The molar mass and formula mass of a compound are numerically equivalent, with only a change in units.
Understanding Formula Mass and Molar Mass
To understand the similarity, it's important to define these two terms:
- Formula Mass: The formula mass (also sometimes called formula weight) of a compound is the sum of the atomic masses of all atoms present in the chemical formula of that compound. The units for formula mass are atomic mass units (amu).
- Molar Mass: The molar mass of a substance is the mass of one mole of that substance. The units for molar mass are grams per mole (g/mol).
Numerical Equivalence
The key similarity lies in the numerical value of these two masses. According to the reference:
Due to the use of the same reference substance in defining the atomic mass unit and the mole, the formula mass (amu) and molar mass (g/mol) for any substance are numerically equivalent (for example, one H2O molecule weighs approximately18 amu and 1 mole of H2O molecules weighs approximately 18 g).
This means that:
- If a water molecule (H₂O) has a formula mass of approximately 18 amu, then one mole of water molecules has a molar mass of approximately 18 g/mol.
- If a molecule of NaCl has a formula mass of 58.44 amu, then one mole of NaCl has a molar mass of 58.44 g/mol.
Table of Examples
| Compound | Formula Mass (amu) | Molar Mass (g/mol) |
|---|---|---|
| H₂O | ~18 | ~18 |
| NaCl | ~58.44 | ~58.44 |
| CO₂ | ~44 | ~44 |
Why This Similarity Exists
The relationship between the atomic mass unit (amu) and the mole is carefully constructed. One mole is defined as the number of atoms in exactly 12 grams of carbon-12. Because the definition of the mole and the amu use the same reference (carbon-12), the numerical values of formula mass and molar mass are equivalent.
Practical Insights
- Calculations: This numerical equivalence makes calculations simple. You can calculate the formula mass (amu) and directly use that same numerical value for the molar mass (g/mol).
- Converting: You can easily convert between the mass of a single molecule/formula unit (amu) and the mass of a large number of molecules/formula units (molar mass in g/mol).
- Lab Work: This knowledge is invaluable when working in a chemistry lab. For example, you can use the molar mass to determine the mass of a substance needed for a specific number of moles.
In essence, the molar mass is simply the formula mass expressed in grams per mole rather than atomic mass units. This crucial equivalence simplifies many chemical calculations and concepts.
