Molecular Weight Lookup — Common Compounds

Pick one of 12 common compounds from the list and this tool returns its molecular weight, its formula and the number of moles in 100 grams, the figures chemists, biologists, and materials scientists rely on for stoichiometry, yield predictions, and solution preparation. The figure underpins a surprising amount of laboratory work.

The list covers water, sodium chloride, carbon dioxide, glucose, ethanol, sodium hydroxide, hydrochloric acid, sulfuric acid, calcium carbonate, ammonia, oxygen and nitrogen. Each molecular weight is stored as a fixed value, rounded to two or three decimals: the sum of the standard atomic weights of the atoms in the formula. For water that is 2 × 1.008 + 15.999 = 18.015 g/mol. Standard atomic weights are averages over the natural mix of each element's isotopes, which is why they are not whole numbers. Moles in 100 g is simply 100 divided by the molecular weight.

There is nothing to type, so capitalization and subscript mistakes cannot creep in, but the list is short: for a compound that is not on it, add up the atomic weights of its atoms yourself. When you convert grams to moles for a solution, use the molecular weight of the exact form you weigh out; a hydrate carries its water molecules in its formula weight, so it is heavier than the anhydrous salt.