Molar Mass Calculator

In short The molar mass of a compound is the sum of the standard atomic weights of every atom in its formula, in grams per mole. Copper sulfate pentahydrate, CuSO4·5H2O, comes to 249.68 g/mol: one copper at 63.546, one sulfur at 32.06, nine oxygens at 15.999 each and ten hydrogens at 1.008 each, because the five waters contribute ten hydrogens and five oxygens on top of the four already in the sulfate. Glucose C6H12O6 is 180.16, sodium chloride 58.44, water 18.015. Two things about that sum surprise people. The first is that almost no atomic weight is a whole number, because each one is an average over the isotopes actually found on Earth — chlorine is 35.45 because natural chlorine runs about three parts Cl-35 to one part Cl-37, and no single chlorine atom weighs 35.45. The second is that capitalisation is not a style choice but part of the notation. CO is carbon monoxide at 28.01 g/mol; Co is cobalt at 58.93. A symbol is one capital letter optionally followed by one lower-case letter, and getting that wrong changes the compound rather than the formatting. Percent composition falls straight out of the same sum: divide each element's total contribution by the molar mass. In copper sulfate pentahydrate the water is 36.1% of the mass, which is why the blue crystals lose over a third of their weight when you drive it off.