The key difference between exact mass and molecular weight is that exact mass is the average mass of the atoms of an element measured in the atomic mass unit, whereas molecular weight is the collection of weights of all the atoms in a molecule.
Exact mass and molecular weight are important terms at the atomic level. We can describe these terms under general chemistry, where we need calculations regarding the physical properties of atoms.
CONTENT
1. Overview and Key Difference
2. What is Exact Mass
3. What is Molecular Weight
4. Exact Mass vs Molecular Weight in Tabular Form
5. Summary – Exact Mass vs Molecular Weight
What is Exact Mass?
Exact mass can be defined as the average mass of the atoms of an element measured in an atomic mass unit. It is also known as weight average molecular weight. Weight average molecular weight is the weight fraction of molecules in a polymer sample. It is another way of determining the molecular mass of a polymer. It gives the average of the molecular masses of the individual macromolecules in the polymer sample. We can find this parameter using the following equation:
Mw = ∑ NiMi2 / ∑ NiMi
Where Mw is the weight average molecular weight, Ni is the number of molecules of molecular mass Mi. We can determine this parameter using static light scattering, small-angle neutron scattering, X-ray scattering, and sedimentation velocity. More importantly, the weight average molecular weight is always greater than the weight average molecular weight since larger molecules in a sample weigh more than smaller molecules.
What is Molecular Weight?
Molecular weight can be defined as the collection of weights of all the atoms in a molecule. The SI unit of this parameter is gmol-1. The molecular weight is the amount of atoms/molecules/compounds present in one mole of the substance. This means that it is the mass of Avogadro’s number of atoms/molecules or compounds.
It is important to measure the weight of atoms and molecules in a practical scenario. But it is difficult to weigh them as individual particles since their masses are extremely small according to the normal weighing parameters (grams or kilograms). Therefore, to fulfill this gap and measure the particles at a macroscopic level, molar mass concept is very useful.
Figure 01: Separating Proteins by Molecular Weight
The definition of molecular weight is directly related to the carbon-12 isotope. The mass of one mole of carbon 12 atoms is exactly 12 grams, so its molar mass is exactly 12 grams per mole. Moreover, we can calculate the molecular weight of molecules containing the same atoms like O2 or N2 by multiplying the number of atoms by the atomic weight of the atoms. However, the molecular weight of the compounds like NaCl or CuSO4 is calculated by adding the atomic weights of each atom.
What is the Difference Between Exact Mass and Molecular Weight?
Exact mass and molecular weight are important terms at an atomic level. We can describe these terms under general chemistry, where we need calculations regarding the physical properties of atoms. The key difference between exact mass and molecular weight is that exact mass is the average mass of the atoms of an element measured in atomic mass unit, whereas molecular weight is the collection of weights of all the atoms in a molecule.
Summary – Exact Mass vs Molecular Weight
Exact mass can be defined as the average mass of the atoms of an element measured in an atomic mass unit. Molecular weight can be defined as the collection of weights of all the atoms in a molecule. The key difference between exact mass and molecular weight is that exact mass is the average mass of the atoms of an element measured in atomic mass unit, whereas molecular weight is the collection of weights of all the atoms in a molecule.
Reference:
1. Boudreaux, Kevin A. “Atomic Mass.” The Parts of the Periodic Table.
Image Courtesy:
1. “Separating Proteins by Molecular Weight” By National Institute of Arthritis and Musculoskeletal and Skin (CC BY-NC-SA 2.0) via Flickr