📚 Amount of Substance | 物质的量
In Cambridge A Level Chemistry, amount of substance underpins almost every quantitative topic, from titrations to gas volumes and enthalpy changes. It is expressed in moles and links the masses we measure to the numbers of atoms, ions or molecules involved in a reaction.
在剑桥 A Level 化学中，物质的量是几乎所有定量主题的基础，从滴定到气体体积和焓变。它以摩尔表示，并将我们测得的质量与反应中涉及的原子、离子或分子数目联系起来。
1. Relative Masses and the Mole | 相对质量与摩尔
Relative atomic mass (Aᵣ) is defined as the weighted average mass of an atom of an element compared with 1/12 of the mass of an atom of carbon-12.
相对原子质量（Aᵣ）定义为元素一个原子的加权平均质量与碳-12 原子质量的 1/12 之比。
Relative molecular mass (Mᵣ) is found by adding the relative atomic masses of all atoms in one molecule or formula unit.
相对分子质量（Mᵣ）是将一个分子或式单元中所有原子的相对原子质量相加得到的。
The mole (mol) is the SI unit for amount of substance. One mole is taken to contain 6.02 × 10²³ specified particles.
摩尔（mol）是物质的量的 SI 单位。1 摩尔被视为包含 6.02 × 10²³ 个指定粒子。
The key relationship connecting mass and amount is:
质量与物质的量之间的关键关系是：
n = m ÷ M
where n is amount in mol, m is mass in g and M is molar mass in g mol⁻¹.
其中 n 为物质的量（mol），m 为质量（g），M 为摩尔质量（g mol⁻¹）。
2. The Avogadro Constant and Molar Mass | 阿伏伽德罗常数与摩尔质量
The Avogadro constant, L, has the value 6.02 × 10²³ mol⁻¹. It gives the number of atoms, ions or molecules in one mole of a substance.
阿伏伽德罗常数 L 的数值为 6.02 × 10²³ mol⁻¹。它表示 1 摩尔物质中含有的原子、离子或分子数目。
To find the actual number of particles N, multiply the amount in moles by L:
要求实际粒子数 N，用摩尔数乘以 L：
N = n × L
Molar mass M has the same numerical value as relative formula mass, but it is expressed in g mol⁻¹. This is why 1 mole of carbon-12 atoms has a mass of exactly 12 g.
摩尔质量 M 的数值与相对式量相同，但单位是 g mol⁻¹。这就是为什么 1 摩尔碳-12 原子的质量恰好为 12 g。
| Quantity | 物理量 | Symbol | 符号 | Common unit | 常用单位 |
|---|---|---|
| Amount of substance | 物质的量 | n | mol |
| Mass | 质量 | m | g |
| Molar mass | 摩尔质量 | M | g mol⁻¹ |
| Number of particles | 粒子数 | N | dimensionless | 无量纲 |
3. Empirical and Molecular Formulae | 实验式与分子式
The empirical formula gives the simplest whole-number ratio of atoms in a compound. The molecular formula gives the actual number of atoms in one molecule.
实验式给出化合物中原子的最简整数比。分子式给出一个分子中原子的实际数目。
To calculate an empirical formula, convert the mass or percentage of each element to moles, then divide each by the smallest number of moles. If necessary, multiply to get whole numbers.
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