Breaking down the formula
Molecular weight (also called molar mass) is the sum of atomic masses for all atoms in a chemical compound. When you enter a formula like H₂O or NaCl, the calculator identifies each element, counts how many atoms are present, and adds up their standard atomic weights from the periodic table.
The result is expressed in grams per mole (g/mol)—the mass of one mole (6.022 × 10²³ particles) of that substance.
The formula
Molar Mass = Σ(atomic weight × atom count) for each element
Worked example
Let's calculate the molar mass of glucose (C₆H₁₂O₆):
- Carbon atoms: 6 × 12.01 g/mol = 72.06 g/mol
- Hydrogen atoms: 12 × 1.008 g/mol = 12.10 g/mol
- Oxygen atoms: 6 × 16.00 g/mol = 96.00 g/mol
- Total: 72.06 + 12.10 + 96.00 = 180.16 g/mol
This means one mole of glucose weighs 180.16 grams. If you had a sample of glucose that weighed 90 grams, you'd have approximately 0.5 moles.
Another example: calcium carbonate (CaCO₃)
- Calcium: 1 × 40.08 = 40.08
- Carbon: 1 × 12.01 = 12.01
- Oxygen: 3 × 16.00 = 48.00
- Total: 100.09 g/mol
Entering your formula
Type the chemical formula exactly as it appears in chemistry notation:
- Use element symbols with correct capitalization (H, not h; Ca, not CA)
- Numbers after an element show atom count (H₂O has 2 hydrogen atoms)
- Use parentheses for polyatomic groups: Ca(OH)₂ means one calcium, two oxygen, and two hydrogen atoms
- Leave out the "1" for single atoms (write O₂, not O₂)
The calculator uses standard atomic weights from the IUPAC periodic table, rounded to two decimal places for practical chemistry work.
Common mistakes
Forgetting parentheses: Writing CaOH₂ instead of Ca(OH)₂ gives the wrong answer—the subscript only applies to the element immediately before it.
Mismatched capitalization: Element symbols are case-sensitive. "CO" (carbon monoxide) is different from "Co" (cobalt).
Skipping the subscript: H2O and HO are different compounds with completely different molar masses.
Typos in formula: Double-check your entry against a reliable source. One wrong atom or number changes the result significantly.
Why this matters
Molar mass is essential for converting between grams and moles—a fundamental step in stoichiometry, solution preparation, and chemical calculations. If you're mixing chemicals, determining reaction yields, or preparing a solution of known concentration, you need the accurate molar mass. It's also used to find empirical formulas and verify compound identity in labs.
This calculator provides an estimate based on standard atomic weights. For specialized work requiring isotope-specific masses or extreme precision, consult NIST or primary chemical databases.