\[x_i = \frac{n_i}{\sum_j n_j}, \qquad \sum_i x_i = 1\]
含义Meaning
组分 i 的物质的量占总物质的量的分数，与温度无关，是热力学中最常用的组成变量。
Fraction of the total amount that is component i; temperature-independent and the natural composition variable in thermodynamics.
符号说明Symbols
| 符号Symbol | 含义 Meaning | 单位 Unit |
|---|---|---|
| \(x_i\) | 摩尔分数mole fraction | 1 |
| \(n_i\) | 组分 i 的物质的量amount of i | mol |
推导Derivation
按定义取比值；各分数之和为 1，所以 C 组分体系只有 C − 1 个独立组成变量。
Defined as a ratio; the fractions sum to 1, so a C-component system has C − 1 independent composition variables.
稀溶液中 \(x_\mathrm{B} \approx n_\mathrm{B}/n_\mathrm{A} = b_\mathrm{B} M_\mathrm{A}\)，与质量摩尔浓度成正比。
For dilute solutions \(x_\mathrm{B} \approx b_\mathrm{B} M_\mathrm{A}\), proportional to molality.
适用条件与局限Assumptions & limitations
电解质溶液要说明是按化学式还是按离子计算。
For electrolytes state whether you count formula units or ions.