Molecular Weight Calculator

The Formula

The molecular weight (or molecular mass) is the sum of the atomic weights of all atoms in a molecule. It is expressed in grams per mole (g/mol).

Variable Definitions

How to Use This Calculator

1. 1

   Enter a chemical formula using standard notation (e.g., H2O, NaCl, C6H12O6).

2. 2

   Use parentheses for polyatomic groups: Mg(OH)2, Ca(NO3)2.

3. 3

   For hydrates, use the · or . notation: CuSO4·5H2O or CuSO4.5H2O.

4. 4

   Unicode subscripts (H₂O) and normal digits (H2O) are both supported.

5. 5

   The calculator shows molecular weight, element breakdown, and mass percentages.

Molecular weight = sum of atomic weights in the molecule

Understanding the Concept

Molecular weight is the sum of the atomic masses of all atoms in a molecule. It is used to convert between mass and moles of a substance, prepare solutions of specific concentration, and determine the composition of chemical compounds. Atomic weights are based on the weighted average of naturally occurring isotopes of each element. Practical example: calculating the molecular weight of glucose (C₆H₁₂O₆). Carbon: 6 × 12.011 = 72.066, Hydrogen: 12 × 1.008 = 12.096, Oxygen: 6 × 15.999 = 95.994. Total = 72.066 + 12.096 + 95.994 = 180.156 g/mol. This means 180.156 grams of glucose contains exactly one mole (6.022 × 10²³ molecules) of glucose. Edge cases: hydrated compounds like copper sulfate pentahydrate (CuSO₄·5H₂O) include water molecules in the crystal structure. The molecular weight must account for the five water molecules: CuSO₄ = 159.609 g/mol, plus 5 × H₂O (5 × 18.015 = 90.075 g/mol) = 249.684 g/mol total. For polymers like polyethylene (C₂H₄)ₙ, the molecular weight varies with chain length (n), so an average molecular weight is reported. For compounds with isotopes like deuterated water (D₂O), use the atomic weight of deuterium (2.014) instead of protium (1.008).

Frequently Asked Questions

Sources & References