Molecular Weight Calculator
Molecular Weight Calculator
Quickly calculate molecular weight by entering a chemical formula.
Loading calculator interface...
Calculator stats
Recent Calculations
Molecular Weight: The Basic Chemistry
Molecular weight = sum of all atomic masses in a formula. Water (H₂O): 2 hydrogen (1.008 each) + 1 oxygen (16.00) = 18.015 g/mol. Glucose (C₆H₁₂O₆): 6 carbon (12.01) + 12 hydrogen (1.008) + 6 oxygen (16.00) = 180.16 g/mol. Units: g/mol (grams per mole) or amu (atomic mass units).
Used everywhere in chemistry: converting grams to moles, calculating reaction yields, determining concentration. One mole = 6.022×10²³ molecules (Avogadro's number). If you have 180.16g glucose, that's exactly 1 mole or 6.022×10²³ glucose molecules.
Common Atomic Weights (Rounded)
Most Common Elements
- H (Hydrogen): 1.008
- C (Carbon): 12.01
- N (Nitrogen): 14.01
- O (Oxygen): 16.00
- S (Sulfur): 32.07
- P (Phosphorus): 30.97
Metals & Halogens
- Na (Sodium): 22.99
- K (Potassium): 39.10
- Ca (Calcium): 40.08
- Cl (Chlorine): 35.45
- Fe (Iron): 55.85
- Cu (Copper): 63.55
Calculation Examples
| Compound | Formula | Calculation | MW (g/mol) |
|---|---|---|---|
| Water | H₂O | 2(1.008) + 16.00 | 18.015 |
| Sodium Chloride | NaCl | 22.99 + 35.45 | 58.44 |
| Carbon Dioxide | CO₂ | 12.01 + 2(16.00) | 44.01 |
| Glucose | C₆H₁₂O₆ | 6(12.01) + 12(1.008) + 6(16.00) | 180.16 |
| Sulfuric Acid | H₂SO₄ | 2(1.008) + 32.07 + 4(16.00) | 98.08 |
| Ethanol | C₂H₅OH | 2(12.01) + 6(1.008) + 16.00 | 46.07 |
Formula Notation Rules
Standard Format
- H₂O: 2 hydrogen, 1 oxygen
- Ca(OH)₂: 1 calcium, 2 oxygen, 2 hydrogen
- Al₂(SO₄)₃: 2 aluminum, 3 sulfur, 12 oxygen
- Number after element = count of that atom
Parentheses Rule
- Multiply everything inside by subscript outside
- Ca(OH)₂: OH group × 2 = 2 O, 2 H
- (NH₄)₂SO₄: NH₄ group × 2 = 2 N, 8 H
- No parentheses = direct subscripts
Practical Applications
Moles ↔ Grams
Grams = Moles × MW
Moles = Grams ÷ MW
Example: 36g H₂O = 36÷18.015 = 2 moles
Concentration
Molarity (M) = moles/liter
Need MW to convert g/L to M
Example: 58.44g NaCl in 1L = 1M solution
Quick Reference: Common Compounds
| Name | Formula | MW (g/mol) |
|---|---|---|
| Water | H₂O | 18.015 |
| Table Salt | NaCl | 58.44 |
| Baking Soda | NaHCO₃ | 84.01 |
| Methane | CH₄ | 16.04 |
| Ammonia | NH₃ | 17.03 |
| Acetone | C₃H₆O | 58.08 |
Frequently Asked Questions
Sum of all atomic masses in a molecule
Units: g/mol (grams per mole) or amu
1. Count atoms of each element
2. Multiply by atomic weight
3. Add all together
Same thing, different terms
Molecular weight and molar mass both use g/mol
Periodic table - weighted average of isotopes
H = 1.008 (not exactly 1) due to deuterium presence
Multiply everything inside by subscript outside
Ca(OH)₂: 1 Ca + 2 O + 2 H
6.022×10²³ molecules (Avogadro's number)
1 mole of water = 18.015 grams
Moles = Grams ÷ Molecular Weight
36g H₂O ÷ 18.015 g/mol = 2 moles
- Convert grams ↔ moles
- Calculate reaction yields
- Prepare solutions (molarity)
- Stoichiometry calculations
Technically different, practically same
Mass (amu) vs weight (g/mol) - numbers identical
Yes, same method
NaCl: 22.99 + 35.45 = 58.44 g/mol (formula weight)