纯净物的元素组成 — AP 化学
1. 质量分数组成 ★★☆☆☆ ⏱ 3 min
质量分数可以直观地告诉你100g化合物样品中所含该元素的质量，这会简化后续实验式的计算，公式为：
\text{Mass percent of X} = \frac{(\text{subscript of X} \times \text{Average atomic mass of X})}{\text{Molar mass of the compound}} \times 100\%
Exam tip: 计算结束后，务必将化合物中所有元素的质量分数相加；如果总和与100%的差值超过0.5%，说明存在计算错误。
2. 由质量数据推导实验式 ★★★☆☆ ⏱ 4 min
要根据实验数据计算实验式，请遵循以下四个核心步骤：
- Convert mass of each element to moles using $n = \frac{m}{M}$
- Divide all mole values by the smallest mole value to get a preliminary ratio
- Multiply all ratios by a whole number to convert any fractional ratios to whole numbers
- Use the whole numbers as subscripts for the empirical formula
Exam tip: 如果比值与整数的差在0.05以内（例如1.98接近2），直接四舍五入为整数；只有当比值明显是1.25、1.33、1.5这类分数时，才需要乘以整数化为整数。
3. 由实验式和摩尔质量推导分子式 ★★☆☆☆ ⏱ 3 min
得到分子化合物的实验式后，如果知道该化合物的实验摩尔质量（AP题目中通常由质谱给出），就可以求出实际的分子式。因为分子式是实验式的整数倍，所以它的摩尔质量也是实验式摩尔质量的相同倍数。倍数$n$的计算公式为：
n = \frac{\text{Molar mass of molecular compound}}{\text{Molar mass of empirical formula}}
$n$ will always be a whole number greater than or equal to 1. If $n=1$, the empirical and molecular formulas are identical. After finding $n$, multiply all subscripts in the empirical formula by $n$ to get the final molecular formula.
Exam tip: Always confirm that the molar mass of your calculated molecular formula matches the given molar mass within rounding error before writing your final answer.
4. Combustion Analysis for Organic Compounds ★★★★☆ ⏱ 4 min
Combustion analysis is an experimental technique used to determine the elemental composition of pure organic compounds (most commonly compounds made of C, H, and O). In the experiment, a known mass of the organic compound is burned completely in excess oxygen, and all $\text{CO}_2$ and $\text{H}_2\text{O}$ produced are absorbed by pre-weighed materials. All carbon in the original compound becomes $\text{CO}_2$, and all hydrogen becomes $\text{H}_2\text{O}$, so we can calculate the mass of C and H in the original compound from product masses. Any remaining mass of the original compound is oxygen, since excess oxygen from the reaction does not contribute to the original sample mass.
Exam tip: Don’t forget to multiply the moles of $\text{H}_2\text{O}$ by 2 to get moles of H; forgetting this step is the most common mistake in combustion analysis problems.
Common Pitfalls
Why: Students implicitly assume a 100 g sample but forget percent values are percentages, not masses, leading to incorrect mole calculations.
Why: Students forget that almost all oxygen in the products comes from the excess oxygen used for combustion, not the original compound.
Why: Students are eager to round to whole numbers and miss common simple fractions that require scaling.
Why: Students mix up the order of division in the multiplier formula.
Why: Students rush and forget to apply the multiplier to all elements.
Why: Students forget the subscript indicates how many atoms of the element are in one formula unit.