A worked example
Glucose, C6H12O6, has a molecular weight of 180.156 g/mol — 6 carbons, 12 hydrogens, and 6 oxygens summed together.
Frequently asked questions
Why do parentheses matter in a formula like Ca(OH)2?
The subscript outside the parentheses multiplies everything inside — Ca(OH)2 means one calcium atom plus two full hydroxide (OH) groups, not just two oxygen atoms. Parsing the parentheses correctly is essential for an accurate molecular weight.
What's the difference between molecular weight and molar mass?
In practice, they're used interchangeably and refer to the same number — the mass of one mole of a substance, expressed in grams per mole (g/mol).