Molecular Weight Calculator
Calculate the molecular weight (molar mass) of any compound from its chemical formula. Supports all elements using IUPAC atomic weights. Essential for chemistry calculations.
Molecular Weight of H2O
18.015 g/mol
| Element | Count | Atomic Weight | Contribution |
|---|---|---|---|
| H | 2 | 1.008 | 2.016 |
| O | 1 | 15.999 | 15.999 |
| Total MW | 18.015 g/mol | ||
About the Molecular Weight Calculator
A molecular weight calculator (also called a molar mass calculator) determines the mass of one mole of any chemical compound by summing the atomic weights of all atoms in the molecular formula. The result, in grams per mole (g/mol), is the essential conversion factor between mass (in grams) and amount of substance (in moles) — the fundamental relationship in quantitative chemistry. For example, water (H₂O) has a molecular weight of 18.015 g/mol (2×1.008 + 15.999), meaning 18.015 grams of water contains exactly one mole (6.022 × 10²³ molecules). Molecular weight calculations are used in solution preparation, stoichiometric calculations, yield determination, limiting reagent analysis, and gas law applications. Atomic weights are the 2021 IUPAC standard values. This calculator is essential for chemistry students at secondary school and university level, laboratory technicians, pharmacists, chemical engineers, and researchers across all fields that use quantitative chemistry.
Formula
MW = Σ (n_i × AW_i) for each element i | moles = mass (g) / MW | mass (g) = moles × MW
How It Works
MW = Σ (element count × atomic weight) summed over all elements in the formula. Step 1: parse the chemical formula to identify each element and its count. Step 2: look up the standard atomic weight for each element from the IUPAC periodic table. Step 3: multiply count by atomic weight for each element. Step 4: sum all contributions. Example — glucose (C₆H₁₂O₆): C: 6 × 12.011 = 72.066; H: 12 × 1.008 = 12.096; O: 6 × 15.999 = 95.994. MW = 72.066 + 12.096 + 95.994 = 180.156 g/mol. Example — sulfuric acid (H₂SO₄): H: 2 × 1.008 = 2.016; S: 1 × 32.06 = 32.06; O: 4 × 15.999 = 63.996. MW = 98.072 g/mol. Moles to mass: mass (g) = moles × MW. Mass to moles: moles = mass / MW.
Tips & Best Practices
- ✓Parentheses in formulas multiply the enclosed group: Ca(OH)₂ = Ca + 2O + 2H. MW = 40.078 + 2×15.999 + 2×1.008 = 40.078 + 31.998 + 2.016 = 74.092 g/mol. Most formula parsers handle standard chemical notation including parentheses with subscripts.
- ✓Hydrated salts include water of crystallisation: CuSO₄·5H₂O (copper(II) sulfate pentahydrate). MW = MW(CuSO₄) + 5 × MW(H₂O) = 159.60 + 5×18.015 = 249.68 g/mol. When preparing solutions from hydrates, always use the hydrated molecular weight.
- ✓Isotopic molecular weight vs average molecular weight: the calculator uses average atomic weights (weighted averages of all natural isotopes). Mass spectrometry measures the exact mass of specific isotopologue species. For most preparative chemistry, average molecular weight is correct; for mass spec interpretation, use exact monoisotopic masses.
- ✓For polymer molecular weight, the formula represents the repeating unit. The degree of polymerisation n multiplies the monomer MW. Polyethylene (-CH₂CH₂-)ₙ has monomer MW = 28.054; a chain of 1000 units has MW ≈ 28,054 g/mol (28 kDa).
Who Uses This Calculator
Chemistry students calculating molar masses for stoichiometry, yield, and limiting reagent problems. Laboratory technicians preparing accurate solution concentrations for analytical and synthetic work. Pharmacists and pharmaceutical chemists calculating drug dosages and formulation concentrations. Chemical engineers performing mass balance calculations in industrial process design.
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Frequently Asked Questions
How do I calculate molecular weight from a chemical formula?
Sum the atomic weights of all atoms in the formula. For H2O: H = 1.008 × 2 = 2.016, O = 15.999, MW = 18.015 g/mol. For NaCl: Na = 22.990, Cl = 35.45, MW = 58.44 g/mol. Atomic weights are from the IUPAC periodic table.
What is the difference between molecular weight and molar mass?
Molecular weight is dimensionless (ratio of average mass of one molecule to 1/12 the mass of carbon-12). Molar mass has units of g/mol. Numerically they are identical: water has molecular weight 18.015 and molar mass 18.015 g/mol. In practice, the terms are used interchangeably in most chemistry contexts.
Why do I need molecular weight for chemistry calculations?
Molar mass converts between mass (grams) and amount (moles): moles = mass / molar mass. This conversion is fundamental to stoichiometry, solution preparation, limiting reagent calculations, and yield calculations in both academic and industrial chemistry.