Molecular Weight
Calculate the molecular weight (MW) of any chemical compound from its formula. Supports parentheses, hydrates, and calculates both average and exact molecular weights.
Sample Compounds
๐ง Water (HโO)
The universal solvent - essential for all known life forms
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๐ฌ Glucose (CโHโโOโ)
Simple sugar - primary energy source for cells
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๐ง Sodium Chloride (NaCl)
Common table salt - essential electrolyte
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๐๏ธ Calcium Hydroxide (Ca(OH)โ)
Used in construction, water treatment, and food processing
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โ๏ธ Iron(III) Sulfate (Feโ(SOโ)โ)
Used in water treatment and as a mordant in dyeing
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๐ Copper(II) Sulfate Pentahydrate (CuSOโยท5HโO)
Hydrate compound - used as fungicide and in electroplating
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๐บ Ethanol (CโHโO)
Alcohol - fuel, solvent, and beverage
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๐ Aspirin (CโHโOโ)
Common pain reliever and anti-inflammatory
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โ Caffeine (CโHโโNโOโ)
Central nervous system stimulant found in coffee
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โ๏ธ Sulfuric Acid (HโSOโ)
Industrial workhorse - most produced chemical
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Enter Chemical Formula
โ ๏ธFor educational and informational purposes only. Verify with a qualified professional.
What is Molecular Weight?
Molecular weight (MW) is the sum of the atomic weights of all atoms in a molecule, expressed in atomic mass units (amu) or grams per mole (g/mol). It represents the mass of one mole of a compound and is fundamental to stoichiometry, solution preparation, and chemical analysis.
Average Molecular Weight
Uses weighted average atomic masses from the periodic table. Accounts for natural isotope abundance. Standard for most calculations.
Exact Molecular Weight
Uses masses of most abundant isotopes. More precise for mass spectrometry. Typically slightly different from average mass.
Formula Weight
Used for ionic compounds. Sum of atomic weights based on empirical formula unit. Same calculation method as MW.
How to Calculate Molecular Weight
- Write the chemical formula (e.g., HโO, CโHโโOโ, Ca(OH)โ, Feโ(SOโ)โ)
- Identify each element and count the number of atoms (handle parentheses by multiplying)
- Find atomic masses from the periodic table for each element
- Multiply each atomic mass by the number of atoms of that element
- Sum all contributions to get the total molecular weight
- For exact mass: Use isotope masses instead of average atomic masses
Example: Water (HโO)
Hydrogen: 2 atoms ร 1.008 g/mol = 2.016 g/mol
Oxygen: 1 atom ร 15.999 g/mol = 15.999 g/mol
Total MW = 18.015 g/mol
Molecular Weight Formulas
Molecular Weight Formula
where: n = number of atoms, A = atomic mass (g/mol)
Exact Mass Formula
where: n = number of atoms, M_isotope = most abundant isotope mass
Percent Composition
Mass from Moles
where: m = mass (g), n = moles, MW = molecular weight (g/mol)
When to Use This Calculator
Molecular weight is essential for stoichiometry, solution preparation, reaction yields, and analytical chemistry. Use exact mass for mass spectrometry and average mass for general calculations.
Stoichiometry
Convert between moles and grams for balanced chemical equations and limiting reagent calculations.
Solution Preparation
Calculate mass of solute needed to prepare solutions of specific molarity or molality.
Mass Spectrometry
Determine exact molecular weight for compound identification and structural analysis.
๐ Official Data Sources
โ ๏ธ Disclaimer: This calculator uses IUPAC-recommended standard atomic weights. For precision work, consult the latest CIAAW tables.