The molar mass and empirical formula mass of a compound are related because the molar mass is a multiple of the empirical formula mass.
Here's a detailed breakdown of their relationship:
Understanding the Key Terms
- Empirical Formula: The simplest whole-number ratio of atoms in a compound. For example, the empirical formula of glucose is CH₂O, even though its molecular formula is C₆H₁₂O₆.
- Empirical Formula Mass: The sum of the atomic masses of all atoms in the empirical formula of a compound.
- Molar Mass: The mass of one mole (6.022 × 10²³) of a substance, usually measured in grams per mole (g/mol).
How They are Related
The key relationship is that the molar mass is always a whole number multiple of the empirical formula mass.
According to the reference:
The difference between molar mass and formula mass is that formula mass is the sum of all the atomic weights of the atoms in its empirical formula while molar mass is just the mass of 1 mol of a substance.
Here's how they connect practically:
- Empirical Formula Mass Calculation: You determine the empirical formula first, then add up the atomic masses of each element present in that formula.
- Molar Mass Determination: The molar mass is typically found experimentally.
- The Multiple Factor: The molar mass will either be equal to or a whole number (1, 2, 3, 4, etc.) multiple of the empirical formula mass. This multiple factor tells you how many times the empirical formula unit exists in a single molecule of the compound.
- Molecular Formula: Multiplying the empirical formula by the multiple factor gives you the molecular formula.
Illustration with Examples
Let's use the glucose example.
| Feature | Empirical Formula (CH₂O) | Molar Formula (C₆H₁₂O₆) |
|---|---|---|
| Atomic Mass (Approximate) | Carbon (12.01) + Hydrogen (1.01 x 2) + Oxygen (16.00) | Carbon (12.01 x 6) + Hydrogen (1.01 x 12) + Oxygen (16.00 x 6) |
| Calculation | 12.01 + 2.02 + 16.00 = 30.03 g/mol | 72.06 + 12.12 + 96.00 = 180.18 g/mol |
| Formula Mass | 30.03 g/mol | 180.18 g/mol |
| Molar Mass | 30.03 g/mol | 180.18 g/mol |
- The empirical formula mass of CH₂O is approximately 30.03 g/mol.
- The molar mass of glucose (C₆H₁₂O₆) is approximately 180.18 g/mol.
180.18 / 30.03 = 6. This shows that the molar mass of glucose is 6 times the mass of its empirical formula unit. In this case the multiple is 6.
Practical Insights
- Knowing the empirical formula and molar mass is essential for determining the actual molecular formula of a compound.
- By comparing the empirical formula mass and molar mass, scientists can deduce the precise molecular structure of a substance.
In Summary
The empirical formula mass represents the mass of the simplest ratio of atoms, while the molar mass is the mass of one mole of the substance. The molar mass is always a whole-number multiple of the empirical formula mass. This relationship provides a key tool for determining molecular formulas in chemistry.
