Molecular formula
A molecular formula lists the atoms that make up one molecule and how many of each are present. For a peptide it reflects the combined composition of its amino acids after they are linked together. It is a compact way of stating what the molecule is made of.
Molecular weight
Molecular weight (or molecular mass) is the mass of one molecule, usually expressed in daltons or grams per mole. It follows directly from the formula. Molecular weight is useful for laboratory calculations — for example, when working out concentrations — and is one of the values an analytical technique such as mass spectrometry can help confirm.
Where you’ll see them
These identifiers commonly appear on specification sheets and certificates of analysis. Where a value is not supplied for a given material, it should not be assumed; documentation states what is known rather than what is estimated.
Frequently asked questions
Why does molecular weight matter for concentration maths?
Concentration links mass and volume, so knowing the molecular weight lets researchers relate a quantity of material to an amount in solution.
Is molecular weight the same as purity?
No. Molecular weight describes the molecule’s mass; purity is a separate measure of how much of a sample is the intended material.