IB Chemistry: Stoichiometry Key Points | IB 化学：化学计量 考点精讲

📚 IB Chemistry: Stoichiometry Key Points | IB 化学：化学计量 考点精讲

Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction. Mastering stoichiometry means understanding how to count particles that are too small to see, using the mole concept, balanced equations, and conversion factors to predict masses, volumes, and concentrations. For IB Chemistry students, this topic is fundamental: it appears across the syllabus from atomic structure to energetics, and is tested consistently on Paper 1, Paper 2, and the internal assessment. The following key points will help you consolidate your knowledge and avoid common pitfalls.

化学计量学（Stoichiometry）是研究化学反应中反应物与产物之间定量关系的化学分支。掌握化学计量意味着理解如何计算那些小到看不见的粒子，运用摩尔概念、配平的化学方程式和换算因子来预测质量、体积和浓度。对于 IB 化学学生来说，这个主题是基础：从原子结构到能量学，它贯穿整个大纲，并在试卷一、试卷二和内部评估中持续考查。以下考点将帮助你巩固知识，避开常见陷阱。

1. The Mole Concept & Avogadro’s Constant | 摩尔概念与阿伏伽德罗常数

A mole is the amount of substance that contains exactly 6.02214076 × 10²³ elementary entities (atoms, molecules, ions, etc.). This number is Avogadro’s constant (Nₐ). The mole allows chemists to count particles by weighing them. The relationship is: number of particles = amount (mol) × Nₐ. In IB, you must be comfortable converting between moles, number of particles, and mass using the relative atomic mass (Aᵣ) or relative molecular mass (Mᵣ). Remember: one mole of carbon-12 has a mass of exactly 12 g.

一摩尔是恰好包含 6.02214076 × 10²³ 个基本单元（原子、分子、离子等）的物质的量。这个数字就是阿伏伽德罗常数（Nₐ）。摩尔使化学家能够通过称重来计算粒子数。关系式为：粒子数 = 物质的量（mol）× Nₐ。在 IB 中，你必须熟练掌握摩尔、粒子数和质量之间的换算，使用相对原子质量（Aᵣ）或相对分子质量（Mᵣ）。记住：一摩尔碳-12 的质量恰好是 12 g。

2. Relative Atomic Mass (Aᵣ) and Relative Molecular Mass (Mᵣ) | 相对原子质量（Aᵣ）与相对分子质量（Mᵣ）

The relative atomic mass (Aᵣ) is the weighted average mass of an atom of an element compared to 1/12 the mass of a carbon-12 atom. It has no units. The relative molecular mass (Mᵣ) is the sum of the Aᵣ values of all atoms in a molecule. For ionic compounds, we use relative formula mass (same principle). IB data booklet provides Aᵣ values; you must be able to calculate Mᵣ from a given formula and use it to find molar mass (M) in g mol⁻¹.

相对原子质量（Aᵣ）是某元素一个原子的加权平均质量与一个碳-12 原子质量的 1/12 的比值，没有单位。相对分子质量（Mᵣ）是分子中所有原子的 Aᵣ 值之和。对于离子化合物，我们使用相对化学式量（原理相同）。IB 数据手册提供了 Aᵣ 值；你必须能够根据给定化学式计算 Mᵣ，并用它求出摩尔质量（M），单位为 g mol⁻¹。

3. Empirical Formula and Molecular Formula | 经验式与分子式

The empirical formula gives the simplest whole‑number ratio of atoms of each element in a compound. The molecular formula shows the actual number of atoms of each element in a molecule. To determine the empirical formula from mass or percentage composition: (1) convert masses to moles using Aᵣ, (2) divide by the smallest number of moles to get the ratio, (3) multiply if necessary to obtain whole numbers. The molecular formula is a whole‑number multiple (n) of the empirical formula, where n = Mᵣ (molecular) / Mᵣ (empirical). Mass spectrometry is often used to determine molecular mass.

经验式给出化合物中各元素原子的最简整数比。分子式显示分子中各元素原子的实际数目。通过质量或百分组成确定经验式：（1）用 Aᵣ 将质量转换为摩尔数，（2）除以最小摩尔数得出比例，（3）如有必要乘以整数得到最简整数比。分子式是经验式的整数倍（n），其中 n = 分子式的 Mᵣ / 经验式的 Mᵣ。质谱常用于测定分子质量。

4. Balancing Chemical Equations | 配平化学方程式

A balanced chemical equation obeys the law of conservation of mass: the number of atoms of each element is the same on both sides. State symbols (s), (l), (g), (aq) must be included where appropriate. Stoichiometric coefficients in a balanced equation indicate the mole ratio of reactants and products. This mole ratio is the core of all stoichiometric calculations. Always balance equations before performing any mole‑based calculation.

配平的化学方程式遵循质量守恒定律：每种元素的原子数目在两边相等。必须酌情标注状态符号（s）、（l）、（g）、（aq）。配平方程中的化学计量系数表示反应物和生成物之间的摩尔比。这个摩尔比是所有化学计量计算的核心。进行任何基于摩尔的计算之前，请务必先配平方程式。

5. Mass‑to‑Mass and Mole‑to‑Mole Calculations | 质量与摩尔的相关计算

Once the balanced equation gives the mole ratio, you can calculate unknown masses or moles. A typical problem: “What mass of CO₂ is produced when 10.0 g of C₂H₆ burns completely?” Steps: (1) write the balanced equation (2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O), (2) calculate moles of given substance (n = m / M), (3) use mole ratio to find moles of desired substance, (4) convert moles back to mass (m = n × M). Always keep units and significant figures consistent.

一旦配平方程式给出摩尔比，你就可以计算未知质量或摩尔数。典型题目：“10.0 g C₂H₆ 完全燃烧产生多少克 CO₂？”步骤：（1）写出配平方程式（2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O）；（2）计算已知物质的摩尔数（n = m / M）；（3）利用摩尔比求目标物质的摩尔数；（4）将摩尔数转换回质量（m = n × M）。始终保持单位和有效数字一致。

6. Limiting Reactant and Excess | 限量试剂与过量试剂

The limiting reactant is the substance that is completely consumed in a reaction, determining the maximum amount of product formed. The other reactants are in excess. To identify the limiting reactant, calculate the moles of each reactant, then divide by its stoichiometric coefficient. The smallest value indicates the limiting reactant. All product calculations must be based on the limiting reactant. Common IB question: “Determine the limiting reactant and the mass of product formed.”

限量试剂是反应中被完全消耗的物质，决定了所能生成产物的最大量。其余反应物则为过量。要识别限量试剂，计算各反应物的摩尔数，然后除以其化学计量系数。最小值指示限量试剂。所有产物计算都必须基于限量试剂。常见的IB考题：“确定限量试剂及生成产物的质量。”

7. Theoretical Yield, Actual Yield, and Percentage Yield | 理论产率、实际产率与产率百分比

The theoretical yield is the maximum mass of product predicted by stoichiometry based on the limiting reactant. The actual yield is the amount obtained experimentally. Percentage yield = (actual yield / theoretical yield) × 100%. Yields are rarely 100% due to incomplete reactions, side reactions, or product loss during purification. IB expects you to calculate any of these three values given the other two, and to suggest reasons for a yield less than 100%.

理论产率是根据限量试剂由化学计量学预测的产物最大质量。实际产率是实验获得的量。产率百分比 = (实际产率 / 理论产率) × 100%。由于反应不完全、副反应或产品纯化过程中的损失，产率很少达到 100%。IB 要求你能够根据其中两个值计算第三个，并能解释产率低于 100% 的原因。

8. Molar Volume of a Gas | 气体的摩尔体积

At standard temperature and pressure (STP: 273 K, 100 kPa), one mole of any ideal gas occupies a volume of 22.7 dm³. (Note: the older value 22.4 dm³ at 1 atm is not used in the current IB syllabus — 22.7 dm³ at 100 kPa is the standard.) The relationship is: amount (mol) = volume (dm³) / molar volume (22.7 dm³ mol⁻¹). This is essential for gas stoichiometry. Remember to convert cm³ to dm³ (÷1000). Ideal gas equation PV = nRT can also be used under non‑standard conditions.

在标准温度和压力下（STP：273 K，100 kPa），一摩尔任何理想气体占据 22.7 dm³ 的体积。（注意：当前 IB 大纲不使用 1 atm 下的旧值 22.4 dm³，标准值为 100 kPa 下的 22.7 dm³。）关系式为：物质的量（mol）= 体积（dm³）/ 摩尔体积（22.7 dm³ mol⁻¹）。这对气体化学计量至关重要。记住将 cm³ 转换为 dm³（除以1000）。在非标准条件下，也可以使用理想气体方程 PV = nRT。

9. Concentration and Molarity | 浓度与摩尔浓度

Concentration (c) is the amount of solute dissolved per unit volume of solution, usually expressed in mol dm⁻³. Equation: c = n / V, where V is in dm³. When preparing standard solutions or performing dilutions, C₁V₁ = C₂V₂ is frequently used. Make sure you can calculate the mass of solute needed to make a solution of known concentration, and the new concentration after dilution or mixing.

浓度（c）是单位体积溶液中溶解的溶质的物质的量，通常以 mol dm⁻³ 表示。方程：c = n / V，其中 V 单位为 dm³。在配制标准溶液或进行稀释时，常用 C₁V₁ = C₂V₂。确保你能计算配制已知浓度溶液所需的溶质质量，以及稀释或混合后的新浓度。

10. Titration and Back Titration | 滴定与返滴定

Titration is an experimental technique to determine the concentration of an unknown solution. Using a known concentration of a standard solution and the balanced equation, the mole ratio at the equivalence point allows calculation. Key steps: (1) record volumes precisely, (2) use concordant titres, (3) apply n = cV and mole ratios. Back titration is used when the substance is insoluble or volatile: it is reacted with an excess of a standard reagent, and the unreacted excess is titrated. Both are common in IB Paper 3 and the Individual Investigation.

滴定是一种测定未知溶液浓度的实验技术。利用已知浓度的标准溶液和配平方程式，等当点处的摩尔比可用于计算。关键步骤：（1）精确记录体积，（2）使用一致的滴定管读数，（3）运用 n = cV 和摩尔比。当被测物难溶或易挥发时，使用返滴定：让它与过量标准试剂反应，然后滴定未反应的过量部分。这两种方法常见于 IB 试卷三和个人探究。

11. Common Stoichiometric Pitfalls | 化学计量常见误区

Even strong students lose marks on these: forgetting to balance the equation first; using molar mass units incorrectly (g mol⁻¹ not g); mixing up dm³ and cm³; using the wrong mole ratio from the equation; ignoring state symbols and significant figures. Also, be careful with diatomic gases: oxygen is O₂ (Mᵣ = 32.00), not O. Memorise the formulae of common polyatomic ions (sulfate SO₄²⁻, nitrate NO₃⁻, carbonate CO₃²⁻, ammonium NH₄⁺).

即使优秀学生也常在以下几点丢分：未先配平方程式；错误使用摩尔质量单位（g mol⁻¹，而非 g）；混淆 dm³ 和 cm³；使用了方程式中错误的摩尔比；忽略状态符号和有效数字。此外，小心双原子气体：氧气是 O₂（Mᵣ = 32.00），而非 O。熟记常见多原子离子的化学式（硫酸根 SO₄²⁻、硝酸根 NO₃⁻、碳酸根 CO₃²⁻、铵根 NH₄⁺）。

12. Linking Stoichiometry to Other Topics | 化学计量与其他专题的联系

Stoichiometry is not an isolated topic. It underpins energetics (enthalpy calculations per mole), kinetics (rate expressed as change in concentration per unit time), equilibrium (concentration changes in ICE tables), and redox (electron transfer mole ratios). In the IA, you will inevitably use stoichiometric principles to calculate yields, concentrations, or enthalpy changes. Building a solid foundation here makes the entire chemistry course more manageable.

化学计量不是一个孤立的主题。它是能量学（每摩尔焓变计算）、动力学（速率表示为单位时间浓度变化）、化学平衡（ICE 表中的浓度变化）和氧化还原（电子转移摩尔比）的基础。在内部评估中，你不可避免地要用到化学计量原理来计算产率、浓度或焓变。在这里打下扎实基础，将使整个化学课程变得更容易掌握。

Published by TutorHao | IB Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）