What is salt's definite chemical composition?

The proper chemical composition of pure salt is NaCl aka Sodium Chloride.

Chemically, it is 60.663% elemental chlorine (Cl) and 39.337% sodium (Na).

Rock salt typically ranges between 95% and 99% NaCl, and mechanically evaporated salt and solar salt normally exceed 99% NaCl. Evaporated salt made with purified brine has the highest purity, in some cases 99.99% NaCl.