Molecular weight (also called molar mass or molecular mass) is the sum of the atomic weights of all atoms in a molecule. It is expressed in grams per mole (g/mol) and represents the mass of one mole (6.022 × 10²³ particles) of that substance. Understanding molecular weight is fundamental to chemistry as it allows scientists to convert between mass and moles, which is essential for stoichiometric calculations.

For example, water (H₂O) has a molecular weight of approximately 18.015 g/mol because it contains two hydrogen atoms (2 × 1.008 = 2.016) and one oxygen atom (15.999), giving a total of 18.015 g/mol. This means that 18.015 grams of water contains exactly one mole of water molecules.

Chemical formulas use element symbols and subscript numbers to represent the composition of compounds. Each element is represented by its one- or two-letter symbol (e.g., H for hydrogen, Ca for calcium), and the subscript number indicates how many atoms of that element are present.

Molecular weight calculations are essential in many areas of science and industry:

• Stoichiometry: Converting between mass and moles for chemical reactions
• Solution Preparation: Calculating how much solute is needed for a specific molarity
• Pharmacology: Determining drug dosages based on molecular composition
• Polymer Chemistry: Characterizing and comparing polymer chains
• Biochemistry: Analyzing proteins, DNA, and other biomolecules
• Industrial Chemistry: Scaling reactions for manufacturing processes

• Check element symbols: Use proper capitalization (e.g., Co for cobalt, CO for carbon monoxide)
• Parentheses matter: Ca(OH)₂ is different from CaOH₂
• Include hydration: Some compounds include water of crystallization (e.g., CuSO₄·5H₂O)
• Verify your formula: Double-check subscripts and element symbols before calculating
• Consider significant figures: Atomic weights have inherent uncertainty, so report results appropriately