Molecular Weight Calculator

Calculate molecular weight and molar mass of chemical compounds from their molecular formulas. Essential tool for chemistry students, researchers, and laboratory professionals.

Molecular Formula Input

Chemical Formula

Use standard chemical notation: H2O, CaCl2, C6H12O6, etc.

Quick Examples

Advanced Options

Include isotope informationShow percent compositionCalculate empirical formula

Common Compound Categories

Calculation Results

18.015

g/mol

Molecular Weight / Molar Mass

Formula Breakdown

HHydrogen

2×

1.008 g/mol

OOxygen

1×

15.999 g/mol

Total Atoms:3

Elements:2

Moles in 1g:0.0555

Mass of 1 mole:18.015 g

Percent Composition

11.1907%

2.016 g/mol

88.8093%

15.999 g/mol

Total:100.00%

Unit Conversions

Grams per mole:18.015 g/mol

Kilograms per mole:0.018 kg/mol

Pounds per mole:0.0397 lb/mol

Atomic mass units:18.015 u

Daltons:18.015 Da

Molar Calculations

Mass (grams)

Moles:1

Molecules:6.022e+23

Atoms:1.807e+24

How to Use the Molecular Weight Calculator

Step-by-Step Instructions

Enter the chemical formula in standard notation
Use numbers to indicate the quantity of each element
Use parentheses for complex ions: Ca(OH)₂
View the calculated molecular weight and breakdown
Enable additional options for detailed analysis

Formula Notation Rules

Elements: Use standard symbols (H, O, C, Na)
Numbers: Write after the element (H2O, not H₂O)
Parentheses: For groups: Ca(OH)2
Case sensitive: Co (cobalt) vs CO (carbon monoxide)
Hydrates: CuSO4·5H2O

Understanding Molecular Weight

Key Concepts

Molecular Weight (MW): The sum of atomic weights of all atoms in a molecule, expressed in g/mol or u (atomic mass units).

Molar Mass: The mass of one mole of a substance, numerically equal to molecular weight but expressed in g/mol.

Avogadro's Number: 6.022 × 10²³ particles per mole, used to convert between moles and number of molecules.

Empirical Formula: The simplest whole-number ratio of atoms in a compound.

Calculation Method

Step 1: Identify each element and its quantity in the formula

Step 2: Look up the atomic weight of each element

Step 3: Multiply atomic weight by the number of atoms

Step 4: Sum all the products to get molecular weight

Example: H₂SO₄
H: 1.008 × 2 = 2.016
S: 32.06 × 1 = 32.06
O: 15.999 × 4 = 63.996
Total: 98.072 g/mol

Example Calculations

Example 1: Water (H₂O)

Formula: H₂O

Breakdown:

• Hydrogen (H): 1.008 g/mol × 2 = 2.016 g/mol

• Oxygen (O): 15.999 g/mol × 1 = 15.999 g/mol

Molecular Weight: 2.016 + 15.999 = 18.015 g/mol

Meaning: One mole of water weighs 18.015 grams

Example 2: Glucose (C₆H₁₂O₆)

Formula: C₆H₁₂O₆

Breakdown:

• Carbon (C): 12.011 g/mol × 6 = 72.066 g/mol

• Hydrogen (H): 1.008 g/mol × 12 = 12.096 g/mol

• Oxygen (O): 15.999 g/mol × 6 = 95.994 g/mol

Molecular Weight: 72.066 + 12.096 + 95.994 = 180.156 g/mol

Frequently Asked Questions

What's the difference between molecular weight and molar mass?

Molecular weight is a dimensionless quantity (ratio of mass to 1/12 of carbon-12), while molar mass has units (g/mol). Numerically, they're the same.

How do I handle hydrated compounds?

For hydrates like CuSO₄·5H₂O, calculate the molecular weight of the main compound and water separately, then add them together.

Why are atomic weights not whole numbers?

Atomic weights are averages of all naturally occurring isotopes, weighted by their abundance. This results in non-integer values.

How accurate are these calculations?

Our calculations use IUPAC standard atomic weights and are accurate to 4 decimal places, suitable for most laboratory and educational purposes.