Molarity Calculator

What molarity actually measures

Molarity is moles of solute per litre of solution, not per litre of solvent. A 1 M solution contains one mole of solute in a total volume of one litre, which is why you dissolve first and make up to the mark afterwards. For dilute aqueous solutions the difference is small; for concentrated ones it is not.

Because moles are mass divided by molar mass, the working equation is m = c × M × V. Every quantity has to be in consistent units before you multiply, which is where most errors creep in. This calculator converts everything to mol/L, grams and litres internally, so mixing µL with mM is safe.

Worked example

To make 500 mL of 100 mM NaCl (molar mass 58.44 g/mol):

• Convert: 100 mM is 0.1 mol/L, and 500 mL is 0.5 L.
• Moles needed: 0.1 × 0.5 = 0.05 mol.
• Mass: 0.05 × 58.44 = 2.922 g.

Weigh 2.922 g, dissolve in roughly 400 mL of water, then make up to the 500 mL mark once it has fully dissolved.

Molarity, molality and percent

Molarity (M) is per litre of solution and changes with temperature, because the solution expands. Molality (mol/kg) is per kilogram of solvent and does not, which is why it appears in colligative-property work like freezing point depression. Percent w/v is grams per 100 mL and carries no information about molar mass at all, so it cannot be compared across different solutes without converting.

Common mistakes

• Adding the solvent to the mark before dissolving. The volume changes as the solid goes into solution, so the final concentration will be off.
• Using the anhydrous molar mass for a hydrate. CuSO₄ is 159.60 g/mol but CuSO₄·5H₂O is 249.68. Using the wrong one makes you weigh about 36% too little.
• Ignoring purity for reagents supplied below 100%. Divide the calculated mass by the fractional purity.
• Assuming molarity is temperature-independent. For work that spans a wide temperature range, use molality.

Converting units first? Use the concentration, volume and mass conversion tables.