Molecular Weight Calculator
Calculate the molecular weight of any chemical compound from its formula. Enter H2O, NaCl, or C6H12O6 for instant molar mass with element breakdown.
Enter Chemical Formula
Use standard element symbols (H, C, N, O, etc.). Numbers after symbols indicate subscripts. Parentheses supported: Ca(OH)2.
Molecular Weight of H2O
18.015
g/mol
Element Breakdown
What Is Molecular Weight?
Molecular weight (MW), also called molar mass, is the sum of the atomic weights of all atoms in a molecule, expressed in grams per mole (g/mol). It connects the microscopic world of atoms to macroscopic measurements we can make in the laboratory—one mole of any substance has a mass equal to its molecular weight in grams.
| Compound | Formula | Molecular Weight | 1 Mole Equals |
|---|---|---|---|
| Water | H₂O | 18.015 g/mol | 18.015 g = 6.022×10²³ molecules |
| Carbon dioxide | CO₂ | 44.01 g/mol | 44.01 g = 22.4 L at STP |
| Glucose | C₆H₁₂O₆ | 180.16 g/mol | 180.16 g |
| Sodium chloride | NaCl | 58.44 g/mol | 58.44 g |
| Aspirin | C₉H₈O₄ | 180.16 g/mol | 180.16 g |
| Caffeine | C₈H₁₀N₄O₂ | 194.19 g/mol | 194.19 g |
Molecular Weight Formula
Where:
- MW= Molecular weight (g/mol)
- n= Number of each type of atom
- Atomic Weight= Element's mass from periodic table
Common Atomic Weights
Knowing common atomic weights speeds up molecular weight calculations. These values come from the weighted average of naturally occurring isotopes.
| Element | Symbol | Atomic Weight | Notes |
|---|---|---|---|
| Hydrogen | H | 1.008 | Lightest element |
| Carbon | C | 12.011 | Basis for organic chemistry |
| Nitrogen | N | 14.007 | Key in amino acids |
| Oxygen | O | 15.999 | Essential for life |
| Sodium | Na | 22.990 | Common in salts |
| Sulfur | S | 32.065 | In proteins (cysteine) |
| Chlorine | Cl | 35.453 | Halogens |
| Calcium | Ca | 40.078 | Bones, signaling |
| Iron | Fe | 55.845 | Hemoglobin |
Tip: For quick estimates, use whole numbers: H≈1, C≈12, N≈14, O≈16, S≈32, Cl≈35.5.
Calculating Molecular Weight Step by Step
Breaking down a molecular formula into its component atoms makes calculation straightforward.
| Step | Action | Example: H₂SO₄ |
|---|---|---|
| 1. Identify atoms | List each element and count | H: 2, S: 1, O: 4 |
| 2. Find atomic weights | Look up each element | H: 1.008, S: 32.065, O: 15.999 |
| 3. Multiply | Atoms × atomic weight | 2×1.008, 1×32.065, 4×15.999 |
| 4. Sum | Add all contributions | 2.016 + 32.065 + 63.996 |
| 5. Report MW | Final answer with units | MW = 98.08 g/mol |
Molecular Weight vs Formula Weight
While often used interchangeably, there's a subtle distinction between these terms.
| Term | Applies To | Definition | Example |
|---|---|---|---|
| Molecular Weight | Covalent compounds | Mass of discrete molecules | H₂O = 18.015 g/mol |
| Formula Weight | Ionic compounds | Mass of formula unit | NaCl = 58.44 g/mol (no molecule exists) |
| Molar Mass | Any substance | General term for both | Both above, plus elements |
| Atomic Weight | Individual elements | Mass of one mole of atoms | Fe = 55.845 g/mol |
Practical note: For calculations, use "molar mass" as the universal term—it applies to everything.
Hydrates and Complex Formulas
Hydrated compounds and complex molecules require including all atoms, including waters of crystallization.
| Compound | Formula | Calculation | MW (g/mol) |
|---|---|---|---|
| Copper sulfate pentahydrate | CuSO₄·5H₂O | Cu + S + 4O + 5(2H + O) | 249.69 |
| Sodium carbonate decahydrate | Na₂CO₃·10H₂O | 2Na + C + 3O + 10(2H + O) | 286.14 |
| Magnesium chloride hexahydrate | MgCl₂·6H₂O | Mg + 2Cl + 6(2H + O) | 203.30 |
Important: When preparing solutions from hydrated salts, use the MW of the hydrate, not the anhydrous form.
Converting Between Moles and Mass
Molecular weight is the conversion factor between moles (amount of substance) and mass (grams).
| Starting With | To Find | Formula | Units |
|---|---|---|---|
| Mass (g) | Moles | n = mass / MW | mol = g / (g/mol) |
| Moles | Mass (g) | mass = n × MW | g = mol × (g/mol) |
| Moles | Molecules | N = n × Nₐ | molecules = mol × 6.022×10²³ |
| Mass (g) | Molecules | N = (mass/MW) × Nₐ | molecules |
Mole-Mass Conversions
Where:
- n= Number of moles
- mass= Mass in grams
- MW= Molecular weight (g/mol)
- N= Number of molecules
- Nₐ= Avogadro's number (6.022×10²³)
Applications of Molecular Weight
Molecular weight calculations are fundamental to nearly every quantitative aspect of chemistry.
| Application | How MW Is Used | Example |
|---|---|---|
| Solution preparation | Calculate mass for target molarity | Make 1 L of 0.5 M NaCl |
| Stoichiometry | Convert mass to moles for reactions | How much product from 10 g reactant? |
| Percent composition | Find mass % of each element | What % carbon in glucose? |
| Molecular formula | Determine formula from analysis | Unknown compound identification |
| Drug dosing | Convert between mass and moles | mg dose to mmol |
| Gas calculations | Molar volume at STP (22.4 L/mol) | Volume of CO₂ produced |
Worked Examples
Calculate MW of Glucose
Problem:
Find the molecular weight of glucose (C₆H₁₂O₆).
Solution Steps:
- 1Identify atoms: 6 Carbon, 12 Hydrogen, 6 Oxygen
- 2Look up atomic weights: C = 12.011, H = 1.008, O = 15.999
- 3Calculate each element's contribution:
- 4Carbon: 6 × 12.011 = 72.066 g/mol
- 5Hydrogen: 12 × 1.008 = 12.096 g/mol
- 6Oxygen: 6 × 15.999 = 95.994 g/mol
- 7Sum: 72.066 + 12.096 + 95.994 = 180.156 g/mol
Result:
MW of glucose = 180.16 g/mol. This means 180.16 grams of glucose contains exactly 1 mole (6.022 × 10²³ molecules).
Calculate Mass from Moles
Problem:
How many grams of calcium chloride (CaCl₂) are in 0.25 moles?
Solution Steps:
- 1First, calculate MW of CaCl₂:
- 2Ca: 1 × 40.078 = 40.078 g/mol
- 3Cl: 2 × 35.453 = 70.906 g/mol
- 4MW = 40.078 + 70.906 = 110.98 g/mol
- 5Now convert moles to mass: mass = n × MW
- 6mass = 0.25 mol × 110.98 g/mol = 27.75 g
Result:
0.25 moles of CaCl₂ = 27.75 grams. This is the mass you'd weigh out to have 0.25 mol of the compound.
Calculate Percent Composition
Problem:
What is the percent composition of carbon in ethanol (C₂H₅OH)?
Solution Steps:
- 1Ethanol formula: C₂H₆O (2 C, 6 H, 1 O)
- 2Calculate MW: 2(12.011) + 6(1.008) + 1(15.999) = 46.07 g/mol
- 3Mass of carbon per mole: 2 × 12.011 = 24.022 g
- 4Percent carbon = (24.022 / 46.07) × 100 = 52.14%
Result:
Ethanol is 52.14% carbon by mass. You can similarly calculate: H = 13.13%, O = 34.73% (total = 100%).
Tips & Best Practices
- ✓Memorize common atomic weights for quick calculations: H≈1, C≈12, N≈14, O≈16, S≈32, Cl≈35.5.
- ✓For hydrated compounds, include the water molecules in your MW calculation.
- ✓Check your answer by ensuring the MW is reasonable for the compound size.
- ✓Avogadro's number (6.022 × 10²³) connects moles to individual molecules.
- ✓For ionic compounds, use 'formula weight' but calculate the same way as molecular weight.
- ✓Always include units (g/mol) in your final MW answer.
- ✓Online periodic tables often provide more decimal places for precise work.
Frequently Asked Questions
Sources & References
- IUPAC Atomic Weights (2024)
- NIST Chemistry WebBook (2024)
- PubChem Compound Database (2024)
- Chemistry LibreTexts - Molar Mass (2024)
Last updated: 2026-01-22
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