No rule for this one
No ending or word group predicts the article of Mund. Learn it as a pair and say it out loud: der Mund.
der Mund
It's der Mund. Mund is masculine, plural die Münder.
Öffnen Sie den Mund.
Open your mouth.
No ending or word group predicts the article of Mund. Learn it as a pair and say it out loud: der Mund.