No rule for this one
No ending or word group predicts the article of Salz. Learn it as a pair and say it out loud: das Salz.
das Salz
It's das Salz. Salz is neuter.
Etwas Salz, bitte.
A little salt, please.
No ending or word group predicts the article of Salz. Learn it as a pair and say it out loud: das Salz.