Molecular Weight Calculator
Calculate the molecular weight (molar mass) of any chemical compound by entering its formula. Get a detailed breakdown of each element's contribution and mass percentage. See also our Percent Composition Calculator and Grams to Moles Calculator for related stoichiometry computations.
Supports parentheses: Ca(OH)2, Mg3(PO4)2
How to Calculate Molecular Weight
Molecular weight (also called molar mass or molecular mass) is the sum of the atomic masses of all atoms in a molecule, expressed in grams per mole (g/mol) or atomic mass units (amu). It is one of the most fundamental quantities in chemistry, essential for converting between mass and moles, preparing solutions of known concentration, and performing stoichiometric calculations. Every quantitative chemistry calculation ultimately depends on accurate molecular weight values.
The concept of molecular weight dates back to Amedeo Avogadro's hypothesis (1811) that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. This insight, combined with John Dalton's atomic theory, allowed chemists to determine relative atomic and molecular masses. Today, atomic masses are measured with extraordinary precision using mass spectrometry, with values referenced to the carbon-12 standard (exactly 12 amu).
- Write the molecular formula identifying each element and its subscript (number of atoms).
- Look up the atomic mass of each element from the periodic table.
- Multiply each atomic mass by the number of atoms of that element in the formula.
- Sum all contributions to get the total molecular weight.
- For compounds with parentheses, multiply the subscript outside by all atoms inside.
- Express the result in g/mol (numerically equal to amu per molecule).
For ionic compounds like NaCl, the term "formula weight" is technically more correct than "molecular weight" since ionic compounds do not exist as discrete molecules. However, the calculation method is identical — sum the atomic masses of all atoms in the empirical formula. The distinction matters in advanced chemistry but not for practical calculations.
Molecular Weight Formula
MW = Σ (nᵢ × Aᵢ)
MW = n₁×A₁ + n₂×A₂ + n₃×A₃ + ...
Example for H₂SO₄:
MW = 2(1.008) + 1(32.065) + 4(15.999)
MW = 2.016 + 32.065 + 63.996
MW = 98.077 g/mol
Where:
MW = molecular weight (g/mol)
nᵢ = number of atoms of element i
Aᵢ = atomic mass of element i (from periodic table)
The molecular weight in g/mol is numerically equal to the mass of one molecule in atomic mass units (amu or Da). One mole of any substance contains Avogadro's number (6.022 × 10²³) of formula units. This means that 98.077 g of H₂SO₄ contains exactly 6.022 × 10²³ molecules of sulfuric acid. This relationship between macroscopic mass and microscopic particle count is the foundation of all quantitative chemistry.
Example Calculation
Problem: Calculate the molecular weight of sulfuric acid (H₂SO₄).
Given:
• H: atomic mass = 1.008, count = 2
• S: atomic mass = 32.065, count = 1
• O: atomic mass = 15.999, count = 4
Solution:
MW = (2 × 1.008) + (1 × 32.065) + (4 × 15.999)
MW = 2.016 + 32.065 + 63.996
MW = 98.077 g/mol
Element breakdown:
• H: 2.016/98.077 = 2.06%
• S: 32.065/98.077 = 32.69%
• O: 63.996/98.077 = 65.25%
Answer: The molecular weight of H₂SO₄ is 98.077 g/mol. Oxygen contributes the most mass (65.25%).
Common Molecular Weights Reference Table
| Compound | Formula | MW (g/mol) | Common Use |
|---|---|---|---|
| Water | H₂O | 18.015 | Universal solvent |
| Sodium chloride | NaCl | 58.443 | Table salt |
| Glucose | C₆H₁₂O₆ | 180.156 | Biological energy |
| Sulfuric acid | H₂SO₄ | 98.079 | Industrial acid |
| Ethanol | C₂H₅OH | 46.069 | Solvent, fuel |
| Carbon dioxide | CO₂ | 44.010 | Greenhouse gas |
| Calcium carbonate | CaCO₃ | 100.087 | Limestone, antacid |
| Aspirin | C₉H₈O₄ | 180.157 | Pain reliever |
| Caffeine | C₈H₁₀N₄O₂ | 194.191 | Stimulant |
| Ammonia | NH₃ | 17.031 | Fertilizer precursor |
| Acetic acid | CH₃COOH | 60.052 | Vinegar |
| Sodium hydroxide | NaOH | 39.997 | Strong base |
Frequently Asked Questions
What is the difference between molecular weight and molar mass?
Molecular weight and molar mass are numerically identical but technically different concepts. Molecular weight (or relative molecular mass, Mr) is a dimensionless ratio comparing a molecule's mass to 1/12 of carbon-12. Molar mass (M) has units of g/mol and represents the mass of one mole of substance. In practice, chemists use the terms interchangeably. For example, water has a molecular weight of 18.015 and a molar mass of 18.015 g/mol — the numbers are the same, only the units differ.
How do I handle hydrates in molecular weight calculations?
Hydrates contain water molecules in their crystal structure, written as a dot formula (e.g., CuSO₄·5H₂O). To calculate the molecular weight, add the mass of the anhydrous compound plus the mass of the water molecules: MW(CuSO₄·5H₂O) = MW(CuSO₄) + 5×MW(H₂O) = 159.61 + 5(18.015) = 159.61 + 90.08 = 249.69 g/mol. The water of crystallization is an integral part of the compound and must be included in all stoichiometric calculations.
Why do atomic masses have decimal values?
Atomic masses are weighted averages of all naturally occurring isotopes of an element. For example, chlorine has two stable isotopes: Cl-35 (75.77%) and Cl-37 (24.23%). The weighted average is 0.7577(34.969) + 0.2423(36.966) = 35.453 amu. Elements with only one stable isotope (like fluorine, F-19) have atomic masses very close to whole numbers. The decimal values reflect the natural isotopic distribution and are essential for accurate molecular weight calculations.
How is molecular weight measured experimentally?
Several methods exist: (1) Mass spectrometry directly measures molecular ions with high precision. (2) Colligative property methods (freezing point depression, boiling point elevation, osmotic pressure) determine molar mass from solution behavior. (3) Gel permeation chromatography separates polymers by size. (4) Light scattering measures weight-average molecular weight of macromolecules. For small molecules, mass spectrometry is most accurate; for polymers, multiple techniques are often combined to characterize the molecular weight distribution.
What is the difference between molecular formula and empirical formula?
The empirical formula shows the simplest whole-number ratio of atoms (e.g., CH₂O for glucose), while the molecular formula shows the actual number of atoms per molecule (C₆H₁₂O₆). The molecular weight of the molecular formula is always a whole-number multiple of the empirical formula weight. To determine the molecular formula from the empirical formula, divide the actual molecular weight by the empirical formula weight: 180.16/30.03 = 6, so the molecular formula is (CH₂O)₆ = C₆H₁₂O₆.
How do I calculate molecular weight for polymers?
Polymers have a distribution of molecular weights rather than a single value. Two averages are commonly reported: number-average (Mn = ΣNᵢMᵢ/ΣNᵢ) and weight-average (Mw = ΣNᵢMᵢ²/ΣNᵢMᵢ). The polydispersity index (PDI = Mw/Mn) measures the breadth of the distribution; PDI = 1 indicates a monodisperse sample. For a specific polymer chain, MW = n × MW(monomer) + MW(end groups), where n is the degree of polymerization.
Importance of Molecular Weight in Chemistry
Molecular weight is the bridge between the atomic world and the laboratory bench. Without accurate molecular weights, chemists could not prepare solutions of known concentration, calculate reaction yields, or determine empirical formulas from elemental analysis data. Every time a chemist weighs a reagent on a balance, they are implicitly using molecular weight to convert between grams (what the balance measures) and moles (what reactions consume and produce).
In pharmaceutical science, molecular weight determines many drug properties. Lipinski's Rule of Five states that orally active drugs typically have molecular weights below 500 g/mol. Larger molecules have difficulty crossing cell membranes by passive diffusion. However, biological drugs (antibodies, proteins) can have molecular weights of 150,000 g/mol or more and must be administered by injection. The molecular weight also affects pharmacokinetics — smaller molecules are generally cleared faster by the kidneys.
Analytical chemistry relies on molecular weight for identification and quantification. Mass spectrometry measures the mass-to-charge ratio of ions, and the molecular ion peak directly reveals the molecular weight. In gas chromatography-mass spectrometry (GC-MS), unknown compounds are identified by matching their molecular weight and fragmentation pattern to spectral databases. Accurate molecular weight determination is also essential for confirming the identity of synthesized compounds in research laboratories.
In materials science and polymer chemistry, molecular weight controls mechanical properties, viscosity, and processing behavior. Higher molecular weight polymers are generally stronger and tougher but more difficult to process due to higher melt viscosity. The relationship between molecular weight and properties is described by empirical equations like the Mark-Houwink equation for intrinsic viscosity and the Fox-Flory equation for glass transition temperature. Controlling molecular weight distribution is a key objective in polymer synthesis.
Environmental monitoring uses molecular weight to predict the behavior of pollutants. Volatile organic compounds (VOCs) with low molecular weights evaporate readily and contribute to air pollution, while high molecular weight compounds tend to adsorb to soil particles and persist in the environment. The octanol-water partition coefficient (log P), which correlates with molecular weight and structure, predicts bioaccumulation potential and helps assess environmental risk.