Calculating the molecular weight of a compound is one of the most fundamental skills in chemistry. It is required for determining reactant concentrations, stoichiometry, molarity, and solving laboratory yields.
Step 1: Identify elements and subscripts
Look at the chemical formula and identify which elements are present by their symbols. Check the subscripts to determine how many atoms of each element are in the compound. If there is no subscript, it means there is exactly 1 atom.
For example, in Water (H₂O):
• Hydrogen (H): Subscript 2 means 2 atoms.
• Oxygen (O): No subscript means 1 atom.
Step 2: Look up atomic weights
Look up the average atomic weight for each element in the periodic table. We use IUPAC standard weights (e.g., H = 1.008 g/mol, O = 15.999 g/mol).
Step 3: Multiply and sum the weights
Multiply the atomic weight of each element by its number of atoms, then sum all results:
Handling Parentheses (Nested Groups)
If a formula contains parentheses, the subscript outside the parentheses applies to every element inside. For example, in Calcium Hydroxide (Ca(OH)₂):
• Calcium (Ca): 1 atom × 40.078 = 40.078 g/mol
• Oxygen (O): 2 atoms × 15.999 = 31.998 g/mol
• Hydrogen (H): 2 atoms × 1.008 = 2.016 g/mol
• Total: 40.078 + 31.998 + 2.016 = 74.092 g/mol.