The key difference between molar mass and empirical formula mass lies in what they represent: molar mass refers to the mass of one mole of a molecule, while empirical formula mass represents the mass of one mole of the simplest ratio of atoms in a compound.
Here's a detailed breakdown:
Understanding the Terms
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Molar Mass: This is the mass of one mole of a substance, expressed in grams per mole (g/mol). It's calculated by adding up the atomic masses of all the atoms present in the chemical formula of the molecule.
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Empirical Formula Mass: This represents the mass of one mole of the simplest whole-number ratio of atoms in a compound, also expressed in g/mol. The empirical formula does not necessarily reflect the actual number of atoms in a molecule.
Key Differences Summarized
| Feature | Molar Mass | Empirical Formula Mass |
|---|---|---|
| Represents | Mass of one mole of a molecule | Mass of one mole of the simplest ratio of atoms |
| Formula Used | Molecular formula | Empirical formula |
| Relationship | Molecular formula mass. | Can be the same or smaller than molar mass. |
Practical Example
Let's consider glucose, a simple sugar, with the molecular formula C6H12O6.
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Molecular Formula: C6H12O6
- Molar Mass: (6 x 12.01 g/mol C) + (12 x 1.01 g/mol H) + (6 x 16.00 g/mol O) = 180.18 g/mol
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Empirical Formula: CH2O (simplest ratio of atoms)
- Empirical Formula Mass: (1 x 12.01 g/mol C) + (2 x 1.01 g/mol H) + (1 x 16.00 g/mol O) = 30.03 g/mol
In this case, the molar mass of glucose (180.18 g/mol) is larger than its empirical formula mass (30.03 g/mol). According to the reference, “the empirical formula will normally have a smaller molar mass as it just represents the ratio of molecules”.
Important Insights
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Same Molar and Empirical Formula Mass: As the reference correctly points out, if the molecular and empirical formulas are equal, their molar masses will be the same. For example, water (H2O) has the same molecular and empirical formula. Therefore, its molar mass and empirical formula mass are both approximately 18.02 g/mol.
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Determining Molecular Formula: If you have the empirical formula and the molar mass of a compound, you can determine the molecular formula. You would divide the molar mass by the empirical formula mass to find a whole number that acts as a multiplier for the empirical formula.
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Ratio Representation: The empirical formula is essentially a ratio, providing information about the relative number of atoms present in a compound but not the exact molecular composition.
In summary, while the molar mass tells you the mass of one mole of a specific molecule, the empirical formula mass tells you the mass of one mole of the simplest ratio of atoms within that molecule. The two values will be equal when the empirical and molecular formulas are the same. Otherwise, the empirical formula mass will usually be smaller.
