Molarity Calculator

Molarity (M) is calculated by dividing the moles of solute by the volume of the solution in liters: M = mol / L. For example, if you dissolve 2 moles of a solute into enough solvent to make 4 liters of solution, the molarity is 2 / 4 = 0.5 mol/L, written as 0.5 M. Molarity always refers to the total volume of the final solution, not the volume of solvent added, since dissolving a solute can change the overall volume slightly.

C1V1 = C2V2 is the dilution equation, where C1 and V1 are the concentration and volume of the starting (stock) solution, and C2 and V2 are the concentration and volume after dilution. Since the number of moles of solute doesn't change when you add solvent — only the volume changes — the product of concentration and volume stays constant. For example, diluting 2 L of a 5 M stock solution up to a final volume of 10 L gives C2 = C1V1 / V2 = (5 × 2) / 10 = 1 M.

Molarity (M) is moles of solute per liter of solution, while molality (m) is moles of solute per kilogram of solvent. Molarity depends on the total solution volume, which can shift slightly with temperature (liquids expand or contract), whereas molality depends on mass, which never changes with temperature. Chemists prefer molality for precise work across temperature ranges, but molarity is far more common in everyday lab dilutions because volumetric flasks and graduated cylinders measure volume directly.

First decide the target concentration (C2) and target volume (V2) you need. Then solve the dilution equation for V1: V1 = C2V2 / C1. This tells you exactly how much of the concentrated stock (C1) to measure out. Pour that volume into a container and add solvent (usually water) until you reach the total final volume V2 — never the other way around, and always add concentrated acid to water, not water to acid, for safety.

Molarity is defined specifically as moles per liter of solution because reactions in solution happen based on how many molecules are present in a given volume — the concentration you'd pipette into a test tube or reaction flask. Two solutions with the same mass of solute but different final volumes will have very different molarities and react at different rates or in different proportions, which is why lab protocols almost always specify concentration in mol/L (M) rather than mass alone.

Divide the mass of the compound in grams by its molar mass (g/mol), which you can find by summing the atomic weights of every atom in its chemical formula. For example, 58.44 g of table salt (NaCl, molar mass ≈ 58.44 g/mol) equals exactly 1 mole. Once you have moles, divide by the solution's volume in liters to get molarity. This calculator assumes you already have the moles value — use a molar mass reference or periodic table to convert from grams first.