Molecular Weight Calculator

Easily compute the molecular weight of any compound by entering its chemical formula.

Options
Examples
H2O C6H12O6 NaCl Ca(OH)2 CuSO4·5H2O
Help

1. Enter a chemical formula (e.g., C2H6O)

2. The tool parses and calculates atomic contributions

3. Results are displayed with detailed breakdown

Tips:

  • Capitalization matters: use Fe, not fe
  • Use parentheses for groups (e.g., Ca(OH)2)
  • Supports hydrates (e.g., CuSO4·5H2O)
Enter a valid chemical formula (case sensitive)

Chemical Theory & Educational Reference

This calculator determines molar mass (molecular weight), a fundamental property in stoichiometry defined as the mass per mole of a substance. The calculation follows the standard formula:

M = Σ(nᵢ × Aᵢ)
  • M = molar mass (g·mol⁻¹)
  • nᵢ = number of atoms of element i in the formula unit
  • Aᵢ = atomic weight of element i (g·mol⁻¹)
  • Σ = summation over all elements in the compound

Example Calculation for Glucose (C₆H₁₂O₆):

M = (6 × 12.011) + (12 × 1.008) + (6 × 15.999) = 72.066 + 12.096 + 95.994 = 180.156 g·mol⁻¹

Molar mass calculations are essential in laboratory practice for:

  • Solution Preparation: Calculating required masses for specific molarities
  • Stoichiometry: Determining reactant masses and product yields in chemical reactions
  • Analytical Chemistry: Converting between mass and moles in titrations and gravimetric analysis
  • Physical Chemistry: Relating mass to colligative properties (boiling point elevation, freezing point depression)
  • Biochemistry: Determining concentrations of biomolecules in biological assays

Real-World Example: Pharmacists use molar mass to calculate exact dosages when preparing medications from active pharmaceutical ingredients.

Atomic Weight Reference Standards

This tool uses atomic weights based primarily on the IUPAC (International Union of Pure and Applied Chemistry) Standard Atomic Weights 2021 values. Key considerations:

  • Isotopic Composition: Atomic weights represent weighted averages of naturally occurring isotopes
  • Uncertainty: Some elements have atomic weight ranges (e.g., Li: [6.938, 6.997]) due to natural variations
  • Conventional Values: For radioactive or synthetic elements, conventional values are used
Accuracy & Rounding
  • Default precision: 2 decimal places (selectable up to 6)
  • Final results are rounded, not truncated
  • For precise analytical work, consult IUPAC's latest atomic weight table

Student Misconceptions
  • Molecular vs. Formula Weight: Ionic compounds (e.g., NaCl) have formula weights, not molecular weights
  • Hydrate Notation: The middle dot (·) indicates coordination, not multiplication (e.g., CuSO₄·5H₂O is copper(II) sulfate pentahydrate)
  • Case Sensitivity: "Co" is cobalt, "CO" is carbon monoxide
  • Isotope Considerations: This calculator uses average atomic masses; for specific isotopes, use different values
Tool Limitations
  • Does not validate chemical formula existence or stability
  • Cannot handle non-integer stoichiometric coefficients
  • Does not distinguish between different allotropes (e.g., graphite vs. diamond both use C: 12.011)
  • For coordination compounds with brackets, use simplified notation

Q: Why is molecular weight expressed in g/mol?

A: The gram-mole relationship is defined by Avogadro's constant (6.02214076×10²³ mol⁻¹). One mole of any substance contains exactly this number of particles, making molar mass numerically equal to molecular weight in daltons.

Q: How does this differ from atomic mass?

A: Atomic mass refers to a single atom (usually in daltons), while molar mass refers to one mole of atoms or molecules (in g·mol⁻¹). Numerically, they are equal but differ in units.

Q: Can I calculate molar mass for ions?

A: Yes, enter the formula as written (e.g., Na⁺ would be entered as "Na", SO₄²⁻ as "SO4"). The calculation ignores charge since electrons contribute negligible mass.

Q: What about polymers or large biomolecules?

A: This tool works for any formula, but very long formulas may exceed input limits. For proteins, use amino acid residue average masses (≈110 Da per residue).

About Molecular Weight

Molecular weight (or molar mass) is the sum of the atomic masses of all atoms in a molecule, measured in grams per mole (g/mol).

This calculator uses the standard atomic weights from the IUPAC periodic table. For elements with no stable isotopes, the atomic weight of the most common isotope is used.

The tool supports complex formulas including parentheses for grouped atoms (e.g., (NH4)2SO4) and hydrates (e.g., CuSO4·5H2O).

Academic Integrity Statement: This tool is designed for educational use and laboratory planning. For publication-quality results, verify against primary literature and IUPAC standards.

Last Formula Verification: October 2025 • Based on IUPAC Standard Atomic Weights 2021