Dextromethorphan

Morphinan,3-methoxy-17-methyl-,(9a,13a,14a)-.
3-Methoxy-17-methyl-9a,13a,14a-morphinan [125-71-3].
Dextromethorphan contains not less than 98.0percent and not more than 101.0percent of C18H25NO,calculated on the anhydrous basis
the formula used in calculating molar mass is the molecular formula, the formula weight computed is the molecular weight. The percentage by weight of any atom or group of atoms in a compound can be computed by dividing the total weight of the atom (or group of atoms) in the formula by the formula weight and multiplying by 100.
Formula weights are especially useful in determining the relative weights of reagents and products in a chemical reaction. These relative weights computed from the chemical equation are sometimes called equation weights.
In chemistry, the formula weight is a quantity computed by multiplying the atomic weight (in atomic mass units) of each element in a chemical formula by the number of atoms of that element present in the formula, then adding all of these products together.