定义Definition
摩尔质量是单位物质的量的物质所具有的质量，M = m/n，常用单位 g mol⁻¹。以 g mol⁻¹ 为单位时，其数值等于该物质的相对原子质量或相对分子质量，如 M(H₂O) = 18.0 g mol⁻¹。
Molar mass is the mass of one mole of a substance, M = m/n, usually in g mol⁻¹. In g mol⁻¹ it is numerically equal to the relative atomic or molecular mass: M(H₂O) = 18.0 g mol⁻¹.
计算Calculation
把化学式中各原子的相对原子质量相加即可，如 M(CaCO₃) = 40.1 + 12.0 + 3 × 16.0 = 100.1 g mol⁻¹。由质量求物质的量：n = m/M，如 25.0 g CaCO₃ 为 0.250 mol。
Add up the relative atomic masses of the atoms in the formula: M(CaCO₃) = 40.1 + 12.0 + 3 × 16.0 = 100.1 g mol⁻¹. The amount from a mass is n = m/M, so 25.0 g of CaCO₃ is 0.250 mol.
测定Measurement
气体的摩尔质量可由一定温度和压强下的密度求出：M = ρRT/p。挥发性液体可用蒸气密度法测定。质谱中分子离子峰的 m/z 直接给出相对分子质量。
The molar mass of a gas follows from its density at known temperature and pressure: M = ρRT/p. Volatile liquids can be measured by vapour density. In mass spectrometry the m/z of the molecular ion gives the relative molecular mass directly.
年表Timeline
- 1860
卡尔斯鲁厄会议后，康尼查罗推广用阿伏伽德罗假说确定分子量
After the Karlsruhe Congress, Cannizzaro promotes Avogadro's hypothesis for finding molecular weights
- 1971
摩尔成为 SI 基本单位，摩尔质量的单位随之规范
The mole becomes an SI base unit, standardising the unit of molar mass
相关公式Formulas
例题Worked example
9.0 g 水：n = 9.0/18.0 = 0.50 mol，含 0.50 × 6.02×10²³ = 3.0×10²³ 个分子。
9.0 g of water: n = 9.0/18.0 = 0.50 mol, i.e. 3.0×10²³ molecules.
例题Worked example
空气约含 78% N₂、21% O₂、1% Ar（体积）：M̄ = 0.78×28 + 0.21×32 + 0.01×40 = 29 g/mol。
Air (78 % N₂, 21 % O₂, 1 % Ar by volume): M̄ ≈ 29 g/mol.