Type a formula — brackets and hydrate dots included — and get the molar mass with each element’s contribution shown.
Symbols are case-sensitive. Use brackets for groups, and · or a full stop before a hydrate: CuSO4·5H2O.
| Element | Atoms | Atomic weight | Contribution | Mass % |
|---|
The molar mass of a compound is the mass, in grams, of one mole of it — that is, of 6.022 × 10²³ formula units. You get it by adding up the standard atomic weight of every atom in the formula. Water is two hydrogens at 1.008 plus one oxygen at 15.999, giving 18.015 g/mol.
It is the number that converts between the two things a chemist actually cares about: mass, which you can weigh, and moles, which is what reactions are counted in. A balanced equation tells you that two moles of hydrogen react with one of oxygen; molar mass tells you that means 4.03 g and 32.00 g respectively.
Case matters, and it matters more than people expect. Co is cobalt, at 58.93 g/mol. CO is carbon monoxide, at 28.01. Cs is caesium; CS would be carbon and sulfur. The calculator rejects lowercase-first input rather than guessing, because guessing is how you get the wrong answer confidently.
Brackets multiply everything inside them. Ca(OH)2 is one calcium, two oxygens and two hydrogens — 74.09 g/mol. Nesting works too. Fe2(SO4)3 gives two irons, three sulfurs and twelve oxygens, at 399.86 g/mol.
Hydrates use a dot. Copper sulfate pentahydrate is CuSO4·5H2O, and the five multiplies the whole water unit that follows it. The anhydrous salt is 159.61 g/mol; with five waters attached it is 249.68 g/mol — a 56% difference. Weighing out the hydrate as though it were anhydrous is one of the most common quantitative errors in a teaching lab.
| Compound | Formula | Molar mass |
|---|---|---|
| Water | H2O | 18.015 g/mol |
| Carbon dioxide | CO2 | 44.009 g/mol |
| Sodium chloride | NaCl | 58.44 g/mol |
| Sulfuric acid | H2SO4 | 98.07 g/mol |
| Glucose | C6H12O6 | 180.16 g/mol |
| Caffeine | C8H10N4O2 | 194.19 g/mol |
| Calcium hydroxide | Ca(OH)2 | 74.09 g/mol |
| Copper sulfate pentahydrate | CuSO4·5H2O | 249.68 g/mol |
| Potash alum | KAl(SO4)2·12H2O | 474.37 g/mol |
An atomic weight is not the mass of a single atom — it is the weighted average across the isotopes found in nature. Chlorine is roughly three parts chlorine-35 to one part chlorine-37, which averages to 35.45. Carbon is 98.9% carbon-12 and 1.1% carbon-13, giving 12.011 rather than a clean 12.
For a few elements the natural mix genuinely varies by source, so IUPAC publishes an interval rather than a single number. The conventional single values used here are the ones intended for exactly this kind of calculation. For elements with no stable isotope — technetium, promethium, the actinides beyond uranium — the figure is the mass number of the most stable known isotope instead.
In practice these are used interchangeably and this calculator treats them as the same number, but they are not identical concepts. Molecular weight is dimensionless — a ratio against one twelfth of a carbon-12 atom. Molar mass carries units of g/mol. Formula weight is the term preferred for ionic compounds like NaCl, which have no discrete molecules at all, only a repeating lattice. The arithmetic is the same in every case.
Almost always capitalisation. Element symbols are one uppercase letter, optionally followed by one lowercase: Na, not NA or na. So "co2" fails while "CO2" works. The other common causes are an unclosed bracket, or a symbol that is not a real element — the calculator names the offending symbol rather than silently ignoring it, because silently ignoring an atom gives a plausible-looking but wrong molar mass.
Use a middle dot or a full stop before the water, with the multiplier in front of it: CuSO4·5H2O or CuSO4.5H2O both work. The number multiplies the entire unit that follows, so 5H2O contributes five waters, not five hydrogens. This matters a great deal in practice — copper sulfate pentahydrate is 249.68 g/mol against 159.61 for the anhydrous salt, so treating one as the other throws a preparation off by more than half.
Numerically nothing, which is why the terms get used interchangeably. Strictly, molecular weight is a dimensionless ratio relative to one twelfth the mass of a carbon-12 atom, while molar mass is a mass per amount of substance and carries units of g/mol. For ionic compounds such as sodium chloride, "formula weight" is the more accurate term, since there is no discrete NaCl molecule — only a lattice.
Because it is an average over the isotopes that occur naturally. Chlorine in nature is about 76% chlorine-35 and 24% chlorine-37, and the weighted average of those lands at 35.45. Only elements with a single stable isotope, such as fluorine or sodium, come out close to a whole number. Carbon-12 is exactly 12 by definition, but natural carbon includes some carbon-13, giving 12.011.
For a repeating unit, yes — enter the monomer and multiply. For a whole polymer chain the concept breaks down, because a real polymer sample contains chains of many different lengths and is described by an average molecular weight with a distribution around it, not a single exact value. Proteins are a similar case: you can compute an exact mass from the sequence, but that is a different calculation from summing a short formula.
Related: Molarity Calculator · Density Calculator · Scientific Notation Calculator · Half-Life Calculator For physics rather than chemistry, Weight on Other Planets applies the same kind of tabulated constants to surface gravity.