Molar mass, molecular weight and monoisotopic mass: what each number means

Molar mass, molecular weight and molecular mass are three names for closely related numbers. For the same compound they usually have the same value, just in different units. This guide explains what each one means, how to work out molar mass from a chemical formula or a peptide sequence, and why a product page and a mass spectrum can show slightly different figures for the same molecule.

To calculate the values for any peptide sequence, use our sequence properties tool. Terms are defined in the glossary.

Three names, one number

• Molar mass is the mass of one mole of a substance. Its unit is grams per mole (g/mol). Chemists use it when weighing out a substance to make a solution.
• Molecular mass is the mass of a single molecule. Its unit is the dalton (Da), also called the unified atomic mass unit (u). Biochemists and mass spectrometrists use it. Large molecules are given in kilodaltons (kDa): 1 kDa is 1,000 Da.
• Molecular weight, strictly called relative molecular mass (Mr), is the same number without a unit. It is the mass of a molecule compared with one-twelfth of a carbon-12 atom.

Water shows how they line up. Its molar mass is 18.015 g/mol. One water molecule has a mass of 18.015 Da on average. Its relative molecular mass is 18.015.

In everyday lab use, "molecular weight" and "MW" are often written with g/mol or Da attached. That is loose, but common, and the meaning is clear.

Calculating molar mass from a formula

Add up the atomic weight of every atom in the formula. Standard atomic weights for the common elements in peptides are (CIAAW):

Worked example: acetic acid, C₂H₄O₂.

• Carbon: 2 × 12.011 = 24.022
• Hydrogen: 4 × 1.008 = 4.032
• Oxygen: 2 × 15.999 = 31.998
• Total: 60.052, so the molar mass is about 60.05 g/mol.

For hydrates and salts, include everything in the formula. Copper(II) sulfate pentahydrate, CuSO₄·5H₂O, has a molar mass of about 249.7 g/mol. The anhydrous salt is about 159.6 g/mol. Weighing out one when a recipe means the other gives a solution that is 36% weaker or 56% stronger than intended.

Calculating the mass of a peptide

A peptide is a chain of amino acids. Each time two amino acids join, a molecule of water is lost. So a peptide's mass is the sum of its residue masses (each amino acid minus one water), plus one water for the two free ends. See what is a peptide for how the bond forms.

Worked example: GHK (Gly-His-Lys).

So GHK has an average molecular weight of about 340.38 g/mol and a monoisotopic mass of 340.186 Da. Modifications change the total. A C-terminal amide subtracts about 0.98 Da, and an N-terminal acetyl group adds about 42.01 Da. See terminal modifications.

Average and monoisotopic mass

Most elements exist as more than one isotope. About 1.1% of carbon atoms are carbon-13 rather than carbon-12, for example. That gives two ways to calculate a mass:

• Average mass uses each element's standard atomic weight, which already allows for the natural mix of isotopes. This is the molecular weight shown on product pages and used for weighing.
• Monoisotopic mass uses only the most common isotope of each element. This is the first peak a high-resolution mass spectrometer sees.

For small molecules the two are close. They drift further apart as molecules grow, because a larger molecule is more likely to contain at least one heavy isotope. For a peptide of 1,500 Da the gap is about 1 Da. A COA should say which one it reports. See mass spectrometry and identity confirmation and LC-MS, electrospray and MALDI.

Why salt forms change the number

Synthetic peptides are usually supplied as salts. The positive charges on the peptide are balanced by counter-ions, most often trifluoroacetate (TFA) or acetate. The molecular weight on a product page normally refers to the peptide alone, without counter-ions or water.

The powder in a vial is heavier per mole than that figure suggests, because it also contains the counter-ions and some water. If you calculate a solution's concentration from the weighed mass and the peptide's molecular weight, the result will be too high. The net peptide content tells you what share of the powder is peptide. For the counter-ions themselves, see TFA counter-ions and salt exchange.

Metal complexes need the same care. GHK-Cu has a different formula and molar mass from GHK, because the copper ion is part of the complex. See GHK-Cu: the chemistry of a peptide–copper complex.

From molar mass to a solution

Molar mass links the mass you weigh to the number of moles:

moles = mass ÷ molar mass

For example, 3.40 mg of GHK is 0.00340 g ÷ 340.38 g/mol = 1.00 × 10⁻⁵ mol, or 10.0 µmol. For working out molarity, mg/mL and dilutions, see molarity, mg/mL and dilutions.

Related guides

For checking a figure against a COA, see how to check a certificate of analysis. For looking up a compound's formula and identifiers, see CAS numbers, PubChem and the literature. Every lot has its own COA, published on the product page and on that lot's own page. See Quality.