The primary difference lies in the type of compound to which each term is usually applied: relative molecular mass is used for molecules, while relative formula mass is generally used for ionic compounds or other substances that don't exist as discrete molecules.
Understanding the Terms
To understand the difference, let's define each term:
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Relative Molecular Mass (Mr): This term refers to the sum of the relative atomic masses of all the atoms present in a molecule. It is applicable to substances that exist as discrete molecules, such as water (H₂O), carbon dioxide (CO₂), or methane (CH₄).
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Relative Formula Mass: This term refers to the sum of the relative atomic masses of all the atoms in a formula unit of a substance. This term is used for compounds that do not exist as discrete molecules, such as ionic compounds like sodium chloride (NaCl) or giant covalent structures like silicon dioxide (SiO₂). In these cases, we refer to the empirical formula unit, as the compound doesn't consist of isolated molecules.
Key Distinctions
Here's a table summarizing the key differences:
| Feature | Relative Molecular Mass (Mr) | Relative Formula Mass |
|---|---|---|
| Applies to | Molecules | Ionic compounds and giant covalent structures |
| Based on | Molecular formula | Empirical formula unit |
| Examples | Water (H₂O), Carbon Dioxide (CO₂) | Sodium Chloride (NaCl), Silicon Dioxide (SiO₂) |
| Describes | Mass of a single molecule | Mass of one formula unit |
Why the Distinction?
The distinction arises from the nature of bonding in different substances. Molecular compounds are formed through covalent bonds where atoms share electrons to form distinct molecules. Ionic compounds, on the other hand, are formed through electrostatic attraction between ions in a lattice structure. There are no discrete "molecules" of NaCl; instead, there's a continuous lattice of Na⁺ and Cl⁻ ions. Therefore, it's more accurate to refer to the "formula mass" of NaCl, based on its empirical formula, rather than its "molecular mass."
In Practice
While the terms are technically distinct, they are often used interchangeably, especially in introductory chemistry. The calculation method is identical in both cases: summing the relative atomic masses of the elements in the given formula. The key is to understand the type of compound you're dealing with and use the more appropriate term.
