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Read privacy policyEnter chemical formula to get molecular weight in g/mol. Free molar mass calculator.
A: Multiply the atomic weight of each element by the number of its atoms, then add the totals. For H2O: 2 x 1.008 + 15.999 = 18.015 g/mol.
A: They are very similar in practice. Molar mass is expressed as grams per mole of substance, while molecular weight usually refers to relative mass without units.
A: Element symbols are case-sensitive. Cl means chlorine, while cl is incorrect. H is hydrogen, and H2 means two hydrogen atoms.
The molecular weight (or molar mass) of a compound is one of the most essential values in chemistry. It bridges the gap between the microscopic world of atoms and molecules and the macroscopic world of grams and liters that we work with in the laboratory. Our free Molecular Weight Calculator instantly computes the molar mass of any chemical compound from its formula, saving you time and eliminating arithmetic errors.
Molecular weight is the sum of the atomic weights of all atoms in a molecule, expressed in grams per mole (g/mol) or unified atomic mass units (u or amu). It tells you the mass of one mole (6.022 × 10²³ particles) of a substance. For ionic compounds that do not exist as discrete molecules (like NaCl), the equivalent term "formula weight" is used, though the calculation is identical.
The atomic weights used in this calculation come from IUPAC's periodic table values, which represent the weighted average of all naturally occurring isotopes of each element. For example, carbon's atomic weight of 12.011 reflects the natural abundance of carbon-12 (98.9%) and carbon-13 (1.1%).
Calculating molecular weight involves three straightforward steps. First, identify each element in the molecular formula and count how many atoms of each are present. Second, look up the atomic mass of each element from the periodic table. Third, multiply each atomic mass by the atom count and sum all the products.
Example — Water (H₂O): Hydrogen has an atomic mass of 1.008, and oxygen is 15.999. Water has 2 hydrogen atoms and 1 oxygen atom: (2 × 1.008) + (1 × 15.999) = 18.015 g/mol.
Example — Glucose (C₆H₁₂O₆): Carbon is 12.011, hydrogen is 1.008, oxygen is 15.999. Calculation: (6 × 12.011) + (12 × 1.008) + (6 × 15.999) = 72.066 + 12.096 + 95.994 = 180.156 g/mol.
| Compound | Formula | Molecular Weight (g/mol) |
|---|---|---|
| Water | H₂O | 18.015 |
| Sodium Chloride | NaCl | 58.443 |
| Glucose | C₆H₁₂O₆ | 180.156 |
| Sulfuric Acid | H₂SO₄ | 98.079 |
| Ethanol | C₂H₅OH | 46.069 |
| Calcium Carbonate | CaCO₃ | 100.087 |
| Aspirin | C₉H₈O₄ | 180.158 |
| Caffeine | C₈H₁₀N₄O₂ | 194.191 |
Atomic masses on the periodic table are not whole numbers because they represent averages weighted by the natural abundance of each isotope. Chlorine, for instance, has an atomic mass of 35.45 because it exists as approximately 75.8% chlorine-35 and 24.2% chlorine-37. When high precision is required (such as in mass spectrometry), monoisotopic masses or exact isotope distributions are used instead.
Many compounds contain parentheses in their formulas to denote repeating groups. For example, calcium hydroxide Ca(OH)₂ means the OH group appears twice: Ca + 2×O + 2×H = 40.078 + 2(15.999) + 2(1.008) = 74.093 g/mol. Our calculator handles nested parentheses, hydrates (like CuSO₄·5H₂O), and complex coordination compounds.
Solution Preparation: To prepare a solution of specific molarity, you need the molecular weight to convert between mass and moles. Mass = Molarity × Volume × Molecular Weight.
Stoichiometry: Balanced chemical equations deal in moles. Molecular weight converts between the masses you measure and the molar ratios required by reaction stoichiometry.
Analytical Chemistry: Techniques like mass spectrometry identify compounds by their molecular weight. Gas chromatography-mass spectrometry (GC-MS) matches detected masses against databases.
Polymer Science: Average molecular weight (Mn, Mw) characterizes polymer chain lengths, affecting material properties like viscosity, tensile strength, and glass transition temperature.
Biochemistry: Protein molecular weights (typically 10,000–100,000+ g/mol) are determined via SDS-PAGE or mass spectrometry and are essential for concentration calculations and structural studies.
These terms are often used interchangeably but have subtle differences. Molecular weight technically refers to a single molecule in atomic mass units (amu). Molar mass is the same numerical value but expressed in g/mol for one mole of substance. Formula weight applies to ionic compounds or empirical formulas where discrete molecules do not exist. For practical laboratory calculations, all three give the same number — the mass of one mole of the substance.
Molecular weight is the sum of atomic masses of all atoms in a molecule, expressed in g/mol. It is calculated from the chemical formula using periodic table values, or measured experimentally via mass spectrometry.
Parse the formula to count atoms of each element, multiply each count by its atomic mass from the periodic table, and sum all values. For NaCl: 22.990 + 35.453 = 58.443 g/mol.
Sodium chloride (NaCl) has a formula weight of 58.443 g/mol, calculated from Na (22.990) + Cl (35.453). Since NaCl is ionic, "formula weight" is the more precise term.
You can calculate the molecular weight of a polymer's repeating unit. For the full polymer, you need the degree of polymerization (n) and multiply: MW_polymer = n × MW_repeat_unit + MW_end_groups.
Atomic masses are weighted averages of all naturally occurring isotopes. Carbon is 12.011 (not exactly 12) because of the small natural abundance of carbon-13. Only carbon-12 is defined as exactly 12 amu.
Atomic weight refers to a single element's average mass. Molecular weight is the sum of atomic weights for all atoms in a molecule or formula unit. Molecular weight ≥ atomic weight always.
For hydrates like CuSO₄·5H₂O, include the water molecules: MW = MW(CuSO₄) + 5 × MW(H₂O) = 159.61 + 5(18.015) = 249.69 g/mol. The dot notation indicates waters of crystallization.
Our calculator uses IUPAC 2021 standard atomic weights, providing accuracy to 3 decimal places for most elements. This precision is sufficient for virtually all laboratory and educational applications.
Guide
Molecular Weight Calculator helps you calculate a result from values, formulas, or measurements without installing extra software. It is designed for students, creators, developers, and everyday users who need a quick, browser-based result with clear input and output.
Molecular Weight Calculator helps you calculate a result from values, formulas, or measurements without installing extra software. It is designed for students, creators, developers, and everyday users who need a quick, browser-based result with clear input and output.
Using Molecular Weight Calculator is simple: (1) Open the tool page, (2) Enter your values, text, or upload your file as prompted, (3) Click the action button or see instant results, (4) Copy, download, or use the output. No technical knowledge required.
Yes — 100% free with no hidden charges. Molecular Weight Calculator is part of WoHoTech's free tools suite. Use it unlimited times without creating an account or providing payment information.
Molecular Weight Calculator uses standard mathematical formulas to deliver reliable results. While suitable for everyday calculations, assignments, and quick estimates, always verify critical financial or academic results with official sources or a qualified professional.
Yes. Molecular Weight Calculator is fully responsive and works on all devices — smartphones, tablets, and desktops. No app download required; just open it in any modern browser and start calculating instantly.
No. All calculations run entirely in your browser. Your input values are never sent to any server, stored, or shared — ensuring complete privacy for every calculation.