= Riboflavin
<Riboflavin> is the <vitamin> commonly called vitamin B2 and a precursor of the <cofactor (biochemistry)>[cofactors] <flavin mononucleotide> and <flavin adenine dinucleotide>. The neutral <riboflavin> structure has <molecular formula> $\mathrm{C_{17}H_{20}N_4O_6}$ and <molar mass> approximately $376.369\,\mathrm{g\,mol^{-1}}$. Its defined chemical structure and its nutritional role describe different aspects of the same substance.<Riboflavin in ChEBI>[CHEBI:17015, Definition, Formula, Average Mass and Ontology]{ref}
<Riboflavin kinase> transfers a phosphoryl group from <ATP> to <riboflavin>, producing <flavin mononucleotide> and <ADP>. <FAD synthase> then uses <ATP> to convert <flavin mononucleotide> into <flavin adenine dinucleotide>, releasing diphosphate. These are distinct chemical transformations; the <vitamin> is not itself the <enzyme> performing them.<Enzyme Nomenclature by the IUBMB>[EC2.7.1.26 and EC2.7.7.2, accepted reactions]{ref}
An ideal solution calculation connects chemical identity with <molar concentration>. Dissolving $0.376369\,\mathrm{mg}$ of pure neutral <riboflavin> and making the final volume $10.0\,\mathrm{mL}$ gives
$$
n=\frac{m}{M}=1.00\,\mathrm{\mu mol},\qquad c=\frac{n}{V}=100\,\mathrm{\mu mol\,L^{-1}}.
$$
This is a preparation calculation using the stated <molar mass>, rather than a measurement of uptake or conversion to either <cofactor (biochemistry)>. A <mass concentration> and a <molar concentration> cannot be equated numerically without the molecular identity and units.
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