Molar Mass Calculator (from chemical formula)

Type a chemical formula and get its molar mass in grams per mole, plus a breakdown of each element's atomic mass, count, mass contribution and percentage by mass. The parser understands parentheses and brackets — like Ca(OH)2 or Fe2(SO4)3 — and hydrate dots such as CuSO4·5H2O. Standard atomic weights for all 118 elements are built in. Pair it with the molarity tool, which takes the molar mass to work out solution concentrations. Everything runs locally in your browser — nothing is uploaded.

The atomic weights are averages over natural isotopic abundance, not the mass of any single atom. Chlorine's 35.45 is a mixture of chlorine-35 and chlorine-37 as they occur on Earth, and no chlorine atom weighs that. For weighing out a reagent the average is exactly right; for interpreting a mass spectrum it is exactly wrong, because the instrument separates the isotopes and you want the monoisotopic mass instead.

Several elements have no stable isotopes and therefore no meaningful average. IUPAC gives them the mass number of their longest-lived isotope in brackets, which is a different kind of quantity, and a formula containing technetium or any transuranic returns a figure that depends on which isotope the table chose rather than on anything about a sample.

Case is chemistry, not typing. CO is carbon monoxide and Co is cobalt, NO is nitric oxide and No is nobelium. The parser follows the standard rule, an uppercase letter optionally followed by a lowercase one, so a formula typed in all caps parses into elements you did not intend rather than failing.

The hydrate dot is part of the formula rather than decoration. CuSO4·5H2O weighs about 56 percent more than the anhydrous salt, so weighing the hydrate against an anhydrous calculation gives a solution well under the concentration you wanted. Which form is on the bottle is the thing to check before the arithmetic.