Molar mass calculator
H₂SO₄ has a molar mass of 98.081 g/mol — two hydrogen at 1.008, one sulfur at 32.06 and four oxygen at 15.999. Case is part of the formula and not a typing convenience: Co is cobalt at 58.93 and CO is carbon monoxide at 28.01, and nothing in the arithmetic can tell you which you meant.
Molar mass and percent composition from a formula — with nested groups, brackets and hydrates parsed the way they are written, and atomic weights taken from NIST rather than from memory.
Atomic weights pulled from NIST; 118 elements · How we check
Molar mass
249.6878
grams per mole. 10 g is 0.040050 moles — 2.412e+22 molecules.
Where the mass is
| Element | Atoms | Atomic weight | Mass | Share |
|---|---|---|---|---|
| O Oxygen | 9 | 15.9994 | 143.9946 | 57.67% |
| Cu Copper | 1 | 63.5460 | 63.5460 | 25.45% |
| S Sulfur | 1 | 32.0675 | 32.0675 | 12.84% |
| H Hydrogen | 10 | 1.0080 | 10.0798 | 4.04% |
O, S, H have no single atomic weight: the isotopic composition varies between natural sources, so IUPAC publishes an interval and the midpoint is used here. For carbon that interval is 12.0096 to 12.0116, which is why a molar mass in the fourth decimal is a convention rather than a measurement.
The parser is the calculator
Adding atomic weights is arithmetic anyone can do. Reading the formula is where the work is: a bracket multiplies everything inside it, a bracket inside a bracket multiplies again, and a hydrate dot means a separate molecule counted alongside. K4[Fe(CN)6] has six carbons and six nitrogens because the 6 applies to the whole CN group, and a parser that scans for element-then-digit gets that wrong every time.
The other half is the table underneath. A single number cannot be checked; a breakdown by element — how many atoms, at what weight, contributing what share — can be compared against your own working line by line, which is what somebody with a periodic table open actually needs.
Questions people actually ask
- What is the difference between molar mass and molecular weight?
- Units, mostly. Molecular weight is a ratio of masses and is dimensionless; molar mass is the same number in grams per mole. In practice they are used interchangeably and the figure is identical, which is why searching for one finds pages about the other.
- Why does case matter in a formula?
- Because Co is cobalt and CO is carbon monoxide — a metal and a gas. Element symbols are one capital letter optionally followed by one lower-case letter, and that rule is what makes a formula unambiguous. A calculator that uppercases what you type has thrown the distinction away before it starts.
- How do I write a hydrate?
- With a dot: CuSO4·5H2O is copper sulfate pentahydrate, and the five waters are part of the crystal rather than a contaminant. They add 90 g/mol — over a third of the total — which is why weighing out the anhydrous mass when your bottle contains the hydrate makes a solution a third too weak.
- Why is carbon 12.011 rather than 12?
- Because natural carbon is a mixture. Carbon-12 is exactly 12 by definition, but about 1.1% of natural carbon is carbon-13, which drags the average up. IUPAC actually publishes an interval — 12.0096 to 12.0116 — because the proportion varies between sources, and the single value everyone quotes is a convention taken from the middle of it.
- Why do some elements have no atomic weight?
- Because there is nothing to average. Above plutonium every element is artificial, has no stable isotope, and has no characteristic natural composition, so IUPAC publishes no standard atomic weight at all. Calculators that show a number for americium are showing the mass number of one isotope and calling it something it is not.