Difference Between Empirical and Molecular Formulas

Thekey differencebetween empirical and molecular formulas is that anempirical formula only gives the simplest ratio of原子而分子公式给出了分子中每个原子的精确数量。

In chemistry, we often use symbols to identifyelements and molecules. Molecular formula and empirical formula are two such symbolical methods we use to represent molecules and compounds in an easy way.

内容

1.Overview and Key Difference
2.What are Empirical Formulas
3.What are Molecular Formulas
4.并排比较 - 表格形式的经验与分子公式
5.概括

What are Empirical Formulas?

经验公式是我们可以为分子编写的最简单的公式。它显示了分子中原子的类型，但并未给出每个原子的实际数量。相反，它给出了分子每个原子的最简单整数比。

For example, C₆H₁₂O₆is the molecular formula of glucose, and CH₂O is its empirical formula. Mostly, we give empirical formulas forionic compounds, which are in the crystalline form. For instance, we cannot say the exact number of Na and Cl in a NaCl crystal. So we just write the empirical formula denoting the ratio of connected atoms.

视频01：经验公式

Further, Ca₃(PO₄)₂is also an empirical formula. In an ionic compound, we can easily write the formula by interchanging the charges of each ion, and that automatically gives the number from each ion in the molecule. Also, we can write empirical formulas for macromolecules. When writing empirical formulas for polymers, we write the repeating unit, and then we use the letter “n” to say that there can be n number of repeating units in the polymer. However, we cannot use the empirical formula to find the mass, structure or isomers of a molecule, but it is useful for analytical purposes.

What are Molecular Formulas?

分子公式是显示分子中与每个原子的原子的类型和数量的公式。因此，它给出了每个原子的正确化学计量法。原子由它们的符号描绘，这些符号显示在元素周期表中。此外，我们应该将原子的数量写为下标。某些分子公式是中性的（无电荷），但是如果有电荷，我们可以将其显示为上标作为上标。

We normally use the molecular formula in the chemical reactions, or when documenting any chemical details. By just looking at the molecular formula, we can get a lot of information about the molecule. For example, we can calculate the molecular mass. Also, if it is an ionic compound, we can predict what are the ions and how many of them will be released when it is dissolved in water.

Video 02: Comparison of Molecular and Empirical Formulas

此外，我们可以预测每个原子的氧化数，它们在反应中的反应方式以及使用分子公式所致的产物。但是，仅从分子公式来看，我们无法预测精确的分子排列。因为有时有几种单个分子公式的结构公式。这些是“异构体”。异构体具有相同的分子式，但与原子（构造异构体）的连通性或原子（立体异构体）的空间排列有所不同。因此，通过查看分子公式，我们可以为分子编写所有可能的异构体。

What is the Difference Between Empirical and Molecular Formulas?

经验公式是制定的最简单形式as that we can write for a molecule while molecular formulas are the formulas showing the type of atoms and number of each atom connected in the molecule. Hence, the key difference between empirical and molecular formulas is that empirical formula only gives the simplest ratio of atom whereas molecular formula gives the exact number of each atom in a molecule. Furthermore, we cannot calculate the exact molecular mass using the empirical formula while we can get the exact molecular mass using the molecular formula.

As another important difference between empirical and molecular formulas, we can say that we can predict the type of atoms in ionic compounds and repeating unit in a polymer whereas we can predict the oxidation numbers of each atom, how they are going to react in a reaction, and the resulted products using molecular formulas.

概括– Empirical vs Molecular Formulas

对于某个分子或大分子，我们可以编写经验公式。但是，分子公式是更详细的形式。经验公式和分子公式之间的关键区别在于，经验公式仅给出原子的最简单比率，而分子公式给出了分子中每个原子的精确数量。

Reference:

1. “Empirical Formula 2.” Transition Metals.Available here
2. Libretexts. “6.9: Calculating Molecular Formulas for Compounds.” Chemistry LibreTexts, National Science Foundation, 20 May 2018.Available here

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