Molarity Calculator
Computes the molarity of a solution from the mass of solute, its molecular weight, and the solution volume. Use it when preparing reagents, buffers, or standards in the lab.
About this calculator
Molarity (M) measures the concentration of a solution as moles of solute per liter of solution. The full formula linking all variables is: M = (mass / molecular_weight) / volume, where mass is in grams, molecular_weight in g/mol, and volume in liters. Breaking it down: moles = mass / molecular_weight first converts grams to moles using the molar mass, then dividing by volume gives concentration. You can rearrange for any unknown: mass = M × molecular_weight × volume; volume = (mass / molecular_weight) / M. Molarity is temperature-dependent because liquid volumes change with temperature, so solutions are typically prepared at and labeled for a standard temperature (usually 25 °C). It is the most common concentration unit in quantitative chemistry.
How to use
You want to prepare 500 mL (0.500 L) of a 1.00 M sodium chloride (NaCl) solution. NaCl has a molecular weight of 58.44 g/mol. Enter Mass = 29.22 g, Molecular Weight = 58.44 g/mol, Volume = 0.500 L. The calculator computes: moles = 29.22 / 58.44 = 0.500 mol; Molarity = 0.500 / 0.500 = 1.00 M. Weigh out exactly 29.22 g of NaCl, dissolve in a small amount of water, then transfer to a 500 mL volumetric flask and fill to the mark.
Frequently asked questions
What is the difference between molarity and molality in chemistry?
Molarity (M) expresses moles of solute per liter of solution, while molality (m) expresses moles of solute per kilogram of solvent. Molarity is easier to measure volumetrically, making it standard for lab solution preparation. Molality is preferred in thermodynamic calculations (boiling point elevation, freezing point depression) because it is independent of temperature — the mass of solvent does not change with temperature the way volume does. For dilute aqueous solutions at 25 °C, the two values are nearly equal.
How do I convert from percent concentration to molarity?
To convert % w/v (grams of solute per 100 mL of solution) to molarity: M = (% w/v × 10) / molecular_weight. For example, a 36% w/v HCl solution with MW = 36.46 g/mol gives M = (36 × 10) / 36.46 = 9.87 M. For % w/w solutions you also need the solution density: M = (% w/w × density × 10) / molecular_weight. This conversion is essential when working with commercially supplied concentrated acids and bases that are labeled in percent purity.
Why does the volume used in the molarity formula refer to the total solution volume, not just the solvent volume?
Molarity is defined as moles per liter of solution, meaning the final, combined volume of solute plus solvent. If you simply added solute to one liter of water, the total solution volume would be slightly more than one liter, giving a slightly lower concentration than intended. Proper volumetric technique involves dissolving the solute first in less solvent, transferring to a calibrated volumetric flask, and then adding solvent to the final volume mark. This distinction matters most for concentrated solutions where the solute contributes significantly to total volume.