📚 IGCSE WJEC Chemistry: Formula Handbook | IGCSE WJEC 化学公式汇总手册
Welcome to your WJEC IGCSE Chemistry formula handbook. Quantitative chemistry can feel daunting, but once you learn to apply the key formulas systematically, you will be able to tackle any calculation question with confidence. This guide collects every essential relationship you need – from the mole concept to energy changes – and explains each one clearly, with notes on units and common pitfalls.
欢迎使用你的 WJEC IGCSE 化学公式手册。定量化学可能会让人望而生畏，但一旦你学会系统地应用关键公式，就能自信地解决任何计算题。本指南收集了从摩尔概念到能量变化等所有重要关系式，并清楚地解释每一个，同时注明单位与常见陷阱。
1. Relative Atomic Mass and the Mole | 相对原子质量与摩尔
The relative atomic mass (Aᵣ) of an element is the average mass of its atoms compared to 1/12 the mass of a carbon‑12 atom. It has no units. The mole is the SI unit for amount of substance; one mole contains exactly 6.02 × 10²³ specified particles (Avogadro constant, Nₐ).
相对原子质量（Aᵣ）是元素原子的平均质量与一个碳‑12 原子质量的 1/12 之比，没有单位。摩尔是物质的量的国际单位；1 摩尔恰好包含 6.02 × 10²³ 个指定粒子（阿伏伽德罗常数，Nₐ）。
To convert between number of particles and moles, use:
在粒子数与摩尔之间进行转换，使用：
number of particles = moles × Nₐ
n = N / Nₐ (Nₐ = 6.02 × 10²³ mol⁻¹)
Remember that ‘particles’ can be atoms, molecules, ions or electrons, depending on the context. When using the Avogadro constant, always check what the question is asking for and give the answer to an appropriate number of significant figures.
请记住，“粒子”可以是原子、分子、离子或电子，依语境而定。使用阿伏伽德罗常数时，务必看清题目要求，并给出适当有效数字的答案。
2. Molar Mass and Moles from Mass | 摩尔质量与质量计算摩尔
Molar mass (M) is the mass of one mole of a substance, expressed in g mol⁻¹. It is numerically equal to the relative formula mass (Mᵣ) for compounds or relative atomic mass (Aᵣ) for elements. The central formula linking mass and moles is:
摩尔质量（M）是一摩尔物质的质量，单位为 g mol⁻¹。它在数值上等于化合物的相对式量（Mᵣ）或元素的相对原子质量（Aᵣ）。连接质量与摩尔的核心公式为：
moles (n) = mass (m) / molar mass (M)
n = m / M
This can be rearranged to m = n × M or M = m / n. On the WJEC examination, you will often be given masses in grams; simply calculate the molar mass from the periodic table and substitute into the formula.
此式可变形为 m = n × M 或 M = m / n。在 WJEC 考试中，质量常以克为单位给出；只需根据周期表计算摩尔质量，代入公式即可。
When working with hydrated salts, remember to include the mass of water molecules in the formula mass. For example, the Mᵣ of CuSO₄·5H₂O is 63.5 + 32.1 + (4 × 16.0) + 5 × (2 × 1.0 + 16.0) = 249.7 g mol⁻¹.
使用水合盐时，请务必将水分子的质量计入式量。例如，CuSO₄·5H₂O 的 Mᵣ = 63.5 + 32.1 + (4 × 16.0) + 5 × (2 × 1.0 + 16.0) = 249.7 g mol⁻¹。
3. Concentration Calculations | 浓度计算
The concentration of a solution in mol dm⁻³ is given by the amount of solute divided by the volume of solution in dm³. If the volume is provided in cm³, you must first convert to dm³ by dividing by 1000.
溶液的浓度（单位为 mol dm⁻³）等于溶质的物质的量除以溶液的体积（以 dm³ 计）。若体积以 cm³ 给出，必须先将体积除以 1000 换算为 dm³。
concentration (c) = moles (n) / volume (V in dm³)
c = n / V
When using V in cm³, the formula becomes: c = (n × 1000) / V. For mass concentration (g dm⁻³), convert using the molar mass: mass concentration (g dm⁻³) = molar concentration (mol dm⁻³) × Mᵣ.
当 V 使用 cm³ 时，公式变为：c = (n × 1000) / V。若需要质量浓度（g dm⁻³），可通过摩尔质量转换：质量浓度 (g dm⁻³) = 摩尔浓度 (mol dm⁻³) × Mᵣ。
A typical mistake is failing to convert cm³ to dm³ before substituting into c = n / V. Always write the units on intermediate steps to avoid this error.
一个典型错误是未将 cm³ 转换为 dm³ 就直接代入 c = n / V。每一步都要写出单位，以避免这一错误。
4. Molar Volume of Gases | 气体摩尔体积
Under room temperature and pressure (RTP, approximately 20 °C and 1 atm), one mole of any gas occupies 24 dm³. This value is provided on the WJEC data sheet and should be used for all gas volume calculations unless stated otherwise.
在室温和常压（RTP，约 20 °C 和 1 atm）下，1 摩尔任何气体占据 24 dm³ 的体积。该数值会提供在 WJEC 数据表中，除特别说明外，所有气体体积计算都应使用该值。
volume (V in dm³) = moles (n) × 24 dm³ mol⁻¹
V = n × Vₘ (Vₘ = 24 dm³ mol⁻¹ at RTP)
Conversely, to find the amount of gas from a measured volume: n = V / 24. If the volume is given in cm³, first convert to dm³. This relationship is particularly useful when linking reacting masses to gas volumes in stoichiometry.
反过来，由测量体积求气体物质的量：n = V / 24。如果体积以 cm³ 为单位，需先转换为 dm³。在将反应质量与气体体积进行计量关系联系时，此关系尤为有用。
5. Reacting Masses and Stoichiometry | 反应质量与化学计量
Stoichiometry uses the balanced chemical equation to find the molar ratios between reactants and products. From a known mass of substance A, you can calculate the mass of substance B expected:
化学计量利用配平的化学方程式找出反应物与产物之间的摩尔比。由已知物质 A 的质量，可以计算预期物质 B 的质量：
Step 1: n(A) = m(A) / M(A)
Step 2: n(B) = n(A) × (coefficient of B / coefficient of A)
Step 3: m(B) = n(B) × M(B)
This can be combined into one expression: m(B) = (m(A) / M(A)) × (b / a) × M(B), where a and b are the balancing numbers from the equation. Always check that the equation is balanced before starting any calculation.
可将其组合成一个表达式：m(B) = (m(A) / M(A)) × (b / a) × M(B)，其中 a 和 b 是方程式中的配平系数。在开始任何计算之前，一定要确认方程式已配平。
Limiting reactant problems are common. Identify the mole of each reactant, divide by its coefficient; the smallest value indicates the limiting reactant. All further calculations must be based on the limiting reactant.
限制反应物问题很常见。求出每种反应物的摩尔数，除以其计量系数；数值最小的即为限制反应物。后续计算都必须基于限制反应物进行。
6. Percentage Yield | 百分产率
The percentage yield compares the actual mass of product obtained from an experiment with the theoretical mass calculated from stoichiometry. It is always less than 100% due to incomplete reactions, side reactions or losses during purification.
百分产率将实验实际得到的产物质量与根据化学计量算出的理论质量进行比较。由于反应不完全、副反应或纯化过程中的损失，百分产率始终低于 100%。
% yield = (actual yield / theoretical yield) × 100
% yield = (actual mass / theoretical mass) × 100
The terms ‘actual yield’ and ‘theoretical yield’ can refer to mass or moles as long as both are in the same units. In WJEC questions, you typically calculate the theoretical mass first, then substitute the given actual mass.
“实际产率”与“理论产率”可以指质量或物质的量，只要两者单位一致即可。在 WJEC 试题中，通常先算出理论质量，再代入所给的实际质量。
7. Atom Economy | 原子经济性
Atom economy measures how efficiently reactant atoms are incorporated into the desired product. A high atom economy indicates a greener process with less waste.
原子经济性衡量反应物原子转化为目标产物的效率。原子经济性高意味着过程更绿色、废物更少。
% atom economy = (Mᵣ of desired product / sum of Mᵣ of all reactants) × 100
% atom economy = (Mᵣ(desired) / Σ Mᵣ(reactants)) × 100
Only the molecular formula masses of the reactants are needed – you do not need to consider balancing coefficients when calculating atom economy. The desired product must be specified in the question.
只需用到反应物的化学式量——计算原子经济性时无需考虑配平系数。题目中必须指明目标产物。
For example, in the production of ethene from ethanol, C₂H₅OH → C₂H₄ + H₂O, Mᵣ(ethene) = 28, Mᵣ(ethanol) = 46, atom economy = (28/46) × 100 = 60.9%. Always show the sum of reactant Mᵣ explicitly.
例如，乙醇制乙烯：C₂H₅OH → C₂H₄ + H₂O，Mᵣ (乙烯) = 28，Mᵣ (乙醇) = 46，原子经济性 = (28/46) × 100 = 60.9%。务必清晰展示反应物式量之和。
8. Energy Changes: q = mcΔT | 能量变化：q = mcΔT
In calorimetry experiments, the heat energy released or absorbed by a reaction is calculated using the temperature change of the surrounding water or solution. The specific heat capacity of water is 4.18 J g⁻¹ °C⁻¹.
在量热实验中，反应释放或吸收的热量通过周围水或溶液的温度变化来计算。水的比热容为 4.18 J g⁻¹ °C⁻¹。
heat energy (q) = mass of water (m) × specific heat capacity (c) × temperature change (ΔT)
q = m × c × ΔT (c = 4.18 J g⁻¹ °C⁻¹)
To find the molar enthalpy change, divide the energy by the number of moles that reacted: ΔH = –q / n. The negative sign indicates that the reaction is exothermic (temperature rises). If the temperature falls, q is still calculated as positive, but ΔH will be positive.
要求出摩尔焓变，需将热量除以反应的物质的量：ΔH = –q / n。负号表示反应放热（温度升高）。如果温度下降，q 仍按正值计算，但 ΔH 为正值。
Always convert the final ΔH to kJ mol⁻¹ if q was in joules. Report your answer to an appropriate number of significant figures and state the sign clearly.
如果 q 的单位为焦耳，最终 ΔH 务必换算为 kJ mol⁻¹。答案应保留适当有效数字，并明确标明正负号。
9. Bond Energy Calculations for ΔH | 键能计算反应热
During a chemical reaction, bonds in the reactants are broken (endothermic) and new bonds are formed in the products (exothermic). The overall enthalpy change can be estimated using average bond energies:
在化学反应中，反应物中的化学键断裂（吸热），产物中新键形成（放热）。总焓变可用平均键能估算：
ΔH = Σ (bond energies of bonds broken) – Σ (bond energies of bonds formed)
ΔH = Σ E(broken) – Σ E(formed)
Draw the displayed formulae of all reactants and products to count every bond. Multiply each bond energy by the number of that type of bond present. The sum for broken bonds is positive (energy in), and the sum for formed bonds is negative (energy out), so the formula gives a negative ΔH for exothermic reactions.
画出所有反应物和产物的结构式，以便计清每一个键。将每种键的键能乘以存在的数目。断裂键的总和为正（能量输入），形成键的总和为负（能量输出），因此放热反应 ΔH 为负值。
Make sure you only use bond energies for the substances in the gaseous state unless the question specifies otherwise. If a liquid or solid appears, the energy changes of state may need to be considered separately, though at IGCSE level this is rarely required.
除非题目另有说明，否则仅使用气态物质的键能。若出现液态或固态，状态变化的能量可能需要另外考虑，但在 IGCSE 层级很少有此要求。
10. Rate of Reaction | 反应速率
The rate of a chemical reaction measures how quickly a reactant is used up or a product is formed. It can be followed by monitoring the change in mass, volume of gas evolved, colour intensity, or the time taken for a precipitate to obscure a mark.
化学反应速率衡量反应物消耗或产物生成的快慢。可以通过监测质量变化、气体出气体积、颜色强度或沉淀遮挡标记所需时间来跟踪。
average rate = change in quantity / change in time
rate = Δ(quantity) / Δt
The ‘quantity’ could be mass lost (g), volume of gas produced (cm³), or concentration decrease (mol dm⁻³). Always state the units: e.g., g s⁻¹, cm³ min⁻¹ or mol dm⁻³ s⁻¹. When plotting a graph, the rate at a specific point is given by the gradient of the tangent.
“量”可以是损失的质量（g）、生成的气体体积（cm³）或浓度降低量（mol dm⁻³）。务必写明单位，例如 g s⁻¹、cm³ min⁻¹ 或 mol dm⁻³ s⁻¹。在绘制曲线图时，某一点的速率由该点切线的斜率给出。
Published by TutorHao | Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导