Molarity Calculator
Work out the molar concentration of a solution in moles per litre from the mass of solute, its molar mass and the volume — or flip it round to find the mass you need to make a target molarity.
Molarity is the most common way chemists describe how concentrated a solution is. It counts the number of moles of dissolved substance in each litre of solution, written as M or mol/L. Because you usually measure out a solid in grams rather than moles, this calculator first converts your mass to moles using the molar mass, then divides by the volume. Switch modes and it runs the sum backwards to tell you how many grams to weigh for a concentration you want.
How the calculation works
molarity (mol/L) = moles ÷ volume (L)
mass needed (g) = molarity × volume (L) × molar mass
Remember to put the volume in litres — 500 mL is 0.5 L. The volume is the total volume of the finished solution, not the volume of water you started with.
Worked example
To find the molarity of 5.85 g of table salt (NaCl, molar mass 58.44 g/mol) dissolved to make 500 mL of solution: moles = 5.85 ÷ 58.44 = 0.1001 mol; volume = 0.5 L; molarity = 0.1001 ÷ 0.5 = 0.20 mol/L. Going the other way, to make 0.5 L of a 0.20 M solution you need 0.20 × 0.5 × 58.44 = 5.84 g.
Molarity, molality and normality
Molarity (mol per litre of solution) is not the same as molality (mol per kilogram of solvent) or normality (equivalents per litre). Molarity is temperature-dependent because liquid volume changes slightly with temperature, whereas molality is not. For most bench chemistry at room temperature, molarity is the practical choice.