ChemistryStepByStep

Molarity Calculator

Enter moles of solute and solution volume, or a mass and chemical formula, to instantly get molarity (mol/L).

Molarity
4.0000 mol/L
4.0000 M

M = mol / L = 2.0000 / 0.5 = 4.0000 mol/L.

What Is Molarity?

Molarity is moles of solute divided by liters of total solution: M = mol / L. It's the standard way to describe how concentrated a solution is, because it directly tells you how many molecules or ions are available to react in each liter, not just how much mass was dissolved.

A key detail: the volume in the denominator is the final solution's volume, not the volume of solvent you started with. Dissolving a solid changes the total volume slightly, so a 1 M solution is made by adding water until the total reaches 1 L, not by adding exactly 1 L of water to the solid.

Worked example

What is the molarity of a solution made by dissolving 58.44 g of NaCl in enough water to make 1 L of solution?

NaCl's molar mass is 58.440 g/mol (22.990 + 35.45, from IUPAC atomic weights), so 58.44 g is 1.0000 mol.

M = mol / L = 1.0000 / 1 = 1.0000 mol/L -- a classic 1 M sodium chloride solution.

Common mistake

Dividing moles by the volume of water added instead of the volume of the final solution. These are only the same if the solute takes up no volume of its own, which is never exactly true -- always use the solution's total, final volume in the denominator.

Keep going

  • Molarity tells you the concentration of a solution as made; the dilution calculator finds what that concentration becomes after adding more solvent. Dilution Calculator
  • Converting a mass of solute into moles for the molarity formula needs the compound's molar mass, which this calculator's formula parser computes directly. Molar Mass Calculator

Frequently Asked Questions

What is molarity?

Molarity (M) is the number of moles of a dissolved substance (solute) per liter of total solution, written mol/L or just M. It's the most common way chemists express concentration because it directly relates to how many molecules or ions are actually reacting.

Is molarity based on the volume of solvent or the volume of solution?

Solution, not solvent. If you dissolve 1 mole of salt in exactly 1 L of water, the final solution's volume is slightly more than 1 L (the salt takes up space), so a 1 M solution is made by dissolving 1 mole and adding water until the total volume reaches 1 L -- not by adding 1 L of water.

How do I find molarity if I only know the mass, not the moles?

Convert mass to moles first using the compound's molar mass (mol = mass / molar mass), then divide by the solution's volume in liters. This calculator's "I know the mass" mode does that conversion for you using the same formula-parsing molar mass calculator.

Does temperature affect molarity?

Yes, slightly. Solution volume expands a little as temperature rises, which lowers molarity by a small amount even though the moles of solute haven't changed. For most classroom and lab work this effect is small enough to ignore, but precise analytical chemistry accounts for it.

What's the difference between molarity and molality?

Molarity divides by the volume of the whole solution (liters); molality divides by the mass of just the solvent (kilograms). Molality doesn't change with temperature since mass doesn't expand or contract, which is why it's preferred for precise work like freezing-point depression.

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