Molecular Weight Formula
Sum atomic masses to find molar mass (g/mol)
Formula
MW = sum(atomic mass of each element × atom count)
Atomic masses come from the periodic table and are averaged by natural abundance.
Step-by-Step
- Write the molecular formula and count atoms of each element.
- Look up atomic masses (g/mol) for each element.
- Multiply atomic mass by the number of atoms for that element.
- Sum all contributions to get molecular weight (g/mol).
Example: Glucose C6H12O6
C: 12.01 g/mol × 6 = 72.06
H: 1.008 g/mol × 12 = 12.10
O: 16.00 g/mol × 6 = 96.00
Total MW = 72.06 + 12.10 + 96.00 = 180.16 g/mol
Report to appropriate significant figures based on atomic masses used.
Common Pitfalls
Forgetting atom counts
Multiply atomic mass by the number of atoms for each element.
Rounding too early
Keep a few extra decimals, round at the end.
Mixing formula units
Ensure the chemical formula is correct before summing masses.
Isotopic masses
Standard tables use average atomic masses; isotopic enrichment changes values.
FAQ
Is molecular weight the same as molar mass?
Yes, both are in g/mol and represent mass per mole of entities.
How is formula weight different?
Formula weight is used for ionic compounds and computed the same way using formula units.
What about hydrates?
Include water of hydration atoms in the formula when summing masses.