Molecular Weight Calculator

Calculating molecular weight (relative molecular mass) and molar mass is a foundational step in general, analytical, and organic chemistry. Whether you are balancing chemical equations, calculating molarity in solution chemistry, or analyzing stoichiometry for laboratory syntheses, determining the exact mass of a chemical compound from its formula is essential.

Our free online Molecular Weight Calculator uses the abridged standard atomic weights published by the International Union of Pure and Applied Chemistry (IUPAC) to compute:

• Molar Mass (g/mol): The mass of exactly one mole ($6.02214076 \times 10^{23}$) of molecules or formula units.
• Relative Molecular Mass: The dimensionless ratio of a molecule’s mass to the atomic mass constant.
• Elemental Mass Composition: The percentage breakdown of each constituent element within a compound.

Key Chemistry Definitions: Atomic Weight, Molar Mass, and Moles

To understand chemical mass calculations, it is helpful to clarify core definitions:

How to Calculate Molecular Weight: Step-by-Step Examples

Calculating molar mass involves three main steps:

1. Identify and count the number of atoms of each element in the chemical formula.
2. Look up the abridged standard atomic weight for each element from the periodic table.
3. Multiply each element’s atomic weight by its atom count and sum all products together.

Example 1: Simple Molecule — Water (H₂O)

Water consists of 2 Hydrogen (H) atoms and 1 Oxygen (O) atom:

• Hydrogen (H): 1.008 g/mol × 2 = 2.016 g/mol
• Oxygen (O): 15.999 g/mol × 1 = 15.999 g/mol
• Molar Mass of H₂O: 2.016 + 15.999 = 18.015 g/mol

Example 2: Polyatomic Parentheses — Aluminum Sulfate [Al₂(SO₄)₃]

Parentheses indicate that everything inside is multiplied by the outer subscript:

• Aluminum (Al): 26.982 g/mol × 2 = 53.964 g/mol
• Sulfur (S): 32.060 g/mol × (1 × 3) = 96.180 g/mol
• Oxygen (O): 15.999 g/mol × (4 × 3) = 191.988 g/mol
• Molar Mass of Al₂(SO₄)₃: 53.964 + 96.180 + 191.988 = 342.132 g/mol

Example 3: Hydrate Crystals — Copper(II) Sulfate Pentahydrate (CuSO₄·5H₂O)

Hydrates contain water molecules bound within their crystal structure:

• Anhydrous CuSO₄: 63.546 (Cu) + 32.060 (S) + 63.996 (O4) = 159.602 g/mol
• Water of Crystallization (5H₂O): 5 × 18.015 g/mol = 90.075 g/mol
• Molar Mass of CuSO₄·5H₂O: 159.602 + 90.075 = 249.677 g/mol

IUPAC Abridged Standard Atomic Weights Table (Elements 1–118)

Below is the complete table of standard atomic weights published by the International Union of Pure and Applied Chemistry (IUPAC), used by this calculator:

To convert molar mass measurements across metric mass units, use our Conversion Calculator. For solution density calculations in chemistry labs, visit our Density Calculator or Volume Calculator.

Frequently Asked Questions (FAQ)

What is the difference between molecular weight and molar mass?

Molecular weight (relative molecular mass) is a dimensionless ratio comparing a molecule’s mass to the atomic mass constant ($1/12\text{ of } {}^{12}\text{C}$). Molar mass is the mass of one mole ($6.022 \times 10^{23}$) of the substance expressed in grams per mole (g/mol). For practical lab calculations, both terms share identical numerical values.

Why is atomic weight a decimal number rather than a whole integer?

Atomic weight represents the natural weighted average of all stable isotopes of an element. Because different isotopes have different numbers of neutrons (e.g., Chlorine-35 and Chlorine-37), the weighted abundance average produces a decimal value (e.g., 35.45 g/mol for Chlorine).

How do I type parentheses or hydrates into the calculator?

Enter formulas using standard uppercase and lowercase chemical symbols:

• Use parentheses for polyatomic ions: Ca(NO3)2 for Calcium Nitrate.
• Use a period or asterisk for hydrates: CuSO4.5H2O or CuSO4*5H2O for Copper(II) Sulfate Pentahydrate.

What is Avogadro’s constant?

Avogadro’s constant is defined as exactly 6.02214076 × 10²³ mol^-1. It represents the exact number of constituent particles (atoms or molecules) contained in one mole of any chemical substance.