#1 Free Molecular Weight & Molarity Calculator 2025

Molecular Weight (MW), also known as Molar Mass (M), is the mass of one mole of a substance (a chemical element or chemical compound). It is expressed in grams per mole (g/mol).

It is calculated by summing the atomic weights (in atomic mass units, amu, which is numerically equivalent to g/mol for molar mass) of all atoms in a molecule. For ionic compounds, the term Formula Weight is often used, calculated in the same way.

• Example for H₂O:
  • 2 Hydrogen atoms: 2 * 1.008 g/mol = 2.016 g/mol
  • 1 Oxygen atom: 1 * 15.999 g/mol = 15.999 g/mol
  • Molecular Weight = 2.016 + 15.999 = 18.015 g/mol

This calculator uses standard atomic weights, typically sourced from IUPAC.

Molarity (M) is a common unit of concentration, defined as the number of moles of solute per liter of solution.

Molarity (M) = Moles of Solute / Volume of Solution (in Liters)

Key Terms:

• Solute: The substance that is dissolved in a solution.
• Solvent: The substance in which the solute is dissolved (often water).
• Solution: A homogeneous mixture of solute and solvent.
• Mole (mol): A unit representing 6.022 x 10²³ entities (Avogadro's number) of a substance. The mass of one mole of a substance is its molar mass (g/mol).

Derived Formulas:

• Moles of Solute = Mass of Solute (g) / Molecular Weight (g/mol)
• Mass of Solute (g) = Molarity (mol/L) * Volume (L) * Molecular Weight (g/mol)
• Volume of Solution (L) = Moles of Solute / Molarity (mol/L)

Accurate molarity calculations are crucial in chemistry for stoichiometry, titrations, and preparing solutions of known concentrations.

1. Select Calculation Mode:

• Choose "Molecular Weight" to calculate the molar mass of a compound.
• Choose "Molarity Calculations" for calculations involving molarity, mass, or volume.

2. For Molecular Weight Calculation:

• Enter the chemical formula (e.g., H2O, C6H12O6, Al2(SO4)3). Case matters for elements (e.g., 'Co' vs 'CO').

3. For Molarity Calculations:

• First, select what you want to calculate: Molarity (M), Mass (g), or Volume (L).
• Then, fill in the required input fields:
  • Chemical Formula / Molecular Weight: You can either enter the chemical formula (and the calculator will determine its MW) or directly input a known Molecular Weight (g/mol). If both are entered, the direct MW input takes precedence. The dynamically calculated MW from the formula will be shown next to the formula input field.
  • Mass: Enter mass in grams (g) or milligrams (mg).
  • Volume: Enter volume in Liters (L) or milliliters (mL).
  • Molarity: Enter molarity in mol/L (M).

4. Calculate:

• Click the "Calculate" button.

5. View Results:

• The calculated value (Molecular Weight, Molarity, Mass, or Volume) will be displayed.
• For molarity calculations, the number of moles will also be shown.
• A detailed step-by-step solution will illustrate how the result was obtained.
• If there are any errors (e.g., invalid formula, non-numeric input), an error message will appear.

6. Reset:

• Click the "Reset" button to clear all input fields and results.