What is Molecular Weight (MW)?
Molecular weight (MW) refers to the mass of one mole of a chemical entity (molecule, ion, etc.). Practically, the MW is the sum of the atomic masses of all atoms in a molecule.
- Main units: dalton (Da) or atomic mass unit (a.m.u)
- Key conversion:
1 a.m.u = 1 Da = 1 g·mol⁻¹
⚗️ You will always find the MW listed on bottles of powdered chemicals in laboratories. It is essential for preparing solutions with precise concentrations (molarity).
Acronyms of Molecular Weight: MW, FW, and PM
Depending on language and context, molecular weight can appear under various acronyms:
- MW: Molecular Weight (English)
- FW: Formula Weight (English, for compounds with a general formula—e.g., hydrates)
- PM: Poids Moléculaire (French, rarely used in English)
Note: FW is often used for salts or hydrated molecules where the total mass includes water molecules.
What is the Molecular Weight Used For?
Molecular weight is vital for:
- Converting the amount of substance (mole) to mass (grams)
- Preparing solutions at the desired concentration (molarity calculations)
- Understanding and predicting chemical reactions (yields, dosages, etc.)
Difference Between Molecular Weight and Molar Mass
These two concepts are closely related:
- Molecular weight = pure number (e.g., 98.08 for H₂SO₄)
- Molar mass (M) = expressed in g·mol⁻¹ (e.g., 98.08 g·mol⁻¹)
- Usage: In practice, they are often used interchangeably, but molar mass specifically indicates the mass/amount of substance relationship.
How to Calculate Molecular Weight? (video)
To illustrate how to compute MW, check out this educational video (in English) from TheChemistryNinja:
