Instantly determine the molar mass, elemental breakdown, and mass percentage of any chemical compound with our professional molecular weight calculation tool.
Enter the chemical symbol (case-sensitive). Supports parentheses.
Invalid chemical formula. Please check syntax.
Total Molecular Weight
0.00 g/mol
Total Atom Count
0
Heaviest Element
–
Element Types
0
Formula Used: Sum of (Atomic Mass of Element × Quantity of Element)
Elemental Breakdown
Element
Atoms
Atomic Mass (u)
Subtotal Mass
Mass %
Mass Percentage Distribution
What is Molecular Weight Calculation?
Molecular weight calculation is the process of determining the sum of the atomic masses of all atoms in a molecule. In chemistry, this value is often referred to as the "molar mass" and is expressed in grams per mole (g/mol) or unified atomic mass units (u). It serves as a fundamental metric for stoichiometry, preparing chemical solutions, and analyzing chemical reactions.
This calculation is critical for chemists, students, and lab technicians who need to convert between the mass of a substance and the number of moles. Using a precise tool for molecular weight calculation eliminates manual errors and accounts for exact atomic weights from the periodic table.
Common misconceptions include confusing atomic number with atomic mass, or neglecting to multiply the mass of an element by its subscript (the number of atoms) in the formula.
Molecular Weight Calculation Formula
The mathematical foundation for calculating the weight of a molecule is a summation formula. To find the total molecular weight ($MW$), you sum the product of the atomic mass ($A$) and the number of atoms ($n$) for each element present in the compound.
MW = Σ (ni × Ai)
Where:
Variable
Meaning
Unit
Typical Range
MW
Total Molecular Weight
g/mol
1.0 – 100,000+
ni
Number of atoms of element i
Count (Integer)
1 – 1000+
Ai
Atomic Mass of element i
g/mol
1.008 (H) – 294 (Og)
Practical Examples of Molecular Weight Calculation
Example 1: Water (H₂O)
To perform a molecular weight calculation for water, we break down the formula:
Hydrogen (H): 2 atoms × 1.008 g/mol = 2.016 g/mol
Oxygen (O): 1 atom × 15.999 g/mol = 15.999 g/mol
Total: 2.016 + 15.999 = 18.015 g/mol. This means one mole of water weighs approximately 18 grams.
Example 2: Glucose (C₆H₁₂O₆)
Glucose is a sugar with a more complex structure. The calculation follows:
Total: 72.066 + 12.096 + 95.994 = 180.156 g/mol. Accurately determining this value is essential for preparing glucose solutions in medical and biological contexts.
How to Use This Molecular Weight Calculation Tool
Enter the Formula: Type the chemical formula into the input field. Use standard case sensitivity (e.g., "Cl" for Chlorine, not "cl" or "CL").
Handle Groups: The calculator supports parentheses for groups, such as Ca(NO3)2.
Review Results: The tool instantly displays the total molecular weight in g/mol.
Analyze Composition: Check the "Elemental Breakdown" table to see how much mass each element contributes.
Visualize: Use the generated chart to understand the mass percentage distribution visually.
This tool is designed to assist in quick lab preparations, homework verification, and industrial chemical planning.
Key Factors That Affect Molecular Weight Results
Isotopic Composition: Standard atomic weights represent an average of natural isotopes. If you are working with specific isotopes (e.g., Deuterium instead of Hydrogen), standard calculations will be slightly off.
Formula Purity: Calculations assume a 100% pure substance. Hydrates (water molecules attached to the crystal structure) significantly increase weight (e.g., CuSO₄ vs CuSO₄·5H₂O).
Significant Figures: The precision of the atomic weights used affects the final result. This calculator uses high-precision standard weights (up to 3-4 decimal places).
Notation Errors: Confusing "Co" (Cobalt) with "CO" (Carbon Monoxide) leads to drastically different results. "Co" is one atom ~58.9 g/mol, while "CO" is two atoms ~28.0 g/mol.
Complex Ions: In salts, forgetting to distribute the subscript outside parentheses to all elements inside affects the count (e.g., in Mg(OH)₂, there are 2 Oxygens and 2 Hydrogens).
Rounding Differences: Different periodic tables may round atomic masses slightly differently, leading to minor variations in the final digit.
Frequently Asked Questions (FAQ)
Does this calculator handle hydrates?
Yes, but you must enter them as part of the formula string. For example, for Copper(II) Sulfate Pentahydrate, enter CuSO4(H2O)5. This treats the water molecules as attached groups.
Why is case sensitivity important in molecular weight calculation?
Chemical symbols are case-sensitive to distinguish elements. "PB" would be Phosphorus and Boron, whereas "Pb" is Lead. Correct casing ensures the parser identifies the correct elements.
What is the difference between molecular weight and molar mass?
Numerically, they are usually identical. Molecular weight is often in atomic mass units (u) describing a single molecule, while molar mass is in grams per mole (g/mol) describing a mole of substance.
Can I calculate the weight of mixtures?
No, this tool calculates the weight of a pure compound defined by a single chemical formula. For mixtures, you would need to calculate each component separately and weigh them by their proportion.
How accurate are the atomic weights used?
We use IUPAC standard atomic weights. However, since isotopic ratios vary slightly in nature, the values are technically "conventional" atomic weights suitable for normal laboratory use.
What is the heaviest element this calculator supports?
It supports the entire standard periodic table up to Oganesson (118). However, elements with very short half-lives often have integer mass numbers representing the most stable isotope.
Does temperature affect molecular weight?
No. Molecular weight is an intrinsic property of the molecule's composition and does not change with temperature, pressure, or state of matter (solid, liquid, gas).
Is this tool suitable for organic chemistry?
Absolutely. It handles long carbon chains (e.g., C18H36O2) and complex organic structures effortlessly, making it ideal for calculating weights of proteins or polymers if the formula is known.