Molecular Weight Calculator
Precision molar mass analysis for chemical formulas and compounds.
Enter the compound formula (e.g., H2O, NaCl, or C6H12O6). Note: Use proper case sensitivity.
Molecular Weight Calculator
Every chemistry calculation involving moles starts with the molecular weight of the compound. Instead of looking up every element on the periodic table and adding them manually, this molecular weight calculator does it for you in seconds.
What Is Molecular Weight?
Molecular weight (also called molar mass) is the sum of the atomic masses of all atoms in a molecule. It's expressed in grams per mole (g/mol) and is essential for converting between grams and moles in stoichiometry.
How to Use This Calculator
- Enter the chemical formula of your compound (e.g., H₂O, NaCl, C₆H₁₂O₆).
- Click Calculate.
- The calculator returns the molecular weight in g/mol and a breakdown by element.
Molecular Weight Formula
MW = Σ (Atomic Mass × Number of Atoms)
For water (H₂O):
- H: 1.008 × 2 = 2.016
- O: 15.999 × 1 = 15.999
- Total MW = 18.015 g/mol
Example Calculations
Glucose (C₆H₁₂O₆):
- C: 12.011 × 6 = 72.066
- H: 1.008 × 12 = 12.096
- O: 15.999 × 6 = 95.994
- MW = 180.156 g/mol
Sodium Chloride (NaCl):
- Na: 22.990 + Cl: 35.453 = 58.443 g/mol
Why Use This Calculator?
Calculating molecular weights by hand is tedious and error-prone, especially for complex organic molecules. This calculator eliminates the manual lookup and arithmetic, saving time and reducing errors in lab work and homework.
Common Mistakes to Avoid
- Forgetting subscripts — H₂O has 2 hydrogen atoms, not 1. Make sure subscripts are read correctly in the formula.
- Using rounded atomic masses — for precise calculations, use the IUPAC standard atomic weights (e.g., H = 1.008, not 1).
- Confusing molecular weight and formula weight — for ionic compounds (like NaCl), the correct term is formula weight, but the calculation method is the same.
Frequently Asked Questions
What is the molecular weight of water?
Water (H₂O) has a molecular weight of 18.015 g/mol.
How do I convert molecular weight to moles?
Moles = Mass (g) ÷ Molecular Weight (g/mol). Example: 36 g of water ÷ 18.015 = 1.999 moles (≈ 2 mol).
What is the difference between molecular weight and atomic mass?
Atomic mass is the mass of a single atom (measured in amu). Molecular weight is the sum of atomic masses of all atoms in one molecule, expressed in g/mol (numerically equal to amu for a single molecule).
Can I calculate the molecular weight of a polymer?
For repeating unit polymers, calculate the weight of one repeating unit first, then multiply by the degree of polymerization.
Where do I find atomic masses?
Atomic masses come from the periodic table. IUPAC publishes the official standard atomic weights used in all scientific calculations.
Conclusion
Molecular weight is a gateway calculation for stoichiometry, solution preparation, and analytical chemistry. This calculator eliminates the tedious math so you can focus on the problem — not the arithmetic.
Related: Molarity Calculator | Percent Calculator | Density Calculator
Expert Strategy
Always check your **Case Sensitivity**. Entering 'co' will result in an error, while 'CO' (Carbon Monoxide) and 'Co' (Cobalt) have drastically different weights!
