📚 A-Level OCR Chemistry: Mole Calculations | 摩尔计算考点精讲
Mole calculations form the bedrock of quantitative chemistry, and they are assessed throughout OCR A-Level Chemistry papers. A solid grasp of the mole concept and its applications — from reacting masses and gas volumes to titrations and yield — is essential for success in both AS and A2 units. This guide walks you through every key calculation technique, with worked examples and tips to avoid common pitfalls.
摩尔计算是定量化学的基础，贯穿OCR A-Level化学的各个试卷。牢固掌握摩尔概念及其应用——从反应质量和气体体积到滴定和产率——对于在AS和A2单元中取得成功至关重要。本指南将带你学习每一项关键计算技巧，附有示例和避免常见错误的提示。
1. The Mole Concept and Avogadro's Constant | 摩尔概念与阿伏伽德罗常数
10²³ specified elementary entities (atoms, molecules, ions, etc.). This number is Avogadro' you must be comfortable using Nₐ to relate the number of particles to the amount in moles.
一摩尔是恰好含有 6.022 × 10²³ 个指定基本单元（原子、分子、离子等）的物质的量。这个数就是阿伏伽德罗常数（Nₐ）。在 OCR 考试中，你必须熟练运用 Nₐ 把粒子数目与摩尔量联系起来。
The relationship is:
关系式为：
Number of particles = n × Nₐ
Where n ds these calculations within questions on redox titrations, percentage composition, or gas volumes. Remember to check whether you need the number of atoms or molecules; for diatomic gases like O₂, 1 mol of molecules contains 2 mol of atoms.
其中 n 为摩尔量。如果题目问 0.450 mol 碳中含有多少个原子，就用 0.450 乘以 6.022×10²³，得到 2.71×10²³ 个原子。OCR 经常把这种计算嵌入到氧化还原滴定、百分组成或气体体积的题目中。务必注意题目要求的是原子数还是分子数；对于 O₂ 等双原子气体，1 mol 分子含有 2 mol 原子。
2. Molar Mass and Mass-Mole Conversions | 摩尔质量与质量-摩尔转换
Molar mass (M) is the mass per mole of a substance, with units g mol⁻¹. It is numerically equal to the relative formula mass (Mr) or relative atomic mass (Ar). The core equation is:
摩尔质量（M）是每摩尔物质的质量，单位为 g mol⁻¹，数值上等于相对式量（Mr）或相对原子质量（Ar）。核心方程为：
n = m / M
To convert mass to moles, divide the mass in grams by the molar mass. For example, 8.55 g of aluminium sulfate, Al₂(SO₄)₃ (Mr 0 mol. OCR frequently tests this in the context of calculating reacting masses or preparing standard solutions. Be careful with hydrated salts — the water of crystallisation must be included in the molar mass (e.g., CuSO₄·5H₂O has Mr = 249.7).
将质量转换为摩尔数时，用质量（g）除以摩尔质量。例如，8.55 g 硫酸铝 Al₂(SO₄)₃（Mr = 342.3），n = 8.55 / 342.3 ≈ 0.0250 mol。OCR 经常在计算反应质量或配制标准溶液的背景下考察这一点。对含结晶水的盐要格外小心——结晶水必须计入摩尔质量（如 CuSO₄·5H₂O 的 Mr = 249.7）。
3. Empirical and Molecular Formulae | 经验式与分子式
The empirical formula shows the simplest whole-number ratio of atoms in a compound. The molecular formula is a multiple of the empirical formula. OCR expects you to calculate these from percentage composition or combustion data.
经验式表示化合物中原子的最简整数比。分子式是经验式的整数倍。OCR 要求能根据百分组成或燃烧数据计算这些。
Worked example: A compound contains 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass. Convert % ula CH₂O. If the relative molecular mass is 180, the molecular formula is C₆H₁₂O₆ (since 12+2+16 = 30, and 180/30 = 6).
计算示例：某化合物含碳 40.0%，氢 6.7%，氧 53.3%（质量分数）。将百分数转换为质量，再转换为摩尔数：C: 40.0/12.0 = 3.33；H: 6.7/1.0 = 6.7；O: 53.3/16.0 = 3.33。除以最小值（3.33）得比例 C:1, H:2, O:1 → 经验式 CH₂O。若相对分子质量为 180，则分子式为 C₆H₁
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