Free base vs salt mass

Two ways of stating a compound's mass — with or without its counterion — which is why a registry's molecular weight and a vial's contents are not the same figure.

The free base (or free acid) is the compound itself, uncharged and unaccompanied: the form a structure drawing describes and the form registry databases compute molecular weight for. The salt is that compound plus its counterion. Both names refer to the same active molecule, but their masses differ by the counterion's contribution, multiplied by however many counterions the sequence binds.

The distinction matters wherever a database value meets a physical vial. The molecular formulas and weights published on the research pages of this site come from each compound's PubChem record and describe the free base; the powder in a vial is a salt, plus residual water. Computing moles from a vial's gross mass using the free-base molecular weight therefore overstates the amount of compound, with the error growing with the counterion fraction.

Careful documents state which form a mass refers to. Where a label or certificate does not, the safe reading is that label mass is gross powder — salt and moisture included — while any quoted molecular weight is the free-base value from the registry.

For research purposes only. Not for human consumption, diagnosis, treatment, or prevention of any condition. Nothing on this page is medical advice.