Molarity Calculator
Calculate the molarity of a solution from solute mass, molar mass, and volume.
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1 mol/L
About this calculator
What Is Molarity?
Molarity (M), also called molar concentration, measures how much of a substance is dissolved in a solution: the number of moles of solute per liter of solution. Its unit is moles per liter (mol/L), usually shortened to M. A 1 M solution contains one mole of solute in each liter. It is the most common way chemists express concentration, because it ties the amount of a substance to the volume you can measure and pour. This molarity calculator solves for molarity, mass, molecular weight or volume when you know the other three.
The Molarity Formulas
- M = n ÷ V — molarity equals moles of solute divided by liters of solution.
- n = mass ÷ molecular weight — moles come from grams divided by the molar mass (g/mol).
- Combined: M = mass ÷ (molecular weight × volume).
Dissolving 2 moles of sodium chloride in enough water to make 1 liter gives a 2 M solution. If you need 0.5 moles of HCl from a 1 M stock solution, the volume required is V = n ÷ M = 0.5 L.
Worked Example: Preparing a Solution
To make 500 mL of 0.5 M sodium chloride (molar mass 58.44 g/mol): moles needed = 0.5 mol/L × 0.5 L = 0.25 mol; mass = 0.25 × 58.44 = 14.61 g. Dissolve that mass in some water, then add water until the total volume reaches 500 mL.
Dilutions
To dilute a stock solution, use M₁V₁ = M₂V₂: the moles of solute stay constant while the volume changes. To get 250 mL of 0.1 M solution from a 1 M stock, you need V₁ = (0.1 × 250) ÷ 1 = 25 mL of stock, topped up to 250 mL with solvent.
Molarity vs. Other Concentration Units
- Molality is moles per kilogram of solvent, and does not change with temperature.
- Percent by mass or volume is common on product labels.
- Parts per million (ppm) describes very dilute solutions.
Because volume changes with temperature, molarity does too. Related quantities are in the Density Calculator and unit changes in the Unit Conversion Calculator.
Worked example
Dissolving 58.44 g of table salt (NaCl, molar mass 58.44 g/mol) in 1 L of solution produces exactly a 1 mol/L (1 M) solution.
Frequently asked questions
What is molar mass, and where do I find it?
Molar mass is the mass of one mole of a substance, in grams per mole — it's calculated by summing the atomic masses of every atom in the compound's chemical formula, and can be found on a periodic table or a compound's reference data.
What's the difference between molarity and molality?
Molarity (mol/L) is moles of solute per liter of solution; molality (mol/kg) is moles of solute per kilogram of solvent. Molarity is more common in general chemistry, while molality is preferred when temperature changes are involved, since it doesn't depend on solution volume, which can expand or contract with temperature.
