Free lab tool
Molarity Calculator
To make a solution of a given molarity, weigh out mass (g) = molarity (mol/L) × volume (L) × molecular weight (g/mol). This free calculator gives the mass needed for a target molarity and volume, or works backwards to the molarity of a known mass, in mg or g and in mM or M.
Calculator
From the reagent label or data sheet. Include water of crystallisation for hydrates.
Mass to weigh out
Enter your values to see the result.
Results are a guide. Check them against your protocol, the reagent data sheet and your instrument manual.
How it works
Molarity is the amount of substance in moles per litre of solution, so 1 M is 1 mol/L. The molecular weight converts moles to grams, which is the quantity you weigh on the balance.
Formula
mass (g) = molarity (mol/L) × volume (L) × molecular weight (g/mol)
molarity (mol/L) = mass (g) ÷ ( molecular weight (g/mol) × volume (L) )
Where
- molarity
- moles of solute per litre of solution
- volume
- final volume of the solution, in litres
- molecular weight
- molar mass of the compound in g/mol, taken from the reagent label or data sheet
Good to know
- Dissolve the solid in less than the final volume, then make up to the mark in a volumetric flask, because molarity refers to the final volume of solution.
- For hydrates, use the molecular weight that includes the water of crystallisation. If the reagent is below 100% purity, divide the mass by the purity as a fraction.
Worked example
Example: 500 mL of 0.1 M sodium chloride (MW 58.44 g/mol)
- 01
Volume = 500 mL = 0.5 L, molarity = 0.1 mol/L.
- 02
Moles needed = 0.1 × 0.5 = 0.05 mol.
- 03
Mass = 0.05 mol × 58.44 g/mol = 2.922 g.
Weigh out 2.922 g and make up to 500 mL. In reverse, 5 mg of a compound with MW 250 g/mol in 10 mL is 20 µmol in 0.01 L, which is 2 mM.
Frequently asked questions
What is molarity?
Molarity is the concentration of a solution expressed as moles of solute per litre of solution. A 1 M solution contains one mole per litre, and a 1 mM solution contains one thousandth of that.
How do I calculate the mass needed for a molar solution?
Multiply the molarity in mol/L by the final volume in litres and by the molecular weight in g/mol. For 500 mL of 0.1 M sodium chloride (58.44 g/mol), that is 0.1 × 0.5 × 58.44 = 2.922 g.
Where do I find the molecular weight?
It is printed on the reagent label and on the supplier's data sheet or certificate of analysis. For hydrated salts, use the weight that includes the water of crystallisation, because that is the form you are weighing.
Should I add the solid to the full volume of solvent?
No. Dissolve the solid in less than the final volume, then add solvent up to the final volume mark. Molarity is defined per litre of finished solution, not per litre of added solvent.
Does reagent purity matter?
Yes. This calculator assumes a pure reagent. If your reagent is, for example, 98% pure, divide the calculated mass by 0.98 to get the mass to weigh out.
Reagents and weighing equipment
Al-Shaker supplies laboratory instruments and consumables across Jordan and MENA, including Vivantis molecular biology reagents and analytical balances for accurate weighing. Browse the catalogue or ask the team for a quotation.
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