What is the Difference Between Exact Mass and Molecular Weight?
🆚 Go to Comparative Table 🆚The main difference between exact mass and molecular weight lies in the consideration of isotopic composition.
- Exact Mass: This is the mass of a molecule calculated with only the most abundant isotopes present, which are usually the lightest isotopes. The exact mass is also called the monoisotopic mass. It is calculated by adding the exact masses of the most abundant isotopes of the constituent elements.
- Molecular Weight: This is the average mass of a molecule, considering all the molecules with different isotopic compositions, each weighted for the natural abundance of the respective isotope(s). The molecular weight is directly related to the carbon-12 isotope, and it is calculated by adding the atomic weights of all the atoms in a molecule.
For example, the molecular weight of propane is 44.0956 g/mol, while its exact molecular mass for the molecules with the lightest isotopes is 44.0626 amu. The difference between molecular weight and exact mass is generally slight for small molecules and increases with the presence of elements having more than one significant isotope or larger numbers of an element with a not-so-significant isotope (e.g., carbon).
Comparative Table: Exact Mass vs Molecular Weight
The following table highlights the key differences between exact mass and molecular weight:
| Property | Exact Mass | Molecular Weight |
|---|---|---|
| Definition | Exact mass is the average mass of the atoms of an element measured in atomic mass units (amu). | Molecular weight is the average mass of a molecule, considering the isotopic distribution of atoms. |
| Calculation | Exact mass is calculated by adding the exact masses of the most abundant isotopes of the constituent elements. | Molecular weight is calculated by multiplying the atomic weights of the constituent elements by their respective abundances. |
| Units | Atomic mass units (amu). | Grams per mole (g/mol). |
| Application | Exact mass is used in mass spectrometry and nuclear physics. | Molecular weight is used in chemistry and biology to describe the average mass of molecules in a sample. |
| Isotopic Distribution | Exact mass considers only the most abundant isotopes of the constituent elements. | Molecular weight takes into account the natural abundance of all isotopes of the constituent elements. |
In summary, exact mass is the precise mass of a molecule considering only the most abundant isotopes, while molecular weight is the average mass of a molecule, taking into account the isotopic distribution of atoms.
- Atomic Mass vs Molecular Weight
- Molar Mass vs Molecular Mass
- Formula Weight vs Molecular Weight
- Mass vs Molar Mass
- Atomic Mass vs Molar Mass
- Atomic Weight vs Atomic Mass
- Atomic Weight vs Mass Number
- Gram Atomic Mass vs Gram Molecular Mass
- Formula Unit Mass vs Molecular Mass
- Number Average vs Weight Average Molecular Weight
- Formula Mass vs Molar Mass
- Mass vs Weight
- Mass vs Weight
- Mass Number vs Atomic Mass
- Atomic Mass Unit vs Atomic Mass
- Gram Molecular Weight vs Gram Equivalent Weight
- Atomic Mass vs Average Atomic Mass
- Atomic Number vs Atomic Weight
- Atomic Number vs Mass Number