Balances measure mass, but chemistry happens in numbers of molecules. The mole connects the two. Once a quantity is in moles you can compare it directly with another compound, set a molar ratio, or make a solution of known molarity.
What a mole is
Since the 2019 revision of the SI, the mole is defined by a fixed number. One mole contains exactly 6.022 140 76 × 1023 elementary entities. That number is the Avogadro constant, NA, and it is exact by definition.
Molar mass connects grams and moles
Molar mass (M) is the mass of one mole of a substance, in grams per mole. Numerically it equals the molecular weight in daltons. A peptide of 340.38 Da has a molar mass of 340.38 g/mol.
n = m / M m = n × M N = n × NA
Here n is amount in moles, m is mass in grams, M is molar mass in g/mol and N is the number of molecules.
Worked example: 5 mg of GHK
M = 340.38 g/mol and m = 5 mg = 0.005 g.
n = 0.005 g ÷ 340.38 g/mol = 1.469 × 10−5 mol = 14.69 µmol.
N = 1.469 × 10−5 × 6.022 × 1023 ≈ 8.85 × 1018 molecules.
Working at peptide scale
Peptide work usually happens in milligrams and micromoles, or smaller. Converting everything to grams and moles first, then choosing a readable unit at the end, avoids most errors:
| Unit | In moles | Typical use |
|---|---|---|
mmol |
10−3 mol |
Synthesis scale |
µmol |
10−6 mol |
Milligram amounts of peptide |
nmol |
10−9 mol |
Microgram amounts, assay plates |
pmol |
10−12 mol |
Analytical injections, mass spectrometry |
A handy shortcut follows from the units: mg ÷ (g/mol) = mmol and µg ÷ (g/mol) = µmol. For example, 5 mg ÷ 340.38 g/mol = 0.01469 mmol = 14.69 µmol.
Which molar mass to use
- Use the average molecular weight (from standard atomic weights) for anything weighed.
- If the material is a salt (a TFA or acetate salt, or a hydrochloride), decide whether you want moles of the salt as weighed or moles of the peptide itself. For the peptide, either use the free-base molar mass and correct the weighed mass for net peptide content, or use the salt-form molar mass from the certificate. Do not mix the two approaches.
Common mistakes
- Dividing milligrams by g/mol and reporting the answer as moles. The result is in millimoles, a factor of 1000 out.
- Using the monoisotopic mass from a mass spectrum to calculate amounts.
- Rounding the molar mass heavily before multiplying. Keep at least four significant figures until the final step.