic masses whole numbers?
Periodic-table masses are weighted averages over an element's naturally occurring isotopes. Chlorine is about 35.45 because natural chlorine is roughly 76% Cl-35 and 24% Cl-37 — no single atom has that mass, but a mole of natural chlorine effectively does.