ChemistryStepByStep

Mole Calculator

Enter a chemical formula and a mass to get moles, or moles to get mass -- converts both directions instantly.

e.g. "H2O", "NaCl"

Moles
2.0000 mol

Molar mass of H2O: 18.015 g/mol. mol = mass / molar mass = 36.03 / 18.015 = 2.0000 mol.

Converting Between Grams and Moles

Moles connect a lab-scale mass you can weigh to the actual number of molecules reacting. The conversion factor is molar mass (g/mol), which is different for every compound -- so converting grams to moles always needs both the mass and the formula, never just the mass alone.

The relationship works both directions: mol = mass / molar mass, and mass = mol x molar mass. This calculator computes the molar mass from whatever formula you type, using the same IUPAC-sourced atomic weight table as the molar mass calculator, so the two always agree.

Worked example

How many moles are in 44 g of CO2?

CO2's molar mass is 44.009 g/mol (12.011 + 2 × 15.999).

mol = mass / molar mass = 44 / 44.009 = 0.9998 mol -- almost exactly 1 mole, a common introductory-chemistry coincidence since CO2's molar mass rounds to 44.

Common mistake

Using a different compound's molar mass out of habit -- for instance, assuming 18 g of any substance is about 1 mole because that's true for water. Always recompute molar mass from the actual formula in the problem.

Keep going

  • Converting mass to moles needs the compound's molar mass first, which this calculator's formula parser computes from any chemical formula. Molar Mass Calculator
  • Moles of solute is one of the two inputs molarity needs; the molarity calculator takes that mole count and a solution volume to get concentration. Molarity Calculator

Frequently Asked Questions

What is a mole in chemistry?

A mole is a counting unit, like "dozen" but for 6.022 x 10^23 particles (Avogadro's number) instead of 12. Chemists use moles because atoms and molecules are far too small and numerous to count individually, but their masses in grams are convenient lab-scale numbers.

How do I convert grams to moles?

Divide the mass in grams by the substance's molar mass in g/mol: mol = mass / molar mass. This calculator computes the molar mass automatically from the formula you enter, using the same IUPAC-sourced periodic table as the molar mass calculator.

How do I convert moles to grams?

Multiply moles by molar mass: mass = mol x molar mass. It's the same relationship as grams-to-moles, just solved in the other direction.

Why does the formula matter for this conversion?

Because molar mass -- the conversion factor between grams and moles -- is different for every compound. 18 g of water is about 1 mol, but 18 g of table salt is a different mole count entirely, since NaCl's molar mass (58.44 g/mol) is more than three times water's (18.015 g/mol).

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