A-Level WJEC Chemistry Formula Handbook | A-Level WJEC 化学公式汇总手册

📚 A-Level WJEC Chemistry Formula Handbook | A-Level WJEC 化学公式汇总手册

This comprehensive handbook draws together the essential equations encountered across the WJEC A-Level Chemistry specification. Each section groups related formulae, explains their symbols, and notes the conditions under which they apply. Mastering these relationships is vital for problem-solving across physical, inorganic, and organic topics, supporting success in both written examinations and practical assessments.

本手册汇集了 WJEC A-Level 化学课程中所有核心公式。每个小节按主题归类，阐明符号含义并指出适用条件。熟练掌握这些定量关系对于解答物理化学、无机化学与有机化学问题至关重要，有助于在笔试与实践考核中稳操胜券。

1. Moles and Stoichiometry | 摩尔与化学计量

All quantitative chemistry rests on the mole, the SI unit for amount of substance. The relationships below convert between mass, gas volume, solution volume, and number of particles.

所有定量化学的基石是摩尔（物质的量的 SI 单位）。下列关系式在质量、气体体积、溶液体积和粒子数之间建立转换桥梁。

n = m / M – The number of moles n is found by dividing the mass m (in grams) by the molar mass M (in g mol⁻¹).

n = m / M – 摩尔数 n 等于质量 m（单位：克）除以摩尔质量 M（单位：g mol⁻¹）。

n = V / Vₘ – For a gas at room temperature and pressure (RTP, approx. 25 °C, 100 kPa), the molar volume Vₘ is taken as 24.0 dm³ mol⁻¹, so n = V(gas in dm³) / 24.0.

n = V / Vₘ – 在室温和常压（约 25 °C、100 kPa）下，气体摩尔体积 Vₘ 取 24.0 dm³ mol⁻¹，因此 n = V（气体体积，dm³）/ 24.0。

n = cV – For a solution, moles equal the concentration c (mol dm⁻³) multiplied by the volume V (dm³).

n = cV – 对溶液而言，摩尔数 = 浓度 c（mol dm⁻³）× 体积 V（dm³）。

N = n × Nₐ – The number of particles N is obtained by multiplying the amount n by Avogadro’s constant Nₐ = 6.02 × 10²³ mol⁻¹.

N = n × Nₐ – 粒子数 N 等于摩尔数 n 乘以阿伏伽德罗常数 Nₐ = 6.02 × 10²³ mol⁻¹。

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2. Empirical, Molecular Formulae and Water of Crystallisation | 经验式、分子式与结晶水

Formulae are deduced from composition data. The empirical formula gives the simplest whole‑number ratio, while the molecular formula shows the actual number of atoms. Hydrated salts contain water of crystallisation that can be quantified by mass loss on heating.

化学式由组成数据推导而来。经验式给出最简整数比，分子式表示实际的原子数。水合盐含有结晶水，可通过加热失重定量。

Empirical formula from % by mass: divide the percentage of each element by its relative atomic mass Aᵣ, then find the simplest whole‑number ratio.

由质量分数求经验式：将各元素的质量分数除以相对原子质量 Aᵣ，再求最简整数比。

Molecular formula = n × (empirical formula), where n = Mᵣ (relative molecular mass) ÷ empirical formula mass.

分子式 = n ×（经验式），式中 n = Mᵣ（相对分子质量）÷ 经验式质量。

Water of crystallisation: heat a hydrated salt, record the mass of the anhydrous solid, and determine the moles of water driven off per mole of salt.

结晶水测定：加热水合盐，记录无水固体质量，求出每摩尔盐脱去的水的摩尔数。

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3. The Ideal Gas Equation | 理想气体方程

When gases behave ideally, pressure, volume, temperature and amount are linked by this single equation. It is used extensively to find one variable when the others are known.

当气体呈现理想行为时，压强、体积、温度与物质的量满足一个统一方程，广泛用于已知其他变量求另一变量。

pV = nRT
p: pressure in pascals (Pa), V: volume in cubic metres (m³), n: amount in moles, T: absolute temperature in kelvins (K), R: gas constant = 8.31 J K⁻¹ mol⁻¹.

pV = nRT
p：压强（Pa），V：体积（m³），n：摩尔数，T：绝对温度（K），R：气体常数，8.31 J K⁻¹ mol⁻¹。

Ensure units are consistent: convert °C to K by adding 273, and convert cm³ or dm³ to m³ (1 m³ = 1000 dm³; 1 dm³ = 1000 cm³).

务必保证单位一致：温度由 ℃ 加 273 转为 K；体积由 cm³ 或 dm³ 转为 m³（1 m³ = 1000 dm³；1 dm³ = 1000 cm³）。

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4. Thermodynamics and Energetics | 热力学与能量学

Energetics deals with heat changes, entropy, and free energy. These equations allow you to predict reaction feasibility and calculate enthalpy changes from experimental data or bond energies.

热力学关注热量变化、熵和自由能。这些公式能预测反应可行性，并通过实验数据或键能计算焓变。

ΔH = q / n – where q = mcΔT (m: mass of water or solution, c: specific heat capacity, usually 4.18 J g⁻¹ °C⁻¹ for water, ΔT: temperature change).

ΔH = q / n – 其中 q = mcΔT（m：水或溶液质量，c：比热容，水常用 4.18 J g⁻¹ °C⁻¹，ΔT：温度变化）。

ΔH = ΣΔHf°(products) – ΣΔHf°(reactants) – uses standard enthalpies of formation.

ΔH = ΣΔHf°(产物) – ΣΔHf°(反应物) – 使用标准生成焓。

ΔH = Σ(bond energies of bonds broken) – Σ(bond energies of bonds formed) – approximate method using mean bond enthalpies.

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