Water molar mass
A simple formula example showing how atomic weights add up in a small molecule.
H2O = 2 × H + 1 × O
Molecular weight = 18.02 g/mol
Molecular Weight Calculator (Molar Mass)
Please enter or select a molecular formula to calculate its molecular weight/molar mass. Note that the formula is case-sensitive. This calculator uses standard atomic weights published by IUPAC with uncertainty ignored. The terms "molecular weight" and "molar mass" are used interchangeably.
Input Parameters
How to Enter Formulas
You can enter simple or complex molecular formulas:
- Simple molecules: H2O (water), CO2 (carbon dioxide), CH4 (methane)
- With brackets: Ca(OH)2 (calcium hydroxide), (NH4)2SO4 (ammonium sulfate)
- Hydrates: CuSO4·5H2O (copper sulfate pentahydrate), enter as CuSO4(H2O)5
Enter a formula and click Calculate button
About Molecular Weight Calculator
What is Molecular Weight?
Molecular weight (MW), also known as molar mass, is the average mass of a molecule, usually expressed in g/mol (grams per mole). It is the sum of the atomic weights of all atoms in a molecule.
In chemistry, molecular weight is a fundamental concept used in stoichiometric calculations, solution preparation, gas law applications, and many other scenarios. It represents the mass of one mole of a substance, or the mass of 6.022×10²³ molecules.
For example, water (H₂O) has a molecular weight of 18.02 g/mol, meaning one mole of water has a mass of 18.02 grams.
How to Use the Molecular Weight Calculator
Using this calculator is straightforward:
- Enter a chemical formula in the input box (e.g., H₂O, C₆H₁₂O₆, CaCO₃). Pay attention to capitalization, as 'CO' represents carbon monoxide, while 'Co' represents cobalt.
- Alternatively, select a common chemical from the dropdown menu.
- Click the "Calculate" button to get results.
- View the detailed results, including total molecular weight and contribution from each element.
Usage Tips
- When entering brackets, use regular parentheses ( ), e.g., Ca(OH)2, (NH4)2SO4
- For hydrates like copper sulfate pentahydrate (CuSO₄·5H₂O), enter as CuSO4(H2O)5
- Results can be exported to CSV format or shared using the share button
Molecular Weight Calculation Formula
The calculation of molecular weight is based on the atomic weights and counts of elements in a chemical formula:
Molecular Weight Formula
$$MW = \sum_{i=1}^{n} (AW_i \times N_i)$$
Where: MW = Molecular Weight (g/mol) AWi = Atomic Weight of element i (g/mol) Ni = Number of atoms of element i in the molecule n = Total number of different elements in the molecule
Percent Contribution Calculation
$$\text{Percent Contribution} = \frac{AW_i \times N_i}{MW} \times 100\%$$
The percent mass contribution of an element in a molecule is calculated as: Percent Contribution = (Atomic Weight × Number of atoms) / Molecular Weight × 100%
Formula Parsing Explanation
This calculator can parse various complex formula formats:
H2O
Simple molecule: 2 hydrogen atoms and 1 oxygen atom
Ca(OH)2
Formula with brackets: 1 calcium atom, 2 oxygen atoms, and 2 hydrogen atoms
CuSO4(H2O)5
Hydrate: 1 copper atom, 1 sulfur atom, 4 oxygen atoms, plus 5 water molecules
Applications of Molecular Weight
Molecular weight is a fundamental concept in chemistry and related disciplines with widespread applications:
Stoichiometry
Molecular weight is used to calculate mass relationships in chemical reactions, balance chemical equations, and determine limiting reagents.
Solution Preparation
In laboratories, molecular weight is used to prepare solutions of specific molar concentrations. For example, to prepare a 1 molar solution of NaCl, you need to know its molecular weight (58.44 g/mol).
Pharmaceutical Development
Drug dosage calculations, active ingredient content determination, and pharmaceutical formulation design all require accurate molecular weight information.
Biochemistry and Molecular Biology
Molecular weights of proteins, nucleic acids, carbohydrates, and other biomolecules are crucial for understanding their structure, function, and interactions.
Environmental Science
Pollutant monitoring, gas emission calculations, and environmental impact assessments involve molecular weight conversions and standardizations.
Examples of Common Compounds and Their Molecular Weights
| Compound | Formula | Molecular Weight | Application |
|---|---|---|---|
| Water | H2O | 18.02 g/mol | Solvent, essential for life, industrial uses |
| Glucose | C6H12O6 | 180.16 g/mol | Energy source, biochemical research |
| Aspirin | C9H8O4 | 180.16 g/mol | Medicine (pain relief, anti-inflammatory, antiplatelet) |
| Caffeine | C8H10N4O2 | 194.19 g/mol | Stimulant, beverage additive |
Frequently Asked Questions (FAQ)
What is the difference between atomic weight and molecular weight?
Atomic weight refers to the average mass of an atom of an element, while molecular weight is the total mass of a molecule composed of multiple atoms. For example, the molecular weight of oxygen gas (O₂) is the sum of the atomic weights of two oxygen atoms.
Are molecular weight and molar mass the same thing?
Yes, in most contexts these terms are used interchangeably. Technically, molecular weight is a dimensionless relative mass, while molar mass has units of g/mol, but they have the same numerical value.
Why aren't atomic weights whole numbers?
Atomic weights are usually not whole numbers because they represent the weighted average mass of all naturally occurring isotopes of an element. Different isotopes have the same number of protons but different numbers of neutrons, resulting in different masses.
How do I calculate the molecular weight of a compound with hydrates?
For hydrates, such as CuSO₄·5H₂O (copper sulfate pentahydrate), you can treat it as a combination of CuSO₄ and 5 molecules of H₂O. In this calculator, you can enter it as CuSO4(H2O)5.
How accurate are the calculated results?
This calculator uses the latest atomic weight data published by IUPAC (International Union of Pure and Applied Chemistry), typically accurate to 4-5 decimal places. This precision is sufficient for most chemical and educational applications.
Molecular Weight Examples
Glucose formula mass
Use a common biochemical compound to check larger atom counts in an organic formula.
C6H12O6
Molecular weight = 180.16 g/mol
Calcium hydroxide with brackets
Bracketed formulas multiply the grouped atoms before summing total atomic mass.
Ca(OH)2
The calculator counts 1 Ca, 2 O, and 2 H atoms
Hydrate notation
Hydrates are useful for validating laboratory compounds and crystalline salts.
CuSO4(H2O)5
The result includes the copper sulfate unit plus five water molecules