The quantitative calculation of pH of monoacid/monobasic solution is a basic problem for inorganic chemistry teaching. The qualitative description and quantitative calculation of pH are connected by approximation conditions. It is very important for the students to understand these approximation conditions, which will help them solve problems about ionic equilibrium in aqueous solution. However, how to use these approximation conditions correctly in solution of low concentration and weak acids is often difficult. Even in some textbooks, the description of concepts is not accurate and not consistent with the associated courses. Thus, to solve the problems mentioned above, the derivations of the approximation conditions are introduced and examples are analyzed in detail in this paper. The problems of quantitative calculation of pH of monoacid/monobasic solution which should be paid attention to in the course teaching of inorganic chemistry are discussed.