Yes, you can if the soap dispenser is designed to allow the dispensation of thinner liquids like mouthwash. You may find some challenges with such a dispenser, however, as liquid soaps are relatively thicker and come with pumps designed to work with thick liquids. The soap dispenser may splash the mouthwash all over your paper cup creating an unwanted mess. They may also have wider nozzles, which is likely to dispense more mouthwash than you need. If you find a perfect fit then you can rely on a soap dispenser but we still recommend buying a dedicated mouthwash dispenser. You won’t be disappointed!