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Molarity Calculator

Work out how much solute to weigh for a molar solution, or solve for the concentration or the final volume from the other two.

Formula

m=C×V×MWm = C \times V \times \mathrm{MW}
C=mMW×VC = \dfrac{m}{\mathrm{MW}\times V}
V=mMW×CV = \dfrac{m}{\mathrm{MW}\times C}
mm
mass of solute, in grams
CC
molar concentration, in mol/L
VV
final volume of the solution, in litres
MW\mathrm{MW}
molecular weight of the solute, in g/mol

How it works

Molarity is the amount of solute, in moles, per litre of finished solution. Multiplying the target concentration by the final volume gives the moles needed, and multiplying by the molecular weight turns moles into a mass you can weigh.

Every input is converted to mol/L, litres and grams before the formula is applied, and the answer is reported in whichever unit reads most naturally.

Worked example

Prepare 250 mL of 150 mM sodium chloride (NaCl, 58.44 g/mol).

  1. 150 mM = 0.15 mol/L, and 250 mL = 0.25 L.
  2. m = 0.15 mol/L × 0.25 L × 58.44 g/mol = 2.1915 g.

Weigh 2.19 g of NaCl, dissolve it in less than 250 mL of water, then make the volume up to 250 mL.

These are the values the calculator opens with, so you can check its output against this example.

Assumptions

  • The molecular weight you enter matches the reagent in the bottle, including any water of hydration or counter-ion.
  • The volume is that of the finished solution, not the volume of solvent added.
  • The reagent is 100% pure. Thermal expansion of the solution is ignored.

Common mistakes

  • Using the anhydrous molecular weight for a hydrate, or the reverse. Use the formula weight printed on the bottle.
  • Adding the solute to the full volume of solvent. Dissolve in less, then bring the solution to the final volume.
  • Forgetting purity. For a reagent that is 95% pure, divide the calculated mass by 0.95.

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