The answer to this question depends on a few factors, including the acid's concentration, temperature, and humidity. Generally speaking, most muriatic acid will dissolve mortar because it is a strong acid. However, if the concentration of muriatic acid is high enough or if the environment is too dry, the acid might not be able to completely dissolve mortar. In cases like these, it may be necessary to use another type of cleaner or water mixture specifically designed for dissolving mortar.