Calculate molarity, the mass of solute needed to prepare a solution, or the volume of solution a known mass makes.
g/mol
Get it from the formula with the molar mass calculator.
g
mol/L
Enter the values to see the result here.
Formula
Molarity M is the amount of solute n per volume V of solution, and n=m/Mr with Mr the molar mass:
M=Vn=MrVmm=MVMrV=MrMm
The volume is in litres, the mass in grams and the molar mass in g/mol.
Worked example
How much sodium chloride is needed for 250 mL of a 0.10 M solution?
Volume: 250mL=0.250 L.
Moles: 0.10×0.250=0.0250 mol.
Mass: 0.0250×58.44=1.461 g.
Dissolve 1.461 g in some water and make it up to exactly 250 mL.
Common mistakes
Leaving the volume in millilitres while the formula needs litres. Use the unit selector.
Adding the solute to 250 mL of water. The final volume of solution must be 250 mL, not the water alone.
Using the molar mass of the wrong form, such as the anhydrous salt when you weighed the hydrate.
Confusing molarity (mol/L) with percent concentration.
Frequently asked questions
What is the difference between molarity and molality?
Molarity is moles of solute per litre of solution (mol/L) and changes slightly with temperature because the volume does. Molality is moles per kilogram of solvent (mol/kg) and does not change with temperature.
Is the volume the solvent or the final solution?
The final solution. To make 0.5 L of solution you dissolve the solute in less water and then top up to the 0.5 L mark in a volumetric flask.
How do I get the molar mass?
Add the atomic weights of all atoms in the formula, or type the formula into the molar mass calculator. For NaCl it is 22.990 + 35.45 = 58.44 g/mol.