Molecular Weight Calculator
Calculate molecular weight, molar mass, and element composition from chemical formulas.
Results
Molecular Weight
18.0150 g/mol
H2O
Chemical Formula
Molecular Weight
18.015
g/mol
Elements
2
unique elements
Total Atoms
3
in formula
Hill Formula
H2O
standard notation
Element Composition
| Element | Symbol | Count | Atomic Mass | Total Mass | Mass % |
|---|---|---|---|---|---|
| Hydrogen | H | 2 | 1.0080 | 2.0160 | 11.19% |
| Oxygen | O | 1 | 15.9990 | 15.9990 | 88.81% |
| Total | 3 | - | 18.0150 | 100.00% | |
Mass Composition
Mass / Moles / Molarity Converter
Molar mass: 18.0150 g/mol
Results
Moles = Mass (g) / Molar Mass (g/mol)
Mass (g) = Moles x Molar Mass (g/mol)
Molarity (M) = Moles / Volume (L)
Molecules = Moles x 6.022 x 10^23
Stoichiometry Quick Reference
1 mole of H2O
18.015 g
1 gram of H2O
0.055509 mol
1 mole contains
6.022 x 1023 molecules
Atoms per mole of H2O:
Common Compounds Reference
| Formula | Name | Molar Mass (g/mol) | Action |
|---|---|---|---|
| H2O | Water | 18.02 | |
| NaCl | Sodium Chloride (Table Salt) | 58.44 | |
| CO2 | Carbon Dioxide | 44.01 | |
| C6H12O6 | Glucose | 180.16 | |
| H2SO4 | Sulfuric Acid | 98.08 | |
| NaOH | Sodium Hydroxide | 40.00 | |
| HCl | Hydrochloric Acid | 36.46 | |
| NH3 | Ammonia | 17.03 | |
| CH4 | Methane | 16.04 | |
| C2H5OH | Ethanol | 46.07 | |
| C6H6 | Benzene | 78.11 | |
| CaCO3 | Calcium Carbonate | 100.09 | |
| Fe2O3 | Iron(III) Oxide | 159.69 | |
| C12H22O11 | Sucrose (Table Sugar) | 342.30 | |
| NaHCO3 | Sodium Bicarbonate (Baking Soda) | 84.01 | |
| C8H18 | Octane | 114.23 | |
| ATP | Adenosine Triphosphate | 507.18 | |
| HNO3 | Nitric Acid | 63.01 | |
| KMnO4 | Potassium Permanganate | 158.03 | |
| Mg(OH)2 | Magnesium Hydroxide | 58.32 |
Formula Input Guide
Basic: Use element symbols with subscripts as numbers. H2O, NaCl, CO2
Parentheses: Use () for groups with multipliers. Ca(OH)2, Mg3(PO4)2
Isotopes: Use D for Deuterium (H-2), T for Tritium (H-3). D2O = heavy water
Hydrates: Use dot or space. CuSO4.5H2O or CuSO4 5H2O
Case sensitive: Co (Cobalt) vs CO (Carbon monoxide), Na (Sodium) vs NA (invalid)
?How Do You Calculate Molecular Weight?
Molecular weight (molar mass) is calculated by summing the atomic masses of all atoms in a formula. For H2O: 2(1.008) + 1(15.999) = 18.015 g/mol. To find mass percentage: (element mass / total mass) x 100. For H2O: H = 11.19%, O = 88.81%.
Definition
Molecular weight (molar mass) is the sum of atomic weights of all atoms in a molecule, expressed in atomic mass units (u) or grams per mole (g/mol). It determines how much of a substance constitutes one mole.
Key Facts About Molecular Weight
- Molecular Weight = Sum of (Atomic Mass x Count) for each element
- Molar mass is expressed in grams per mole (g/mol)
- Mass percentage = (Element mass contribution / Total mass) x 100
- One mole contains 6.022 x 10^23 particles (Avogadro's number)
- Moles = Mass (g) / Molar Mass (g/mol)
- Molarity (M) = Moles of solute / Liters of solution
- Isotopes like Deuterium (D) have different atomic masses than standard elements
- Parentheses in formulas multiply all enclosed elements: Ca(OH)2 = Ca + 2O + 2H
Frequently Asked Questions
Find each element's atomic mass from the periodic table, multiply by the number of atoms of that element, and sum all values. For glucose C6H12O6: 6(12.011) + 12(1.008) + 6(15.999) = 72.066 + 12.096 + 95.994 = 180.156 g/mol.
Molecular weight (relative molecular mass) is dimensionless - a ratio compared to 1/12 of carbon-12. Molar mass is the mass of one mole in grams per mole (g/mol). Numerically, they are equal: water's molecular weight is 18.015, and its molar mass is 18.015 g/mol.
Parentheses group atoms, with the subscript multiplying everything inside. Ca(OH)2 means: Ca + 2 x (O + H) = Ca + 2O + 2H. For Mg3(PO4)2: 3 Mg + 2 x (P + 4O) = 3 Mg + 2P + 8O.
Mass % = (mass of element / total molecular mass) x 100. For H2O, hydrogen: (2 x 1.008 / 18.015) x 100 = 11.19%. Oxygen: (15.999 / 18.015) x 100 = 88.81%.
Moles = Mass (g) / Molar Mass (g/mol). Molarity (M) = Moles / Volume (L). Mass = Moles x Molar Mass. To find mass from molarity: Mass = Molarity x Volume (L) x Molar Mass.
Elemental analysis determines the percentage composition of elements in a compound. It can verify compound identity and purity. For empirical formula: convert mass % to moles, divide by smallest, and round to whole numbers.
Standard atomic masses are weighted averages of natural isotopes. For specific isotopes like Deuterium (D, mass 2.014) or Carbon-13, use the isotope's actual mass. Heavy water D2O = 2(2.014) + 15.999 = 20.027 g/mol vs. regular water = 18.015 g/mol.
Last updated: 2025-01-15
Results
Molecular Weight
18.0150 g/mol
H2O