Instantly determine the molecular weight (molar mass) and elemental composition of any chemical compound.
Enter standard chemical notation (case sensitive, e.g., 'Co' is Cobalt, 'CO' is Carbon Monoxide).
Number of moles to calculate total mass.
Molar Mass (Molecular Weight)
180.156 g/mol
Formula: Sum of atomic masses for C₆H₁₂O₆
Total Mass
180.156 g
Total Atoms
24
Element Count
3
Mass Composition Breakdown
Element
Atomic Mass (u)
Count
Total Mass (g/mol)
% by Mass
Table 1: Detailed elemental breakdown of the calculated molecule.
What is "Calculate Weight of a Molecule"?
To calculate weight of a molecule, commonly referred to as molecular weight or molar mass, implies determining the sum of the atomic masses of all atoms present in a single molecule of a chemical substance. This fundamental concept in stoichiometry allows chemists, students, and engineers to convert between the mass of a substance and the amount of substance (moles), bridging the gap between the microscopic world of atoms and the macroscopic world of laboratory scales.
This metric is essential for preparing chemical solutions, determining reactant stoichiometry, and analyzing chemical purity. While the term "weight" is scientifically distinct from "mass" (weight depends on gravity), in common chemical parlance, "molecular weight" and "molar mass" are often used interchangeably to represent the mass of one mole of a substance expressed in grams per mole (g/mol).
This tool is designed for students, laboratory technicians, and researchers who need a quick, error-free method to process complex organic or inorganic formulas without manual periodic table lookups.
Molecular Weight Formula and Mathematical Explanation
The mathematical basis to calculate weight of a molecule is a summation formula. The molecular weight ($MW$) is the sum of the atomic weights ($A_i$) of each constituent element multiplied by the number of atoms ($n_i$) of that element in the formula.
The General Formula:
MW = Σ (Aᵢ × nᵢ)
Where:
MW = Molecular Weight (or Molar Mass)
Σ = Summation symbol (add all components)
Aᵢ = Atomic Mass of the i-th element (from the periodic table)
nᵢ = Number of atoms of the i-th element in the formula
Variables Table
Variable
Meaning
Common Unit
Typical Range
Molar Mass
Mass of 1 mole of substance
g/mol
1.0 – 1,000,000+
Atomic Mass
Mass of a single atom
amu or u
1.008 (H) – 294 (Og)
Moles (n)
Amount of substance
mol
0.001 – 100+
Table 2: Key variables used in molecular weight calculations.
Practical Examples (Real-World Use Cases)
Example 1: Water (H₂O)
Scenario: A student needs to calculate weight of a molecule of water to determine how many grams make up 1 mole.
Input Formula: H₂O
Hydrogen (H): Atomic Mass ≈ 1.008, Count = 2. Mass = 2.016
Oxygen (O): Atomic Mass ≈ 15.999, Count = 1. Mass = 15.999
Calculation: 2.016 + 15.999 = 18.015 g/mol
Interpretation: One mole of water weighs approximately 18.015 grams.
Example 2: Glucose (C₆H₁₂O₆)
Scenario: A biochemist needs to prepare a solution requiring 0.5 moles of glucose. First, they must determine the molar mass.
Input Formula: C₆H₁₂O₆
Carbon (C): 12.011 × 6 = 72.066
Hydrogen (H): 1.008 × 12 = 12.096
Oxygen (O): 15.999 × 6 = 95.994
Total Molar Mass: 180.156 g/mol
For 0.5 moles: 180.156 × 0.5 = 90.078 grams
How to Use This Molecular Weight Calculator
Follow these simple steps to obtain accurate results:
Identify the Formula: Locate the chemical formula (e.g., from a textbook or reagent bottle).
Enter the Formula: Type the formula into the "Chemical Formula" field. Ensure correct capitalization (e.g., use "Cl" for Chlorine, not "cl" or "CL").
Set Quantity: If you need the mass for a specific number of moles, adjust the "Quantity (Moles)" field. The default is 1.
Analyze Results: View the "Molar Mass" for the per-mole weight. Check the "Mass Composition Breakdown" chart to see which elements contribute most to the total mass.
Copy Data: Use the "Copy Results" button to paste the data into your lab report or spreadsheet.
Key Factors That Affect Molecular Weight Results
When you calculate weight of a molecule, several nuances can influence the precision and application of the result:
Isotopes: Standard atomic weights are averages of natural isotopic abundance. For specific isotopes (e.g., Carbon-13 labeled compounds), the standard weight will be inaccurate.
Atomic Mass Precision: Different periodic tables may list atomic masses to varying decimal places (e.g., H as 1.01 vs 1.00784). This tool uses high-precision standard values.
Formula Notation: Correct capitalization is vital. "Co" is Cobalt (mass ~58.9), while "CO" is Carbon Monoxide (mass ~28.0). Misreading the case leads to drastic errors.
Hydration: Many salts exist as hydrates (e.g., CuSO₄·5H₂O). You must include the water molecules in the formula string for accurate weighing of physical crystals.
Purity: In practical settings, reagents are rarely 100% pure. The calculated theoretical weight assumes 100% purity, so real-world measurements usually require a purity correction factor.
Significant Figures: The precision of your final mass usually cannot exceed the precision of the least precise atomic mass used in the calculation.
Frequently Asked Questions (FAQ)
What is the difference between molecular weight and molar mass?
Technically, molecular weight is the mass of a single molecule (measured in amu), while molar mass is the mass of one mole of substance (measured in g/mol). However, numerically they are identical, and this calculator provides the value used for both.
Does this calculator handle parentheses?
Currently, this calculator supports standard linear formulas. For complex nested structures with parentheses (e.g., Ca(NO3)2), please expand them manually to CaN2O6 for the most accurate result.
Why is capitalization important in chemical formulas?
Chemical symbols are case-sensitive. "PB" would be interpreted as Phosphorus (P) and Boron (B), whereas "Pb" is Lead. Always use standard periodic table capitalization.
Can I calculate the weight of mixtures here?
No, this tool calculates the weight of a pure compound defined by a single chemical formula. Mixtures require weighted averages based on composition percentage.
How are electrons accounted for?
The mass of an electron is negligible compared to protons and neutrons. Therefore, the mass of an ion (e.g., Na+) is treated effectively the same as the neutral atom (Na) for general laboratory calculations.
Is this applicable to polymers?
For simple polymers with a known formula, yes. However, most commercial polymers have a distribution of chain lengths, requiring number-average or weight-average molecular weight analysis, which is more complex.
What unit does the result use?
The primary result is displayed in grams per mole (g/mol), which is the standard SI-compliant unit for molar mass in chemistry.
Does this tool account for deuterium?
By default, "H" refers to the natural isotopic mix of Hydrogen. To calculate specifically for Deuterium, you would need to manually adjust the mass, as standard calculators use the weighted average.