A molecular formula counts atoms in a defined structure. Molecular weight and measured mass describe different quantities; neither is the milligram amount printed on a vial.
Separate the fields
| Field | Meaning |
|---|---|
| Molecular formula | Numbers of each element in the defined molecule |
| Relative molecular mass (molecular weight) | A relative quantity, without a unit |
| Molecular mass in Da | Mass of a molecule on the atomic mass scale |
| Molar mass in g/mol | Mass per amount of substance |
| Vial amount in mg | A bulk amount of supplied material, not a molecular mass |
One kilodalton is 1,000 daltons: an illustrative 2,500 Da molecular mass is 2.5 kDa. This conversion does not establish how many milligrams a vial contains.BIPM — The International System of Units (opens in a new tab)
Average and monoisotopic values
Average mass weights isotope contributions by isotopic composition. Monoisotopic mass uses the most abundant isotope of each element. The two conventions can give different values for the same molecular formula; a difference is not automatically a transcription error.IUPAC — Average mass (opens in a new tab)IUPAC — Monoisotopic mass (opens in a new tab)
The existing dossier’s glycylglycine record is a small peptide example with formula C4H8N2O3. Its molecular-weight field is approximately 132.12 g/mol, while its monoisotopic mass is approximately 132.0535 Da. Preserve the property labels and precision supplied by the record.PubChem — Glycylglycine, CID 11163 (opens in a new tab)
In mass spectrometry the instrument measures ions by mass-to-charge ratio. A charged ion, a reconstructed neutral mass and an average formula-based mass must be compared on a stated basis.Waters MS primer (opens in a new tab)
Apply the distinction to a catalogue entry
The existing BPC-157 PubChem record (CID 9941957), rechecked for this revision, gives formula C62H98N16O22, a displayed molecular weight of 1419.5 g/mol and sequence GEPPPGKPADDAGLV. Those fields describe the reference structure without added counterion or water; they do not describe the complete composition of a supplied powder.PubChem — BPC-157, CID 9941957 (opens in a new tab)Bachem — Frequently asked questions (opens in a new tab)
If the specification does not say whether a value describes the peptide moiety, a salt or a complex, ask for that basis before adjusting the number. Keep uncertain fields visibly unresolved rather than adding an assumed counterion mass.Bachem — Frequently asked questions (opens in a new tab)
Sources and further detail
- IUPAC Gold Book — Molecular formula (opens in a new tab)
Entry M03987, molecular rather than empirical formula.
- IUPAC Gold Book — Relative molecular mass (opens in a new tab)
Entry R05271: relative molecular mass and molecular-weight terminology.
- BIPM — The International System of Units (opens in a new tab)
Ninth edition, English version 4.01 (2026), tables 7 and 8 and section 5.4.7. Definitions and notation; examples in this article are independently constructed.
- IUPAC — Monoisotopic mass (opens in a new tab)
Terminology definition; most abundant isotope is not replaced with a universal lightest-isotope rule.
- IUPAC — Average mass (opens in a new tab)
Definition based on isotopic-composition weighting.
- PubChem — Glycylglycine, CID 11163 (opens in a new tab)
Computed formula and mass properties. The record's molecular-weight unit and monoisotopic-mass unit are retained and distinguished.
- Waters — What is mass spectrometry and how does it work? (opens in a new tab)
Instrument primer; ion measurement, mass-to-charge ratio and interpretation of spectra.
- PubChem — BPC-157, CID 9941957 (opens in a new tab)
Existing PubChem CID 9941957 identity source rechecked via official PUG REST on 3 October 2026 for formula and displayed molecular weight. Reference structure without added counterion or water.
- Bachem — Frequently asked questions (opens in a new tab)
Salt-form explanation. Bachem's stated supply defaults describe Bachem, not every peptide supplier.
Revised from the existing source dossier on 3 October 2026. Supplied-report values are document transcriptions, not independent authentication or new measurements. Source notes identify the scope of each reference; this article has not undergone independent scientific peer review.