Chemistry · Solutions
Molarity calculator.
Solve molarity, the mass to weigh out for a target molarity, or the final volume. Formula in, molar mass filled.
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Molarity formula
Molarity (M) is the moles of solute per litre of solution: M = mol / L. Moles come from mass: mol = grams / molar mass. Combine them to get grams = M x litres x molar mass.
Worked example 1. Dissolve 5.85 g of NaCl (molar mass 58.44 g/mol) and make it up to 500 mL. Moles = 5.85 / 58.44 = 0.1001 mol. Molarity = 0.1001 / 0.500 L = 0.2002 M.
Worked example 2. To make 250 mL of 0.1 M NaOH (molar mass 39.997 g/mol): moles = 0.1 x 0.250 = 0.025 mol, mass = 0.025 x 39.997 = 0.9999 g. Put the solute in a volumetric flask, dissolve, and fill to the mark, so the volume is that of the finished solution, not of the water you start with.
Use the molar mass printed on your reagent bottle if it is a hydrate or a stated purity. Real reagents (for example NaOH, which absorbs water and carbon dioxide) are rarely exactly 100% pure, so a solution for quantitative work is normally standardised.
Frequently asked questions
How do I calculate molarity?
Divide the moles of solute by the litres of solution. If you have grams, divide by the molar mass first to get moles.
How many grams do I need for a given molarity?
Grams = molarity (mol/L) x volume (L) x molar mass (g/mol).
Is the volume the solvent volume or the solution volume?
The solution volume, the total after the solute is dissolved. That is why you dissolve the solute and then fill to the mark of a volumetric flask.
What is the difference between molarity and molality?
Molarity is moles per litre of solution. Molality is moles per kilogram of solvent. This tool only does molarity.
Sources
Checked
- OpenStax Chemistry 2e, 3.3 Molarity (molarity and the dilution equation)
- OpenStax Chemistry 2e, 3.1 Formula Mass and the Mole Concept
- CIAAW: Abridged Standard Atomic Weights 2024
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Lab safety and scope
This is arithmetic, not professional advice. Check every result against your own procedure, the safety data sheet (SDS) for each chemical, and your lab supervisor or instructor before you prepare, mix or dispose of anything. Verify values on the reagent bottle, since purity and hydration state change the real molar mass.