Molarity calculator
To make a solution of a given molarity, weigh mass (g) = concentration (mol/L) × volume (L) × molar mass (g/mol). Enter any two of mass, concentration and volume plus the molar mass, and the calculator gives the third, with units from kilograms to nanograms and molar to picomolar.
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What it does
How to use the molarity calculator
- Choose what to calculateMass to weigh, the resulting concentration, or the final volume.
- Enter the molar massPick a reagent or type the formula weight from the bottle (it includes any water of hydration).
- Enter the known valuesType the other two values with their units; the result updates as you type.
How it works
m = C × V × M
C = m / (V × M)
V = m / (C × M)
where m is the mass in grams, C the molar concentration in mol/L (M), V the volume in litres and M the molar mass in g/mol. All inputs are converted to these base units first.
Worked example
To make 500 mL of 150 mM NaCl (M = 58.44 g/mol): m = 0.150 mol/L × 0.500 L × 58.44 g/mol = 4.383 g. Dissolve in about 400 mL of water and make up to 500 mL.
Frequently asked questions
How do you calculate molarity?
Molarity is moles of solute divided by litres of solution: M = n / V, and n = mass / molar mass. So M = mass / (molar mass × volume).
What is the difference between molarity and molality?
Molarity is moles per litre of solution; molality is moles per kilogram of solvent. Molality does not change with temperature, molarity does slightly.
Should I use the molar mass of the hydrate?
Yes, if you weigh the hydrate. MgCl₂·6H₂O is 203.30 g/mol, while anhydrous MgCl₂ is 95.21 g/mol; use the formula weight printed on your bottle.
Why make up to volume instead of adding solvent to the volume?
The solute takes up space, so dissolving in the full volume of solvent gives slightly more than the target volume and a lower concentration.
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