Molecular Weight Calculator
Calculate molar mass from any chemical formula, with per-element percent composition and a grams-to-moles conversion.
Comment utiliser cette calculatrice
- 1Type the chemical formula, respecting capitalisation — Co is cobalt, CO is carbon monoxide.
- 2Use parentheses for groups, as in Ca(OH)2, and a dot for hydrates, as in CuSO4·5H2O.
- 3Optionally enter a sample mass to convert grams to moles and molecules.
Comment ça marche
Molar mass and the mole
molar mass = Σ (atomic weight × number of atoms) moles = mass (g) ÷ molar mass (g/mol) molecules = moles × 6.02214076 × 10²³ percent by mass = (element contribution ÷ molar mass) × 100
Molar mass is simply the sum of every atom's weight in the formula. Its usefulness comes from the mole: one mole of any substance contains Avogadro's number of particles and weighs its molar mass in grams. That single bridge is what lets you weigh out a reagent on a balance and know exactly how many molecules you have put in the flask.
Exemple détaillé
Glucose, C₆H₁₂O₆: carbon contributes 6 × 12.011 = 72.066, hydrogen 12 × 1.008 = 12.096, and oxygen 6 × 15.999 = 95.994 — a molar mass of 180.156 g/mol. A 10 g sample is therefore 0.0555 moles, or about 3.34 × 10²² molecules.
Molecular Weight Calculator : le guide complet
Why capitalisation is not optional
Element symbols are always one capital letter, optionally followed by one lowercase letter. Co is cobalt; CO is carbon monoxide — one atom of carbon and one of oxygen. Their molar masses are 58.93 and 28.01 g/mol, so a typo here is a factor-of-two error in a reaction.
The same trap catches Ni (nickel) versus NI, Hf (hafnium) versus HF (hydrogen fluoride), and Bi (bismuth) versus BI. This calculator parses strictly rather than guessing, so an ambiguous formula is rejected rather than silently misread.
Writing formulas with groups and hydrates
Parentheses group a unit that repeats: Ca(OH)₂ is one calcium with two hydroxide groups, giving one Ca, two O, and two H. Writing it CaO2H2 gives the same molar mass but loses the structural information, which matters as soon as you are talking about the chemistry rather than just the mass.
A centre dot marks a hydrate — water held in the crystal lattice. Copper(II) sulfate pentahydrate is CuSO₄·5H₂O, and those five waters are 90 g/mol of the 249.68 total. Weighing out the anhydrous mass when you have the hydrate on the shelf under-doses your reaction by 36%.
This parser accepts a middle dot, an asterisk, or a period as the hydrate separator, and handles nested parentheses such as Fe₂(SO₄)₃ and K₄[Fe(CN)₆].
Percent composition and empirical formulas
Percent composition tells you what fraction of a compound's mass each element contributes. It is how fertiliser is graded (the N-P-K numbers), how ore quality is quoted, and how a purity calculation is checked.
It also runs in reverse. Elemental analysis returns percentages, and dividing each by its atomic weight then normalising to the smallest ratio recovers the empirical formula. Glucose comes back as CH₂O — the simplest whole-number ratio, which is why empirical formulas alone cannot distinguish glucose from formaldehyde.
Average versus monoisotopic mass
The atomic weights used here are averages weighted by natural isotopic abundance. Chlorine's 35.45 is not any real atom's mass — it reflects roughly 76% chlorine-35 and 24% chlorine-37. That average is exactly right for weighing bulk material on a balance.
Mass spectrometry sees individual molecules and therefore individual isotopes, so it reports monoisotopic mass — computed from the single most abundant isotope of each element. For a small molecule the difference is under 1%, but across a protein it accumulates to several daltons, which is enough to misidentify a peak.
Use the average mass here for stoichiometry and solution preparation, and a dedicated monoisotopic calculation for MS interpretation.
Questions fréquentes
What is the difference between molecular weight and molar mass?
Numerically nothing. Molecular weight is a dimensionless ratio relative to 1/12 the mass of a carbon-12 atom; molar mass is the mass of one mole in grams. They have the same value, which is why the terms are used interchangeably in practice — molar mass is the more precise term.
How do I convert grams to moles?
Divide the mass in grams by the molar mass. 10 g of glucose at 180.156 g/mol is 0.0555 moles. Enter a sample mass above and the calculator does this along with the molecule count.
How do I write a hydrate formula?
Put the anhydrous compound first, then a dot, then the number of waters: CuSO₄·5H₂O. The waters count toward the molar mass — they are 90 of the 249.68 g/mol for copper sulfate pentahydrate, which is why using the anhydrous figure badly under-doses a solution.
Why do the elements have decimal atomic weights?
Because they are averages across the naturally occurring isotopes, weighted by abundance. Chlorine's 35.45 reflects a mix of chlorine-35 and chlorine-37. Only carbon-12 has an exact integer mass, because it is the definition the scale is built on.