Molecular Weight Calculator
Calculate molecular weight (molar mass) from chemical formulas
How to Use This Molecular Weight Calculator
- Enter the molecular formula in the input field (case-sensitive: Ca not CA)
- Use standard chemical notation: H2O, NaCl, C6H12O6, Ca(OH)2
- Parentheses work for groups: Mg3(PO4)2, Ca(NO3)2
- Click a quick example button to load common compounds
- Click 'Calculate' to see molecular weight and element breakdown
Example: Glucose (C6H12O6): Carbon 6 x 12.01 = 72.06, Hydrogen 12 x 1.008 = 12.10, Oxygen 6 x 16.00 = 96.00. Total molecular weight = 180.16 g/mol.
Tip: Element symbols are case-sensitive. Use 'Ca' for calcium and 'Co' for cobalt, not 'CA' or 'CO' (which would be interpreted as Carbon + something).
Why Use a Molecular Weight Calculator?
Molecular weight is essential for converting between mass and moles, preparing solutions, and performing stoichiometric calculations in chemistry.
- Calculating moles from mass for reaction stoichiometry
- Preparing molar solutions in the laboratory
- Determining percent composition by element
- Identifying unknown compounds from mass spectrometry
- Calculating empirical formulas from elemental analysis
- Converting between mass and moles in homework problems
Understanding Your Results
Results show total molecular weight in g/mol and the contribution of each element with percentage composition.
| Result | Meaning | Action |
|---|---|---|
| Under 100 g/mol | Small molecules | Common solvents, gases, and simple compounds. Often volatile. |
| 100-500 g/mol | Medium molecules | Most laboratory reagents, sugars, many drugs fall in this range. |
| 500-5000 g/mol | Large molecules | Peptides, complex organics. May have limited solubility. |
| Over 5000 g/mol | Macromolecules | Proteins, polymers. Usually expressed in kDa (kilodaltons). |
Meaning: Small molecules
Action: Common solvents, gases, and simple compounds. Often volatile.
Meaning: Medium molecules
Action: Most laboratory reagents, sugars, many drugs fall in this range.
Meaning: Large molecules
Action: Peptides, complex organics. May have limited solubility.
Meaning: Macromolecules
Action: Proteins, polymers. Usually expressed in kDa (kilodaltons).
Note: Atomic weights are based on natural isotopic abundance. Isotopically enriched compounds will have different actual masses.
About Molecular Weight Calculator
Formula
MW = Sum of (atomic weight x number of atoms) for each element Add up contributions from each element: count atoms of each type, multiply by that element's atomic weight from the periodic table, and sum all contributions.
Current Standards: Atomic weights are based on IUPAC 2021 standard atomic weights, which reflect natural isotopic abundance. The unified atomic mass unit (u or Da) equals 1/12 of carbon-12's mass.
Frequently Asked Questions
What's the difference between molecular weight and formula weight?
Molecular weight applies to discrete molecules like H2O or glucose. Formula weight applies to ionic compounds like NaCl, which don't exist as individual molecules but as crystal lattices. The calculation is identical - the term 'formula weight' acknowledges there's no actual 'NaCl molecule' in the solid.
Why does the calculator require proper capitalization?
Element symbols follow strict rules: first letter capitalized, second lowercase. 'Co' is cobalt (element 27), while 'CO' would be interpreted as carbon + oxygen. 'Ca' is calcium, but 'CA' is invalid. This prevents ambiguity in chemical formulas.
How do I handle hydrates like CuSO4-5H2O?
Enter the complete formula including water of hydration. For copper sulfate pentahydrate, you'd calculate CuSO4 separately (159.6 g/mol) plus 5 x H2O (5 x 18.02 = 90.1 g/mol), totaling 249.7 g/mol. Or enter Cu1S1O4H10O5 (awkward but works).
Why are atomic weights not whole numbers?
Atomic weights are averages of natural isotopes weighted by abundance. Carbon is 12.01 because 98.9% is C-12 (mass 12) and 1.1% is C-13 (mass 13). Chlorine is 35.45 because it's 75.8% Cl-35 and 24.2% Cl-37. Only pure isotopes have near-integer masses.
How is molecular weight used in mass spectrometry?
Mass spectrometers measure mass-to-charge ratio (m/z). For singly charged ions, this equals molecular mass. Comparing measured mass to calculated molecular weight helps identify compounds. The difference reveals molecular ions, fragments, and adducts, enabling structure determination.