A monoprotic acid is an acid that contains one ionisable hydrogen atom and so can donate only one proton (H⁺) per molecule. It reaches the stoichiometric point in a titration after one mole of base has been added per mole of acid. Hydrochloric acid (HCl) is a monoprotic acid, in contrast to a polyprotic acid, which can donate more than one proton.

一元酸是含有一个可电离氢原子的酸,因而每个分子只能给出一个质子(H⁺)。在滴定中,每摩尔酸加入一摩尔碱后即达到化学计量点。盐酸(HCl)是一元酸;与之相对,多元酸可给出不止一个质子。