定义Definition
相对分子质量是一个分子的平均质量与一个 ¹²C 原子质量的 1/12 之比，符号 Mr，没有单位。它等于分子中各原子相对原子质量之和。
Relative molecular mass, Mr, is the ratio of the average mass of one molecule to one-twelfth of the mass of a ¹²C atom; it has no units. It equals the sum of the relative atomic masses of the atoms in the molecule.
计算与相关概念Calculation and related terms
Mr(CO₂) = 12.0 + 2 × 16.0 = 44.0。离子化合物没有分子，用相对式量表示，如 NaCl 为 58.5。以 g mol⁻¹ 为单位时，摩尔质量的数值与相对分子质量相同。
Mr(CO₂) = 12.0 + 2 × 16.0 = 44.0. Ionic compounds have no molecules, so relative formula mass is used, e.g. 58.5 for NaCl. Molar mass in g mol⁻¹ has the same numerical value as the relative molecular mass.
测定Determination
现代方法主要是质谱：分子离子峰 M⁺ 的 m/z 给出相对分子质量。早期用气体密度（理想气体方程）、凝固点降低或沸点升高等依数性测定。
Today it is found mainly by mass spectrometry, from the m/z of the molecular ion M⁺. Earlier methods used gas density (the ideal gas equation) or colligative properties such as freezing-point depression and boiling-point elevation.
年表Timeline
- 1858
康尼查罗利用阿伏伽德罗假说系统确定分子量
Cannizzaro uses Avogadro's hypothesis to fix molecular weights systematically
- 1882
拉乌尔用凝固点降低测定分子量
Raoult measures molecular weights by freezing-point depression