Molecular Weight Calculator Online Free
Molecular Weight Calculator
Formula Input
Use element symbols (H, C, O, etc.) and numbers for subscripts
Choose from 158 pre-loaded chemicals (67 organic, 91 inorganic)
Formula Guidelines
- • Element symbols are case-sensitive (H, not h)
- • Numbers go after element (H2, not 2H)
- • Use parentheses for groups: Ca(OH)2
- • No spaces in formula
Enter a Chemical Formula
Type a chemical formula or select a common molecule to calculate its molecular weight and analyze elemental composition.
Molecular weight (also called molar mass) is the sum of the atomic masses of all atoms in a molecule, measured in grams per mole (g/mol). This calculator looks up atomic masses by element symbol, computes molecular weight from a chemical formula, and converts between moles, grams, and number of molecules.
How to Calculate Molecular Weight
Look up the atomic mass of each element from the periodic table. Multiply by the number of times that element appears in the formula. Sum all values.
Example: Water (H₂O) H: 1.008 × 2 = 2.016 O: 15.999 × 1 = 15.999 Molecular Weight = 18.015 g/mol Example: Glucose (C₆H₁₂O₆) C: 12.011 × 6 = 72.066 H: 1.008 × 12 = 12.096 O: 15.999 × 6 = 95.994 MW = 180.156 g/mol
1 mole = 6.022 × 10²³ molecules (Avogadro's number).
Mole Conversions
Moles = Mass (g) / Molecular Weight (g/mol) Mass (g) = Moles × MW Molecules = Moles × 6.022 × 10²³ Example: 36 g of water Moles = 36 / 18.015 = 2.0 mol Molecules = 2.0 × 6.022 × 10²³ = 1.20 × 10²⁴
Frequently Asked Questions
What is the difference between molecular weight and molar mass?⌄
Technically, molecular weight is the mass of a single molecule in atomic mass units (u). Molar mass is the mass of one mole of a substance in grams per mole (g/mol). Their numerical values are identical: water has a molecular weight of 18.015 u and a molar mass of 18.015 g/mol. The terms are often used interchangeably in chemistry.
What is a mole in chemistry?⌄
A mole is Avogadro's number (6.022 × 10²³) of particles. Just as a dozen always means 12, a mole always means 6.022 × 10²³ -- whether atoms, molecules, ions, or electrons. The mole bridges atomic-scale masses (in atomic mass units) and laboratory-scale masses (in grams), because the atomic mass in u equals the molar mass in g/mol.
How do you read a chemical formula to count atoms?⌄
Subscripts after an element symbol indicate how many atoms of that element are in one formula unit. Parentheses group elements with a subscript applied to the whole group. Ca(OH)₂: Ca=1, O=2, H=2. Mg₃(PO₄)₂: Mg=3, P=2, O=8. Work from inside out: parentheses are resolved first, then outer subscripts are applied.
What are isotopes and how do they affect molecular weight?⌄
Isotopes are atoms of the same element with different numbers of neutrons (and thus different masses). The atomic mass listed in the periodic table is the weighted average of all naturally occurring isotopes. Carbon's atomic mass (12.011) reflects mostly C-12 (98.9%) with some C-13 (1.1%). For most chemical calculations, the average atomic mass is appropriate.